Laser melting, evaporation, and fragmentation of nanoparticles: Experiments, modeling, and applications Open Access

c₀

heat capacity of particle material [J·g⁻¹·K⁻¹]

c₁

heat capacity of environment [J·g⁻¹·K⁻¹]

E

laser fluence [J·m⁻²]

E_M

threshold fluence of melting [J·m⁻²]

E_EV

threshold fluence of evaporation [J·m⁻²]

E_FR

threshold fluence of fragmentation [J·m⁻²]

I(I₀)

radiation intensity [W·m⁻²]

K_abs

particle absorption efficiency factor

$KabsM$

absorption factor for a particle of molten gold

k₀

thermal conductivity coefficient of particle material [W·m⁻¹·K⁻¹]

k₁

thermal conductivity coefficient of environment (water) [W·m⁻¹·K⁻¹]

L_M

specific melting energy of mass unit [J·g⁻¹]

L_EV

specific evaporation energy of mass unit [J·g⁻¹]

Q_abs

radiation energy absorbed by particle [J]

Q_EV

energy spent on particle evaporation [J]

Q_M

energy spent on particle melting [J]

Q_T

thermal energy of particle [J]

Q_T∞

initial thermal energy of particle [J]

Q_C

energy spent on thermal conduction [J]

r_∞

initial radius of particle [m]

r₀

radius of nanoparticle [m]

r_f

radius of fragmented nanoparticle [m]

t

time [s]

t_P

pulse duration [s]

T₀

particle temperature [K]

T_∞

initial particle temperature [K]

T_M

melting temperature (point) [K]

T_EV

evaporation temperature [K]

$T0max$

maximum particle temperature [K]

α

part of molten volume of a particle

β

part of evaporated volume of a particle

λ

wavelength [m]

ν

pulse frequency [Hz]

ρ₀

density of particle material [g·cm⁻³]

ρ₁

density of environment (water) [g·cm⁻³]

χ

thermal diffusivity [m² s⁻¹]

τ₀

characteristic time [s]

EV

evaporation

M

melting

0

parameter of nanoparticle

1

parameter of environment

∞

initial value

NP

nanoparticle

TEM

transmission electron microscopy

SEM

scanning electron microscopy

SMP

submicrometer particle

MD

molecular dynamics

Over the past two and a half decades, there has been increased interest in laser radiation interacting with nanoparticles (NPs) and the subsequent processes, leading to the emergence of the field of laser processing of NPs. The mid-1990s were when experimental studies of the interaction of laser pulses with NPs were first reported. In 1999, Koda and colleagues^1,2 studied the size reduction of gold NPs in an aqueous solution using pulsed laser irradiation with a nanosecond duration and a wavelength 532 nm. The initially nonspherical particles became spherical, which was caused by their heating during a very short time of laser irradiation, i.e., the melting of the NPs.¹ The size reduction ceased after 5 min of irradiation, and the maximum diameter in the size distribution decreased significantly.² It was believed that the change in shape and reduction in size occurred because of the melting and evaporation of the gold NPs; the high temperature of the NPs causing melting and evaporation is the result of thermal mechanisms due to the strong absorption of laser energy by the particles and the low heat transfer to the surrounding water. In work by Fujiwara et al.,³ thionicotinamide-capped gold NPs underwent fusion as well as fragmentation when excited by a nanosecond laser pulse (532 nm), and it was established that the morphological changes caused by thermal and photochemical effects affected the optical properties of these particles. Link et al.⁴ found that gold nanorods changed their shape after excitation by intense pulsed laser irradiation, depending on the energy of the laser pulse and its width, with durations of 100 fs and 7 ns. The shape transformations of the gold nanorods were monitored using visible absorption spectroscopy and transmission electron microscopy (TEM) to analyze the final shape and size distribution; fragmentation of the gold NPs occurred at high laser fluence (∼1 J·cm⁻²), while melting of the nanorods into spherical NPs was observed with decreasing energy of the femtosecond laser pulses.

Inasawa et al.⁵ synthesized gold NPs (average diameter: 8.3 nm) from a solution of tetrachloroaurate complex by laser irradiation at a wavelength of 308 nm without the use of any stabilizers. The combination of photochemical particle growth with photothermal particle size reduction under the 308-nm laser irradiation resulted in a narrow size distribution. A thermal model was proposed to estimate the maximum diameter of gold NPs formed in the system under the irradiation of nanosecond laser pulses. Pustovalov et al.⁶ published one of the first theoretical reviews of the thermal processes of laser–NP interaction, with a theoretical model that determined how the NP temperature depends on time and the NP and laser parameters. The thermal processes that happen during the nanosecond, picosecond, and femtosecond laser irradiation of various NPs continue to be actively studied.

In 1998, Kamat et al.⁷ studied the picosecond dynamics of silver nanoclusters under laser irradiation, accompanied by photoejection of electrons and fragmentation of NPs. The fragmentation of silver clusters with a size 40–60 nm into smaller clusters (5–20 nm) under laser-pulse excitation with a wavelength 355 nm was discovered, which ensures size selectivity in cluster fragmentation. Picosecond pump/probe studies confirmed that electron ejection is one of the main photochemical events leading to the photofragmentation process, and this nonthermal mechanism is the main reason for the sharp decrease in NP size in this case and was newly discovered. In another study, gold NPs with an average diameter of ca. 8 nm were irradiated by a pulsed laser with a wavelength of 355 nm in an aqueous solution.⁸ The observed transient absorption can be attributed to the transition of solvated electrons resulting from multiple ionizations of gold NPs, and the resulting charge state of the NPs was estimated from the transient absorption. TEM measurements after laser irradiation revealed that the gold NPs had fragmented into smaller ones. The observed correlation between the resulting charge states and the degree of size reduction of the gold NPs after laser treatment indicates that the size reduction was caused by the Coulomb explosion of highly charged NPs, as explained by the liquid-droplet model. Over the past decade, some debate has emerged regarding the explanation of NP fragmentation processes, calling into question the validity of some experimental observations and existing theoretical models.

So, there are two mechanisms for the laser processing of metal NPs depending on the pulse duration and laser wavelength, one thermal and the other nonthermal. In their own experiments, various authors have realized the main advantage of using metal NPs: upon laser irradiation, the enhanced light–matter interaction at plasmon resonance creates very high and time-localized temperature increments of the NPs. Subsequently, different laser wavelengths and pulse durations and sophisticated irradiation and diagnostic methods have been used to study the complex processes involved in the laser processing of various NPs, and Refs. 9–11 contain the latest results on the laser processing of NPs. All these developments based on using various plasmonic NPs and laser sources have recently expanded the range of direct applications. The ability to change the parameters and properties of NPs at the nanoscale has already impacted a wide range of research and industrial activities. The laser processing of NPs is now entering an important phase, with some studies reaching completion while others are moving on to industrial applications. The current research situation is focused specifically on overcoming fundamental challenges and realizing applications in many different areas of laser nanoscience and industry.

The melting of a metal NP is a physical process that results in a solid-to-liquid phase transition. When exposed to a laser pulse, the temperature of the NP increases to the melting temperature T_M, the order of the ions in the solid is broken to a less ordered state, and the solid melts into liquid metal. Under moderate laser pulse duration and fluence, melting of the entire volume of the NP is achieved when heat transfer equalizes the temperature throughout the entire volume of the NP and the melting temperature is reached. Melting of the NP is realized before its subsequent laser evaporation upon reaching the evaporation temperature T_EV, this being because T_EV is usually higher than T_M. Melting of the surface layer of the NP occurs under the action of short laser pulses without heat transfer throughout the entire volume of the NP during the pulse duration and with predominant absorption of laser energy in a thin surface layer. Melting of the entire volume of the NP and surface melting can be used in laser nanotechnology to spheroidize NPs that are initially nonspherical. Rapid cooling of the molten full volume or surface layer of the NP can change the internal structure of its material to form new material states.

Evaporation is predominantly the process of the thermal ejection of atoms with sufficiently high energy to allow them to overcome the potential barrier on the NP surface¹² and so reduce the NP size. A mass flow is created through the surface of the NP at approximately the boiling (evaporation) point of the NP metal (material) at T_EV by laser pulses of sufficient intensity and duration. The evaporation of atoms forms a hydrodynamic gas plume or diffusion of evaporated atoms into the liquid or gas surrounding the NP.

Laser ablation is the process of removing material from the solid surface of an NP; it includes the influence of material evaporation, the formation of plasma on the surface of the NP, and the ejection of clusters from the surface when using a laser of picosecond or femtosecond duration. Under extreme conditions, phase explosion and explosive boiling can occur; this is when the material is heated rapidly to well beyond its boiling point and overheats into a metastable thermodynamic state.¹² With rapid heating of the NP from ultrashort high-power laser heating, a temperature very close to the critical temperature can be reached.

The fragmentation of NPs under the influence of ultrashort laser pulses is usually associated with the formation of very small NPs resulting from a rapid and significant reduction in the size of the initial NPs. This reduction may be due to evaporation, a possible phase explosion of the NPs, or a Coulomb explosion triggered by the emission of electrons from the metal NPs, followed by fission due to electrostatic repulsion. However, the mechanisms underlying NP fragmentation are not yet fully understood.

The processes of NPs interacting with laser pulses—which can be used to control the size, shape, and structure of NPs in various laser nanotechnologies—have become an increasingly important research topic over the past 20 years.^9–11 Typically, laser pulses are used with a wavelength close to the plasmon resonance of the NPs (i.e., the energy is absorbed readily by the NPs, which then heat up) and a pulse duration short enough to minimize the heat losses of the NPs. This localization of energy inside the NPs initiates various thermal threshold processes. An important feature of the laser processing of NPs is heating so that the NP temperature T₀ equals or exceeds T_M or T_EV.

In this review, results pertaining to nanosecond, picosecond, and femtosecond laser pulses are presented and analyzed, with attention also paid to how CW radiation affects NPs. The pulse duration and laser wavelength and fluence have direct influences on the mechanisms and results of laser–NP interaction, and so these influences are studied especially herein. This review is focused on the latest publications, with an emphasis on results from recent years and especially 2021–2024, naturally taking into account the results of previously published work. The present analysis of experimental and theoretical results can be used to estimate the laser and NP parameters during laser–NP interactions in various laser technologies.

This section presents the main results of experimental studies of laser melting, evaporation, fragmentation, and breakdown of NPs as currently available. In many studies, experiments were carried out with single laser pulses; such experiments are more reliable and precise than those with laser pulse trains, and their data are directly comparable with theoretical results. The energy balance must be maintained during and after exposure to the laser pulse, taking into account possible changes in NP metals (materials), such as melting and evaporation. The problems involved in using single pulses to determine the threshold laser fluence include differences in the laser energy and pulse duration from pulse to pulse, as well as interaction with only a small part of the NP ensemble.

In some situations, a train of pulses is used to melt or evaporate NPs because of the impossibility of achieving results under single-pulse action. This is due to the low energy of the laser source used, the need to achieve results for all the particles in the studied assembly or colloidal volume, using the movement of the NP ensemble, and variable laser parameters (fluence, beam parameters, pulse duration) from pulse to pulse, among other issues.

If the time between adjacent pulses is sufficient for the NPs to cool to their initial temperature T_∞ before the start of the next pulse, then the effects of a pulse train on the NPs are the same as those of single pulses, taking into account the statistical nature of the effects of a train of laser pulses. On the other hand, using multi-pulse exposures provides the advantage of a more homogeneous radiation exposure by reducing the influence of pulse-to-pulse energy variation and spatial heterogeneity of the laser beam, and it provides more consistent and reproducible application conditions. Consequently, the single-pulse fluence threshold exceeds that for a train of ca. 10² to 10⁴ pulses.

Sections II A and II B analyze the results of experiments on the laser melting, evaporation, and fragmentation of gold and silver NPs, which are used widely in the reviewed sources. Section II C reports experimental studies of surface melting with ultrashort laser pulses without heat transfer throughout the entire NP volume during the pulse duration and subsequent melting of the entire NP volume after the end of the laser pulse.

Below are the main results obtained to date for the melting and evaporation of NPs by using nanosecond, picosecond, and femtosecond laser irradiation. In pioneering work by Takami et al.,² pulse trains from an Nd:YAG laser with a duration of t_P = 7 ns, a wavelength of λ = 532 nm, and a frequency of ν = 10 Hz were used to irradiate nonspherical NPs for 10 min (6000 pulses), the initial size distribution being 11–45 nm (Fig. 1). At a fluence of E = 14 mJ·cm⁻², neither the size nor the shape changed, and the particles remained nonspherical. Shape change began at a fluence of 16 mJ·cm⁻², and as the fluence was increased in the range of 30–500 mJ·cm⁻², the diameter of the gold NPs decreased.

The mechanism of photothermal evaporation of NPs by laser radiation—when the temperature of the NPs reaches or slightly exceeds the boiling point of the bulk material—was used to explain the decrease in the size of Au NPs under the action of nanosecond irradiation.² The degree of size reduction is proportional to the laser fluence, the irradiation period, and the number of pulses. Only the shape of the particle has changed, as can be seen from Fig. 1(b). Figure 1(c) shows the diameters obtained from TEM photographs as a function of the laser fluence. In the case of nanosecond laser pulses, the thermal heat loss of the NPs due to thermal conduction plays the main role, and thermal conduction acts during the entire duration of the pulse action (and after its end) and determines the heating of the NPs. Studies of nanosecond melting and evaporation of metal NPs are still ongoing.^13–21 

In experiments by Fales et al.,^16,17 multiple laser pulses were delivered to ensure that the entire volume of the sample was exposed uniformly to the laser radiation. The experimental testing techniques included TEM, dynamic light scattering, and spectrophotometry to evaluate the damage thresholds experimentally. This systematic study of the melting and evaporation thresholds was carried out using a nanosecond pulsed laser with a wavelength of 532 nm for NP diameters in the range of 20–100 nm. The advantage was the use of gold NPs with a strict nominal diameter of 20 nm, 40 nm, 60 nm, 80 nm, and 100 nm, which differs from many other studies that used randomly shaped NPs with a wide range of sizes obtained predominantly by laser ablation. In this case, the threshold values were determined based on a statistical probit analysis, with size uniformity allowing repeatable experiments and accurate measurements. These experimental results provide both a comprehensive characterization of the damage threshold for gold NPs when using nanosecond pulses with a wavelength of 532 nm and the basis for developing a method for determining the thresholds of laser-induced damage to NPs.

Inasawa et al. and others have studied the effects of picosecond pulses on NPs.^22–25 In particular, the evolution of the size distribution of gold NPs under irradiation with an Nd:YAG laser (λ = 355 nm, t_P = 30 ps) has been studied by TEM.²² Ellipsoidal NPs with an aspect ratio of 1.3 and a mean diameter of 25 nm were irradiated with pulse trains with a melting threshold fluence of E_M = 5 mJ·cm⁻² per pulse for 10 min at a frequency of 10 Hz to give them a spherical shape. The initial monomodal size distribution of NPs under the action of four, 10, and 50 laser pulses with an evaporation threshold fluence of E_EV = 23 mJ·cm⁻² per pulse turned into a bimodal one, with two peaks in the number of NPs: one at 6 nm and the other at 16–24 nm. This change is associated with laser-induced evaporation (size reduction) of the initial NPs with the subsequent formation of small NPs due to the cooling of gold vapor and its condensation and growth. This is the first time that a similar mechanism for the formation of small NPs has been proposed. NPs with gold strings were observed after single-pulse irradiation with a laser fluence of 43 mJ·cm⁻² per pulse, sufficient for boiling. It is believed that such NPs with gold strings form as a result of the projection of gaseous gold from liquid droplets. During the action of laser pulses with a pulse duration of 3 × 10⁻¹¹ s, the NP heat loss due to thermal conduction from the surfaces of the NPs is insignificant.

Figure 3 shows changes in gold NPs under laser pulses with t_P = 30 ps and a wavelength of 355 nm.²⁴ Fluences below 6.3 mJ·cm⁻² did not lead to a change in the transient absorption. Changes in gold NPs were observed as a result of their melting when the fluences changed from 6.3 mJ·cm⁻² to 17 mJ·cm⁻². Above 17 mJ·cm⁻², evaporation of gold NPs was observed at both wavelengths. The laser fluences of 15 mJ·cm⁻² [Fig. 3(b)] and 32 mJ·cm⁻² [Fig. 3(c)] correspond to the melting and evaporation of gold NPs, respectively. Spherical NPs form during melting, and small NPs form upon evaporation and reduction in the size of the original gold NPs.

Results for the irradiation of gold NPs with femtosecond pulses have been presented.^26–31 The interaction of a single femtosecond pulse (t_P = 50 fs, λ = 550 nm) with gold NPs with an average particle diameter of 16 nm, 23 nm, and 45 nm was studied experimentally.²⁶ Laser energy is absorbed by gold NPs without significant thermal losses to the environment due to thermal conductivity during the pulse.⁹ NPs were irradiated with a series of one, two, five, and 10 pulses of 550 nm with a fluence of 75 mJ·cm⁻² per pulse. About 5000 particles were visualized in each experiment. Before irradiation [Fig. 4(a)], the solution contained mainly nanospheres as well as a certain number of particles of various shapes, including triangles, rods, etc. After the first pulse, a more uniform set of NPs was observed with a lower content of nanorods, triangles, etc. [Fig. 4(b)]. This trend continued with each additional pulse [Fig. 4(d)] until almost all particles became spheres with a narrower size distribution. Using TEM and optical spectroscopy, it was established that NPs irradiated with a single femtosecond pulse at a resonant wavelength undergo morphological changes (melting, evaporation), and the pulse fluences are sufficient for the melting and evaporation of particles.

Note the results of a femtosecond laser with t_P = 35 fs, which led to modification of the optical properties of a composite material, i.e., gold NPs embedded in a borosilicate glass host.³¹ Also interesting are studies of the structural transformation of cubic³² and octahedral NPs³³ and nanorods³⁴ to spherical shapes. Photothermal reshaping of gold NPs in a plasmonic absorber using nanosecond pulses could be used for temperature-related applications.³² Structural modifications of gold nanorods under femtosecond excitation were recorded by the pump–probe method.³⁴ Single-pulse reactions include an irreversible change in the shape of nanorods at 30 J·m⁻². Monodisperse octahedral NPs with an average edge length of 72 nm and a narrow size distribution were irradiated with a nanosecond pulsed laser with λ = 532 nm and a laser fluence of 3.84 mJ·cm⁻² for 60 s.³³ 

Irradiation of gold-nanorod colloids with a femtosecond laser can cause a controlled change in the shape of the nanorods, obtaining colloids with narrow surface plasmon resonance bands.^35,36 The process uses multi-shot reductions in the aspect ratio while leaving the rod shape and volume virtually unaffected, and it provides gold nanorods with exceptional optical response. These methods^32–36 can be used to obtain highly monodispersed gold nanospheres by pulsed laser irradiation of polyhedral nanocrystal colloids.

The Koshizaki group carried out a set of studies on the fabrication of spherical submicrometer particles (SMPs).^18–20 In the framework of this approach, initial raw NPs dispersed in a liquid are heated by a pulsed laser to above the melting point to form molten nanodroplets, followed by their fusion and quenching to form SMPs. The interaction processes of laser pulses with E = 60 mJ·cm⁻², λ = 532 nm, t_P = 7 ns, ν = 10 Hz, and a laser exposure duration of 15 min with Au NPs of size 65 ± 4 nm were studied.¹⁹ The size-dependent agglomeration mechanism of Au NPs is applicable for increasing the growth efficiency of Au SMPs by laser-melting Au NPs prepared by laser ablation in liquids [Fig. 6(a)] and removing small Au NPs with a size of ca. 10–20 nm by centrifugation [Fig. 6(b)]. Figures 6(c) and 6(d) show Au SMPs prepared by laser melting from these Au NPs, and larger Au SMPs are most common in Fig. 6(d).

Four types of gold NPs—including nanostars, nanoantennas, and nanorods—moving in a capillary flow were irradiated by one-shot modes while adjusting the capillary flow and the pulse repetition rate of nanosecond pulses with a particle resonance wavelength of 900 nm.²¹ Measurements of extinction spectra and electron microscopy were used to evaluate the changes in the particle morphology and associated thresholds.

A study was carried out on the application areas of the three classical thermodynamic melting models due to Pawlow, Rie, and Reiss.³⁷ The calculated melting temperatures of Au, Al, and Sn NPs agreed well with experimental results.

Laser pulsed fragmentation of gold NPs has been studied^38–47 as a process in which many much-smaller NPs are generated from laser action on large NPs. Size reduction of Au NPs under laser action (532 nm, 9 ns, 10 Hz) with a fluence of 10–500 mJ·cm⁻² was carried out, and the average size of the gold NPs was reduced to a few nanometers by using laser treatments with different strategies.³⁹ Increasing the fluence from 88 mJ·cm⁻² to 442 mJ·cm⁻² decreased the average diameter of the Au NPs from 24 nm to 4.6 nm because of evaporation, and the subsequent cooling of vaporized Au atoms led to their condensation and coagulation into smaller NPs. TEM analysis carried out on the initial Au NP solution and that irradiated at 88 mJ·cm⁻² and 442 mJ·cm⁻² fully confirmed the significant NP size reduction.

A study was reported of the laser fragmentation of colloidal gold NPs driven by a one-step mechanism using nanosecond pulses.⁴⁵ Regarding the thermal processes, the sequence of heating–melting–evaporation–condensation was realized as the particle temperature reached the melting and evaporation temperatures, and the evaporated atoms underwent subsequent nucleated growth to form small particles. Two nanosecond lasers with a maximum intensity of 2.8 × 10¹² W·m⁻² and a wavelength of 532 nm were used in the experiment, one with a pulse duration of t_P = 9 ns and a frequency of 100 Hz (low) and the other with t_P = 7 ns and 2 kHz (high), as well as a picosecond laser with a maximum intensity of 1.6 × 10¹⁵ W·m⁻² (532 nm, t_P = 10 ps, 80 kHz). Single-pulse laser fragmentation of gold NPs in liquid by nanosecond pulses is a one-pulse and one-stage event. Up to a laser intensity of 1.6 × 10¹² W·m⁻², fragmentation forms a bimodal particle size distribution with diameters of 3.2 ± 0.2 nm and 18.8 ± 2.5 nm (Fig. 8), which can be explained by thermal evaporation. Above 1.6 × 10¹² W·m⁻², the first fraction of particles becomes smaller (2.9 ± 0.02 nm) and the second disappears. In this case, large particles evaporate and the formation of a vapor phase is thermodynamically favorable. Unfortunately, only two experimental points were provided for the nanosecond and picosecond cases in the second regime, which is insufficient for understanding the processes in this time range. In Fig. 8, the data point marked with an asterisk (*) shows statistically significant differences from the previous data, but there is no explanation for the jump in laser intensity at 1.6 × 10¹² W·m⁻².

The formation of particles from the vapor phase is influenced successively by two main phenomena: nucleation and particle growth. To understand the results, the evaporation–heat–energy balance was used, but unfortunately without heat loss due to thermal conductivity from the NP surface.

Irradiation of gold NPs (with initial size greater than 25 nm) with near-IR pulses at 800 nm, 1200 nm, and 1350 nm with t_P = 56 fs and E = 1.3–5.3 J·cm⁻² leads to the formation of very small NPs of 2.5 nm with a narrow size distribution (±0.5 nm).⁴⁴ The final size of the fragmented NPs depends on the laser pulse fluence and irradiation time, but it depends little on the initial size and shape of the particles, as well as on the laser wavelength.

An experimental continuation of Ref. 45 is the study of the kinetics of picosecond laser-induced heating and fragmentation of gold spheres.⁴⁶ Excitation by ultrashort pulses can lead to the formation of a nonequilibrium state of the NP material, which can result in an explosion or spinodal destruction with the formation of small particles. Fragmentation of gold NPs with a diameter of 54 nm by picosecond pulses was recorded by time resolved X-ray scattering. Almost complete conversion to ca. 2–3-nm particles was observed at a maximum applied fluence of 1800 J·m⁻², which heats the particles close to complete evaporation of the original particles, and the evaporation process is followed by the nucleation and growth of secondary particles. Laser-induced fragmentation was identified as a one-step instantaneous reaction, and the obtained results mean that the scenario of fragmentation due to evaporation with complete atomization of NPs and subsequent nucleation growth of atomic clusters cannot be excluded. The present level of experimental techniques does not yet allow us to establish unambiguously a quantitative boundary between phase explosion, charge instability, or thermal evaporation during laser fragmentation of NPs.

The threshold fragmentation fluences of spherical Au NPs were studied in a wide size range of 5–100 nm using single picosecond and nanosecond laser pulses.⁴⁷ A plate reader was used to measure the extinction spectrum from 400 nm to 800 nm. For 15-nm Au NPs under the action of a picosecond laser (λ = 532 nm, t_P = 28 ps), fragmentation occurs for E ≥ 1 mJ·cm⁻² [Fig. 9(a)]. By contrast, fragmentation under the action of a nanosecond laser (532 nm, t_P = 6 ns) requires a much higher fluence of 300–400 mJ·cm⁻² [Fig. 9(b)], which is confirmed by TEM data. Importantly, note that instead of the formation of small spherical fragments under the action of a picosecond laser, nanosecond laser-induced fragmentation forms continuous extrusions that attach to the original particles. Previous studies have shown that the entire particle fragments when the laser fluence is sufficiently large, being only ca. 180 mJ·cm⁻² for larger gold NPs of size 54 nm⁴⁶ but ca. 1 J·cm⁻² for NPs of size 15–25 nm.⁴⁵ These results for 15 nm contradict sharply those presented in Ref. 47.

The fragmentation behavior due to the action of picosecond and nanosecond lasers shows obvious differences, including threshold fluence (one to two orders of magnitude lower for picosecond) and change in morphology (discrete particles vs extrusion). While fragmentation induced by nanosecond lasers can be elucidated through a photothermal mechanism, neither the photothermal and Coulomb mechanisms nor thermomechanical forces adequately account for picosecond laser-induced fragmentation, which occurs when Au NPs are in a solid state. This perspective aligns with the findings presented in Ref. 47, but it is challenging to reconcile these claims with the analyses conducted in previous studies. The author posits that picosecond laser stimulation leads to particle fragmentation below the melting point of the entire particle and beneath the threshold for nonthermal mechanisms. It is hypothesized that near-field enhancement and NP surface melting explain picosecond laser-induced fragmentation at low laser intensity,⁴⁷ but this hypothesis awaits confirmation and discussion by subsequent independent studies.

A set of experimental studies was carried out on the evaporation and fragmentation of NPs by laser pulses with various pulse durations.^11,25,41,42 However, those studies provided no values of the threshold fluences, and it is impossible to connect the realized modes with concrete mechanisms and present a quantitative description of the experimental results. Stable and contaminant-free crystalline colloidal gold NPs with a very narrow size distribution of ca. 3–5 nm have the potential for use in various nanotechnologies.

Determining the laser thresholds for the melting (E_M), evaporation (E_EV), and fragmentation (E_FR) of NPs is a very complex task that depends on the sensitivity, correctness, and reproducibility of the methods used. This is determined by the methods of processing the experimental results (probit analysis, the influence of experimental errors, etc.). The influences of the environment and the various laser parameters (e.g., wavelength, pulse duration, beam characteristics) and NP parameters (e.g., shape, size, material) must be taken into account and strongly influence the final results. Moreover, the difference between nonthreshold and threshold fluence values is determined only from TEM images in Figs. 1–9 and leads to additional errors in the values. Tables of experimental thresholds are presented very rarely in articles. Finally, the number of reported values of threshold parameters is currently insufficient, and further research is absolutely necessary. The threshold fluence values themselves are presented herein in Tables I and II as approximate values.

A brief quantitative analysis of the numerical data in Tables I and II on the laser thresholds leading to melting, evaporation, and fragmentation of NPs leads to the following conclusions. An increase in r₀ leads to an increase in the threshold fluences approximately in proportion. Note that in Ref. 15, experiments were carried out under pressure up to ca. 100 MPa and beyond, and this affected the threshold data. At the same time, the thresholds for nanosecond pulses^2,13,15,17 differ from each other by no more than a factor of ca. 2. On the other hand, the results of Refs. 33 and 47 contradict these results. In Ref. 33, the threshold values are lower, but in Ref. 47 for r₀ < 15 nm, they significantly exceed the results given above. More-solid results have been obtained by using multiple laser irradiation, but experiments with single laser pulses were also used, which showed corresponding results. Note also the influence of the laser wavelength used at a fixed pulse duration, which is closely related to the change in the absorption of laser energy by NPs when the laser wavelength changes. Threshold laser fluences leading to the initiation of complete evaporation (fragmentation) of gold NPs by nanosecond pulses also showed differences in their values, determined by different numbers of pulses, NP radii, etc. Note that there is a slight increase in the results⁴⁵ for t_P = 7 × 10⁻⁹ s compared to our own results for t = 9 × 10⁻⁹ s. The general conclusion that can be drawn from the presented results is to continue experimental studies of laser interaction with NPs and more-correctly determine the threshold parameters for the possible use of the results in industrial applications.

Silver is one of the best metals available because of its low optical loss over a wide wavelength range encompassing the entire visible optical spectrum. Based on their optical and thermo-optical properties, silver NPs can be used in novel nanomaterials, biosensors, and optoelectronic and nanophotonic devices, as well as in laser nanobiomedicine and for cancer treatment using their antimicrobial and antibacterial action.

Experimental studies have been carried out on laser-induced modification of the size and shape of silver NPs in various media.^7,48–61 In early work, Kamat et al.⁷ proposed the nonthermal mechanism of Coulomb fragmentation of silver NPs of size 40–60 nm by picosecond laser pulses in an aqueous medium, and this mechanism is discussed in detail in Sec. II D.

The melting of silver NPs by laser irradiation has been studied.^50,54,55 The temperature dependence of the surface plasmon energy of silver NPs behaves nonmonotonically, indicating the melting of silver NPs of size 8–30 nm.⁵⁰ This process was suggested for use as a probe test for changing the plasmonic properties of NPs. Plasmonic nanostructures including Ag NPs can be heated efficiently without collateral damage to the surrounding material when they are irradiated in a special way by multiple femtosecond pulses with low fluence.⁵⁴ Femtosecond laser irradiation with E ∼ 7.2 mJ·cm⁻² makes it possible to heat and melt individual Ag NPs with a size of ca. 50 nm contained in extended plasmonic nanostructures. It was found that this effect of selective heating is most evident in clusters of two, three, four, and seven Ag NPs. Melting is triggered by a localized enhancement of the electric field by surface plasmons at “hot spots” on NP aggregates, and after irradiation for 10 s (10 000 pulses), some spherical particles of ca. 80 nm and 160 nm in diameter were formed. Ultrafast laser-induced melting of silver NPs was studied experimentally using a femtosecond laser pulse.⁵⁵ A sintered Ag structure fabricated from Ag NP printing ink using a femtosecond laser (1064 nm; 300 fs; repetition rate: 1 MHz; maximum laser power: 3 W) has been studied experimentally. Silver NPs with a radius of 5 nm were printed on a glass substrate using a femtosecond fiber laser to sinter the printed lines of Ag NPs.

The fragmentation of silver NPs dispersed in various media has been studied experimentally.^52,53 The effect of laser pulse durations of 164 fs, 5 ps, 4 ns, 36 ns, 64 ns, and 100 ns at a wavelength of 1070 nm and with a fluence 0.10–0.13 mJ·cm⁻² on the generation of NPs by the fragmentation method was studied.⁵² Silver NPs with a size of ca. 25 nm were produced by laser ablation of an Ag target in distilled water according to a scanning scheme for 10 min, as well as by laser fragmentation. Ablation and fragmentation acted simultaneously on the NPs, and it is impossible to separate the role of each mechanism. NP microstructure was imaged by TEM and SEM, and the size distribution of the NPs was estimated by a particle size analyzer. In the nanosecond region, the NPs had diameters of 25–200 nm and formed linked clusters.

Seifert and colleagues^56–61 carried out a set of studies on the action of high-power femtosecond laser pulses with silver NPs embedded in glass, including their intensive heating and shape transformation with the formation of small silver fragments. Spherical silver NPs with an average diameter of 30 nm and embedded in float glass were irradiated with linearly polarized laser pulses (λ = 400 nm, t_P = 150 fs, ν = 1 kHz, maximum intensity I = 4 TW·cm⁻²), and several hundred pulses transformed the initially spherical Ag NPs into prolate spheroids.⁵⁷ After single-shot irradiation with intensity I₂ = 2 TW·cm⁻², a spherical particle was formed; at high intensity (>I₂), there was a nonspherical central Ag particle and much smaller fragments, which was interpreted as complete destruction of the Ag NPs. Laser-induced transformation of silver NPs can lead to the fabrication of optical structures with specific optical properties, fully optical data storage, and opto-electronic devices using metal–glass nanocomposites.

Results have been presented on the response of borosilicate glass doped with a certain concentration of noble (Ag, Au) metals to femtosecond and nanosecond laser pulses.⁵⁹ Optical and morphological changes in glass containing photoactive silver agents can induce the direct femtosecond generation of noble-metal NPs and various nanostructures in glass.⁶⁰ The laser-induced processing of silver-containing oxide glasses was discussed for fabricating integrated waveguides and optical circuits with innovative nonlinear optical and plasmonic properties. Laser annealing of metal silver NPs synthesized in glasses by ion implantation was studied.⁶¹ 

Some fundamental mechanisms of solid NP melting under femtosecond laser pulses at the nanoscale remain unclear, and their ultrafast phenomena have revived interest in unraveling NP melting processes. Surface melting and evaporation of NPs by laser pulses were studied experimentally.^23,62–67 However, direct experimental verification of various theoretical models is limited because of the difficulty of visualizing the internal structures of materials undergoing ultrafast and irreversible transitions.

Inasawa et al.²³ found that ellipsoidal gold NPs in solution undergo a transformation from ellipsoids to spheres at a temperature of ca. 940 °C, which is significantly lower than their melting point of ca. 1060 °C. Transformation of the shape of gold NPs induced by a single pulse (λ = 355 nm, t_P = 30 ps) was observed directly using TEM. It was revealed that the thickness of the molten layer on the surface of the solid NP core is 1.4 nm. It was mentioned for the first time that transformation of the shape of NPs occurs at a temperature significantly lower than their melting point because of the surface melting of the particles.

A study was carried out of the photothermal reshaping of single gold nanorods irradiated with femtosecond pulses at a temperature below the melting point caused by surface diffusion.⁶³ The melting of Bi NPs was studied by electron diffraction during ultrafast laser heating by a pulse with λ = 800 nm, t_P = 110 fs, and at a rate of ca. 10¹⁵ K·s⁻¹, and the melting temperature of NPs was ca. 44 K lower than the melting temperature T_M of bulk Bi.⁶⁴ This result is explained by the round shape of Bi NPs, which are bounded by the surfaces melting below T_M.

Studies have been carried out on the melting of single spherical Au NPs with a diameter of 100 nm, irradiated with single IR laser pulses with λ = 800 nm, E = 870 mJ·cm⁻², and t_P = 50 fs and using X-ray pulses for high spatiotemporal resolution.^65,67 Single-shot time-resolved imaging was used to provide direct evidence of irreversible melting with a spatial resolution of 10 nm and a temporal resolution of 10 ps.⁶⁵ Images of single Au NPs directly visualized heterogeneous surface melting and the ion distribution, followed by density fluctuations deep within the particle. The images showed the irreversible process of melting and disintegration of single Au NPs irradiated with a single femtosecond pulse.

The degree of variation in inhomogeneous density depending on the delay time is shown. The process began with a decrease in density as the NP started to melt and expand from the surface, which is often observed during heterogeneous melting. Reduced surface density was observed for images at 10 ps and more clearly at 40 ps because of the display of the density difference from a perfect sphere, showing inhomogeneous surface melting at 40 ps with a layer thickness of ca. 10 nm, indicated by two arrows in the inset in Fig. 10. As melting continued, regions of low density were formed, which subsequently increased and turned into a void inside the sphere [marked as arrows in Fig. 10(a)]. Penetrating toward the center of sphere, the void expanded with increasing density variation, which ultimately led to the complete destruction of the NP. The size of the spherical Au NPs was estimated using the collected images for each delay time [Fig. 10(b)]. The faster expansion of NPs particle size during the melting becomes pronounced after 40 ps with the time evolution of the sample radius [Fig. 10(b)], and complete disintegration occurs after ca. 100 ps. The radial expansion rate was estimated to be ca. 700 m·s⁻¹, slightly less than the speed of sound in water.

A single-shot coherent X-ray imaging instrument was modified into a hard X-ray beamline to perform various experiments including diffraction imaging, X-ray photon correlation spectroscopy, and coherent scattering.⁶⁶ An X-ray imaging instrument was used as an example of its application for diffraction imaging of Ag NPs. The surface melting and evaporation of NPs by ultrashort pulses have been studied with hitherto unprecedented picosecond temporal and nanometer spatial resolution,^65,66 which makes it possible to reveal new features of these processes.

To study the ultrafast energy transfer associated with the photoinduced solid-state melting of NPs with a diameter of 100 nm, Au nanosphere cores coated with a 30-nm-thick SiO₂ shell were used in conjunction with a multiplex femtosecond X-ray probe.⁶⁷ An infrared laser (λ = 800 nm, t_P = 100 fs) was used to photoexcite electrons in Au NPs at a laser fluence of 0.6 J·cm⁻². Anisotropic melting of the shell was initiated at the interface, and the removal of the shell was observed after ca. 3 ps. A reduced density developed from the surface, more clearly after 4 ps, which was revealed by studying density changes in the Au nanosphere. Volumetric expansion of the NPs began only after ca. 3 ps at a speed of 1.0 km·s⁻¹, which is slightly less than the speed of sound for Au (3.2 km·s⁻¹). Regions of molten surface formed randomly within the crystal early in the transition, before the lattice was completely thermalized, to facilitate the void formation. A possible contribution of Coulomb explosion cannot be completely excluded, but it is very small given the low ionization level of ca. 0.02%. Real-time images of the electron density distribution with associated lattice structures show that the energy transfer begins with subpicosecond melting at the sample boundary before lattice thermalization and proceeds through void formation.

A study of laser-induced solid–solid phase transitions in Co NPs was carried out by means of wide-angle X-ray scattering with a time resolution of less than 100 ps.⁶⁸ The experimental results indicate a phase transition within the first 100 ps, suggesting the formation of a solid intermediate phase on a time scale of less than 100 ps.

In this part, the nonthermal mechanism of Coulomb fragmentation of NPs by ultrashort laser pulses is analyzed. Silver particles of 40–60 nm in size in an aqueous medium were subjected to fragmentation under picosecond laser pulsed excitation at a wavelength of 355 nm or 532 nm (energy: 2–3 mJ per pulse; t_P = 18 ps).⁷ A suspension of colloidal silver was irradiated with 355-nm laser pulses (energy: 1.5 mJ, ν = 10 Hz) for 3 min, and clusters of 40–60 nm in size were observed to break up into smaller ones (5–20 nm). Fragmentation was confirmed by both the absorption spectra and TEM images. For λ = 532 nm, only larger (or irregularly shaped) particles were broken up.

In the case of femtosecond laser pulses, the pulse energy is absorbed by electrons, which increases their kinetic energy and temperature significantly.⁶⁹ The electron–electron and electron–NP surface collisions and subsequent electron–phonon relaxation occur after the end of the femtosecond laser pulse and end at ca. 10⁻¹² s. These processes were analyzed comprehensively in Ref. 69. Compared to nanosecond pulses or continuous lasers, the advantages of using femtosecond laser pulses to heat metal NPs are (i) the absence of thermal losses when exposed to the laser pulse and (ii) the possibility of using nonthermal and thermal processes in the time range of 10⁻¹⁴–10⁻¹² s for various possible applications; e.g., the emission of electrons from the surfaces of NPs can be used to inject them into semiconductor materials. The heating of NPs by picosecond pulses stops after electron–phonon relaxation and subsequent processes occur when the electron and ion temperatures equalize.

Larger silver nanoclusters fragment into smaller ones when using 355-nm laser-pulse excitation. The primary event following the laser excitation is the photoexcitation of electrons causing intensive plasmon absorption. Photoelectron ejection dominates at high laser excitation intensities, and two photons with a wavelength of 355 nm are needed for an electron ejection event; they accumulate at or near the colloid surface. This ultrafast process is completed under the influence of a laser pulse with t_P = 18 ps and leads to the charging of the surface of silver nanoclusters with a diameter of 50–60 nm. This in turn leads to their disintegration and the formation of smaller particles (diameter: 5–20 nm). These results lead to the important conclusion that the size of silver and other NPs reduces because of the possible Coulomb explosion of highly charged particles. The fact that fragmentation occurs is evident from both the absorption spectra of UV-irradiated samples and the TEM pictures. Picosecond pump/probe studies confirm that electron ejection is one of the primary photochemical events leading to the photofragmentation process.

After this work, several studies were performed to confirm and understand the possible processes of NP fragmentation under ultrashort laser pulses.^70–74 

For an NP, if the peak intensity (laser fluence) exceeds a certain threshold, then electrons can overcome the ejection work of the metal and escape from the NP, which can cause its Coulomb explosion. Because of this lack of electrons, the positively charged particle undergoes spontaneous fission when the Coulomb forces exceed the cohesive forces of the particle because of the internal charge repulsion. The exchange of hot electrons with the lattice—which promotes an increase in the lattice temperature up to the melting point—causes a phase transition of the NP from solid to liquid. The second mechanism of NP fission is Rayleigh instability: when the critical charge of a liquid Au NP exceeds the instability threshold because of the thermionic emission of conduction electrons, the liquid droplet breaks up into many smaller nanodroplets.

A study was performed on femtosecond fragmentation of gold NPs with a diameter of 60 nm, and the use of transient absorption spectroscopy revealed the reduction of NPs to small particles with a diameter of 3.5 nm.⁷¹ It was reasoned that the results show that femtosecond laser fragmentation is dominated by the Coulomb explosion mechanism. However, the absence of intermediate sizes of NPs and the presence of only the initial (∼60–50 nm) and very small (∼3 nm) NPs also admits the suggestion that the initial NPs are thermally evaporated and small NPs are formed from the condensing vapor.

In the absence of direct high-resolution dynamic observations on the picosecond scale, the mechanism of femtosecond laser fragmentation of plasmonic NPs has remained a subject of discussion until recently. Visualizing nanoscale dynamics and interactions in nanoscale systems using time-resolved electron microscopy requires high-speed studies with a time resolution that is high enough to match the characteristic time scale of the system, as performed by Lorenz and colleagues.^72–74 An electron microscope was used to directly observe the mechanism for the fragmentation of gold NPs enclosed in a silica shell, which allowed elucidation of the underlying mechanism.

Figure 11 shows the experimental fragmentation of a silica-coated Au NP with a core diameter of 20 nm and a shell thickness of 20 nm induced by a femtosecond pulse with t_P = 240 fs and λ = 515 nm [Figs. 11(a)–11(h)] and the proposed fragmentation mechanism based on experimental observations of Coulomb fission of an ionized liquid gold core [Figs. 11(i)–11(l)]. Under the influence of ultrashort laser irradiation, electrons are emitted and trapped in the shell, causing the accumulation of negative charge therein [Fig. 11(i)]. The electron–phonon coupling heats the lattice and melts the gold core, ejects a small particle [Figs. 11(b) and 11(j)], and then other fragments containing ca. 30–200 atoms are ejected [Figs. 11(c), 11(d), and 11(k)]. The resulting ionized liquid gold then undergoes Coulomb fission to emit a highly charged daughter droplet, a process that takes ca. 100 ps [Figs. 11(c), 11(d), 11(j), and 11(k)]. These fragments subsequently coalesce, accumulate, and fuse together to form a second core [Fig. 11(e)]. This continues to grow [Figs. 11(f) and 11(g)] until it begins to undergo fission itself and release fragments [Figs. 11(g) and 11(h)]. Obviously, fragmentation occurs through the stepwise ejection of daughter particles (which contradicts the thermal mechanism), which causes evaporation and condensation with the formation of a large number of fragments. This was the first direct experimental confirmation of Coulomb fission of plasmonic gold NPs under femtosecond laser pulses.

The Coulomb fragmentation of gold NPs immersed in water has been studied,⁷⁴ and a typical process is shown in Figs. 12(a)–12(f). A suspension of gold NPs (15 nm and 50 nm in diameter) in distilled water was placed in a liquid cell and irradiated with femtosecond laser pulses with λ = 515 nm, t_P = 200 fs, and ν = 10 kHz, and the fragmentation process was observed with an electron beam [Fig. 12(a)]. Irradiation with femtosecond laser pulses (fluence: 53 mJ·cm⁻²; ca. 20 s) caused most particles to fragment extensively. A range of fragment sizes is visible, with the smallest being ca. 2.5 nm in diameter. In all our experiments, we used the minimum fluence necessary to induce fragmentation. As can be seen, the two particles in Fig. 12(b) melt and begin to gradually eject several small fragments with no preferred direction [Fig. 12(c)]. During irradiation, the fragments increase in size [Fig. 12(d)] and begin to fragment themselves [Figs. 12(e) and 12(f)]. In addition to the fragmentation of newly formed particles, small fragments are ejected from the opposite side of the original particle with respect to the second particle [Figs. 12(e) and 12(f)]. Progeny particles are ejected in a stepwise manner, similar to the fragmentation of core–shell particles⁷² (see above), so it can be assumed that the main mechanism of fragmentation is Coulomb fission.

A proposed fragmentation mechanism based on experimental observations is presented [Figs. 12(g)–12(j)]. A femtosecond laser ionizes the NP and release electrons into the water [Fig. 12(g)], and the solvated electrons diffuse away so that an anisotropic charge distribution in the water is established before the next laser pulse arrives 100 μs later. Continued laser irradiation ionizes the particle until it reaches its stability limit. When a particle is melted by a laser pulse, it undergoes Coulomb fission, emitting a charged progeny droplet in a random direction [Fig. 12(h)]. With prolonged laser irradiation, the descendant particles grow and divide themselves [Fig. 12(i)]. Finally, a quasi-stationary state is reached in which repeated cycles of fission and fusion-growth continuously move mass between fragments [Fig. 12(j)]. The significance of Coulomb fragmentation is considerable for femtosecond laser pulse action in the general field of laser fragmentation of NPs. Our experiments highlight the importance of in situ observations for understanding and disentangling the multitude of interactions of plasmonic particles with laser radiation. Compared to laser evaporation and various chemical synthesis methods, fragmentation is a more useful method for obtaining very small NPs of sizes down to 1–5 nm. Such small NPs can be used in nanobiomedicine as drug carriers and in nanoelectronics for the development of novel devices.

It has long been known that plasmonic NPs in an aqueous solution irradiated with intense ultrafast laser pulses fragment according to the Coulomb mechanism, producing progeny particles that are several nanometers in diameter. Recent studies have done much to understand and unravel many of the interaction processes between plasmonic NPs and laser radiation. Experiments^72–74 provided the first direct evidence to date of the existence of a Coulomb mechanism for the fragmentation of plasmonic NPs under femtosecond laser pulses. On the other hand, understanding and describing picosecond and femtosecond effects on NPs are still far from being completely resolved. It is necessary to conduct the following systematic experimental and theoretical work in order to resolve the boundary between thermal and nonthermal (Coulomb, thermomechanical, ablative) mechanisms and transition regions in which several mechanisms can operate simultaneously depending on the pulse duration, laser intensity, and wavelength for femtosecond pulses, as well as the NP metal, sizes, etc. This is a very important task for possible applications of laser fragmentation of NPs in various fields.

Studies of the interaction of high-power laser pulses with aerosol particles to form plasma and the effect—facilitated by the presence of micrometer-sized particles suspended in the gas—on laser-induced breakdown were carried out decades ago.^75–77 This effect was explained mainly by the local heating of the surfaces of metal particles, leading to thermoelectric emission, and the effect depends on the particle and laser parameters. This field has expanded considerably in recent decades, as evidenced by recent reviews and book chapters on the subject.^78,79

In the early 2000s, experimental studies began on laser-induced breakdown facilitated by NPs. One of the first studies on the ionization of gold NPs in a liquid was carried out under irradiation with a nanosecond laser pulse as a result of repeated photoexcitation–relaxation cycles.⁸⁰ Plasma-mediated effects were studied under ultrafast irradiation of Au NPs in water.⁸¹ Laser optical breakdown of colloidal Ag, Al₂O₃, and TiO₂ NPs in distilled water was induced by a laser with λ = 1064 nm and t_P = 30 ns.⁸² The NP and laser characteristics have a significant impact on the breakdown process. In Ref. 83, the breakdown of triangles with a size of 150 nm placed on a substrate by laser pulses of 40-fs duration with a peak intensity of I = 1.4 × 10¹² W·cm⁻² was studied. The morphology of NP damage varies significantly depending on the intensity. A high-voltage nanosecond pulse was discharged through two electrodes in argon with inclusions of metal NPs (Au and Pt) both without and with laser excitation (continuous 532 nm, 0.5–2.5 W·cm⁻²), which led to a 200-fold increase in the intensity of plasma radiation and a lower threshold of plasma discharge.⁸⁴ Plasma emission was tested under laser irradiation with wavelengths of 633 nm and 785 nm. The optical breakdown of colloidal Ni NPs during laser action with nanosecond pulses of ca. 650 mJ in energy was studied.⁸⁵ Using a combined aerodynamic lens and velocity map spectrometer, the electronic response of the nanoplasmon–nanoplasma transition in a laser field—mediated by various mechanisms including thermionic emission—was demonstrated using Cu NPs.⁸⁶ The mechanism of femtosecond laser-induced breakdown mediated by Al/SiO₂ core/shell nanostructures was studied

The formation of shock waves and bubbles as a result of the laser breakdown of NPs has been subjected to many studies. Cavitation occurring during the laser breakdown of single gold NPs with a diameter of 100 nm has been studied.⁸⁸ The energy breakdown threshold of Au NPs under the action of a single nanosecond pulse with λ = 532 nm and an energy of 10 nJ is three orders of magnitude lower than that of water, which leads to nanocavitation, ensuring the transfection of single cells. Direct observations of localized light absorption in a single nanostructure irradiated with a femtosecond laser were made by recording plasma explosion images.⁸⁹ The plasma was formed by irradiating the NP with a 40-fs-long laser pulse (wavelength: 400 nm or 800 nm) with I = 3 × 10¹³ W·cm⁻² to 4 × 10¹⁴ W·cm⁻². Individual isolated NPs in vacuum were used to observe small changes in the characteristics of the NPs, leading to different absorption of light and the process of transformation of the solid-state NP on a femtosecond time scale into a dense plasma. The current state of laser-induced cavitation is presented as an effective method for generating controlled cavitation bubbles.⁹⁰ The structure of the breakdown plasma and subsequent bubble dynamics were analyzed using high-speed imaging and measurements of the intensity of the shock wave triggered by the breakdown.⁹¹ 

Note the results on the wavelength dependence of nanosecond and femtosecond breakdown in water, which can be used for the comparison with experiments with NPs.^92–94 Unfortunately, there are still no systematic studies of the thresholds and other parameters of laser breakdown for NPs, which hinders other applications of this phenomenon in various nanotechnologies.

Note that to date, many studies have been performed on laser melting, evaporation, fragmentation, and breakdown of NPs, and many fundamental dependencies and features have been revealed and established. On the other hand, a more correct determination of the threshold fluences and the elimination of discrepancies—sometimes up to an order of magnitude—in the values of thresholds and other parameters require the continuation of these studies, taking into account the growing interest in using these results in various nanotechnology applications.

To understand and describe the processes of heating, melting, evaporation, and fragmentation of NPs, various theoretical research methods have been used, including computer and analytical calculations and molecular dynamics (MD). The goal of theoretical studies is to investigate the time dynamics of NP temperature and estimate the laser threshold parameters leading to melting and reduction in NP size, depending on the pulse duration, laser wavelength, and NP radius at different exposure durations. It is necessary to compare theoretical data on the threshold laser fluences of NPs with experimental data, which can confirm the results of theoretical modeling.

Numerical modeling of laser pulsed heating, melting, and evaporation of NPs was carried out using the equations describing the behavior of a continuum medium and processes in the electron–phonon system of metal NPs.^9,95–98 Laser heating of NPs and its heat exchange with the environment were calculated numerically using the equation of nonstationary thermal conductivity and the thermal transformation of the environment.^9,95 Heating the NP environment ensures its thermochemical transformation and the formation of vapor bubbles described by hydrodynamic equations. Computer simulation confirmed the heat localization of absorbed laser-pulse energy in the NP, which leads to its significant heating above the environment. A model based on a two-temperature model related to the evaporation mechanism of Au NPs via laser excitation was proposed.⁹⁶ To clarify the influence of electron emission on the decomposition of NPs, numerical calculations were carried out,⁹⁷ and it was established that small particles melt at lower fluences than those required for electrostatic decomposition. The dynamics of gold NPs and their environment under the action of a nanosecond pulse were simulated taking into account the decrease in NP size.⁹⁸ 

Note that all previous calculations used uniform absorption of laser radiation inside the NP volume. The experimental results presented in Refs. 23 and 65 (see Sec. II C) revealed heating of NPs below the melting temperature but the implementation of NP melting. This fact can be associated with the absorption of laser radiation by the surface layer of NPs of certain sizes and heat transfer inside NPs during ultrashort laser pulses. Computer modeling was carried out of the intensity distribution of laser radiation with λ = 400 nm, 532 nm, and 800 nm inside spherical gold and silver NPs with r₀ = 10–100 nm and immersed in water and air.^99–101  Figure 13 shows how the relative radiation intensity I_n normalized by that at the entrance to the NP depends on the main diameter inside a spherical Au NP at r₀ = 10–100 nm for radiation wavelengths of λ = 400 nm, 532 nm, and 800 nm. The intensity distributions inside the gold NP at different values of r₀ and λ are significantly inhomogeneous. For λ = 400 nm, the intensity I_n decreases along the main NP diameter for small NPs with r₀ = 10 nm and 20 nm, but for r₀ = 40–100 nm, I_n has a minimum value in the central part of the NP and then increases. For λ = 532 nm close to NP plasmon resonance, I_n decreases slightly for r₀ = 10 nm and 20 nm; however, at r₀ ≥ 40 nm, I_n has a minimum value in the central part of the NP and then increases in the shadow hemisphere and reaches relative values exceeding I_n on the irradiated surface. For λ = 800 nm, the distributions of I_n are almost symmetric about the NP center at r/r₀ = 0, with a minimum value there.

Experimental results⁶⁵ confirmed the absorption of laser energy at λ = 800 nm and t_P = 50 fs by Au NPs with a diameter of 100 nm in predominantly in a near-surface layer with a thickness of ca. 5–10 nm and its subsequent melting (Fig. 13). The results of Ref. 99 for λ = 800 nm can be used to interpret the results of Ref. 65. Absorption by electrons of the laser energy of a femtosecond pulse with a duration of t_P = 5 × 10⁻¹⁴ s is concentrated mainly in a thin near-surface layer with a thickness of ca. 0.1r₀ (Fig. 14). After the characteristic time τ_eph ∼ 10⁻¹² s of electron–phonon coupling,⁶⁹ the lattice in this layer is heated to its maximum temperature exceeding T_M, and the residual NP volume is heated to a lower temperature than T_M. The characteristic time of temperature equalization in the entire NP volume at r₀ = 50 nm is t_T0 ∼ 1 × 10⁻¹¹ s. By this moment, the melting and spheroidization of a thin layer are complete, which is recorded experimentally as possible melting of the entire NP volume. Only from this moment does the entire NP volume—as a result of heat transfer—acquire a temperature below the high initial temperature of the thin layer, taking into account the absorbed energy consumed in the layer for its melting and the small volume of the layer compared to the entire NP volume. Note the possible effects of the nonhomogeneous distributions of absorbed energy inside NPs⁹⁹ in experiments²³ and inside core–shell NPs¹⁰¹ on the melting of NPs in experiments.^72,73

Recently, atomistic and molecular models have been used for numerical studies of the processes of laser fragmentation of gold NPs and the laser synthesis of metal NPs in liquids by ablation. Inogamov and colleagues^102,103 conducted a numerical simulation of droplet fragmentation and ablation in a liquid by a femtosecond laser pulse based on hydrodynamic and MD methods. The instantaneous absorption of laser energy in the skin layer and the corresponding increase in temperature form a compression wave that propagates deep into the particle and is focused toward its center, which creates large tensile stresses and can fragment the particle mechanically; also, a cavity can form in the center of the particle. These results can be used to describe experiments.^72,73 The melting of gold NPs was studied using MD simulations, and it was found that at a high heating rate, this process is very different from near-equilibrium melting, followed by the formation of a pre-melting layer near the surface and solid–liquid interface.¹⁰⁴ The atomistic continuum model (TTM-MD)—which combines MD and the Two-Temperature Model (TTM)—was used to characterize the growth of metal NPs¹⁰⁵ and the femtosecond laser sintering of Al NPs.¹⁰⁶ Simulation results can describe processes with atomic resolution and elucidate possible mechanisms. The advantages and disadvantages of this combined model for simulating laser interaction with NPs were discussed.¹⁰⁷ Huang and Zhigilei^108,109 developed a computational model that facilitates realistic descriptions of various interrelated processes that occur during the fragmentation of NPs by short-pulse laser irradiation. Explosive phase decomposition of a superheated NP leads to two channels for the formation of fragmentation products with a central NP surrounded by smaller fragments.

Recently, there have been a few theoretical studies of the laser breakdown of NPs. The model of femtosecond breakdown mediated by aluminum NPs on deposited substrates in a vacuum¹¹⁰ includes the near-field amplification of the NPs, their electron and lattice temperatures, and a plasma model. The presence of NPs in water (gas) may include additional mechanisms for initiating optical breakdown, such as the generation of seed electrons due to multiphoton ionization and the tunneling effect,^111,112 photothermal emission of electrons from the NP surface,¹¹³ and the inhomogeneous distribution of laser intensity inside the NP.⁹⁹ After generating a threshold number of seed electrons via a combination of the aforementioned processes, the plasma receives sufficient kinetic energy from the laser pulse and grows because of electron avalanche, which is accompanied by subsequent processes of generation and recombination of electrons, the formation of bubbles, and a shock wave into the environment.

By contrast, numerous studies have used simpler and more-adequate analytical calculations to study the processes of laser–NP interaction. Analytical solutions can be used to determine the threshold laser and NP parameters for initiating various processes, as well as for comparisons with experimental data. References 94 and 114 reported analytical studies of the threshold laser intensities for the melting and evaporation of spherical and spheroidal NPs by short laser pulses based on the model reported in Refs. 115 and 116; a comparison with experimental data was carried out, and satisfactory agreement was obtained. Furthermore, the threshold laser fluence for initiating the laser explosion of absorbing NPs by an ultrashort laser pulse has been determined analytically.¹¹⁷

A model of NP photothermal heating–melting–evaporation was proposed in Ref. 2, taking into account the balance between the laser energy absorbed by the particles and the energies used for the processes; these energies include that expended on heating the NPs to their melting or evaporation temperature and that spent on melting or evaporation. The model of NP heating–melting–evaporation was applied to the processes of NPs interacting with pulsed lasers,¹⁴ and recently this model was used to calculate the laser fluence for phase transitions in the reduction of iron oxide.¹¹⁸ Unfortunately, this model does not account for the nonstationary stage of laser–NP interaction and assumes that there is no heat loss due to thermal conductivity from NPs during all processes, which can lead to an energy loss of 30%–40% for nanosecond laser pulses. Consequently, this model has low accuracy and can only be used for an integral qualitative description of the heating–melting–evaporation processes.

The NP cooling process after the end of the laser pulse has been studied numerically and analytically,^9,95 but considering this process in connection with NP laser melting and evaporation does not seem necessary for the following reason. Upon cooling, the NP temperature decreases rapidly, and the NP size and morphology cannot change because of the strong (usually exponential) dependence of the surface melting and evaporation processes on the NP temperature.^9,95

A further analytical model and results of heating–melting–evaporation and direct comparisons with experimental data are presented and analyzed.

Preliminary knowledge of laser threshold parameters for various processes is important for analyzing experimental data and planning future applications. In this subsection, a simple analytical model^95,114 developed for laser NP heating is used to describe the laser transformation and size reduction of NPs. The model uses an analytical solution for the time dependence of the NP temperature and the law of conservation of energy to describe these processes. The results of a theoretical study and determination of the threshold fluences for melting, evaporation, and size reduction of solid spherical NPs by laser pulses are presented. The obtained threshold fluences for spherical Au NPs are compared with experimental results, and their satisfactory agreement confirms the accuracy of the model.

The state of a metal NP changes because of melting and evaporation when it is heated above its melting (T_M) and boiling (T_EV) points. The values of T_M and T_EV significantly exceed the initial NP and environment temperature T_∞, i.e., T_M, T_EV ≫ T_∞ (e.g., for gold, we have T_M = 1336 K and T_EV = 3150 K,^119,120 whereas T_∞ ∼ 300 K). Two main characteristic laser fluences are studied: (i) E_M, which leads to achieving the NP melting temperature T_M, followed by melting and spheroidization; (ii) E_EV, which leads to achieving the NP evaporation temperature T_EV, followed by a decrease in size compared to the initial NPs.

In Fig. 14, it is interesting to compare the results of analytical [Eq. (1)] and computer [Eq. (2)] calculations of the time dependences of the temperature T_max(r = 0, t) at the center of the particle on t/t_P for an Ag NP with r₀ = 25 nm, immersed in silica, and heated by laser pulses, taking into account the melting of the NP and the use of Δt_M and Δt_S [Eq. (2)]. After melting, the NP temperature reaches a maximum; after the end of the pulse, the NP temperature drops rapidly, and solidification occurs. The maximum temperatures in the computer solutions are lower than in the analytical ones at t_P = 10⁻⁸ s and 10⁻¹⁰ s [Fig. 14(b)], and the nonstationary (computer) heat exchange of the NP with its environment determines the difference between the analytical (quasi-stationary) temperature and the computer time-dependent temperature. Heating of the NP by a pulse with t_P = 10⁻¹² s [Fig. 14(c)] leads to an increase in the NP temperature over time with almost complete absence of heat exchange with the environment during t_P, and the maximum temperatures are close to each other for both solutions.

Figure 15 compares experimental results for E_M and E_EV^2,13,16,71 and the theoretical r₀ dependences [Eqs. (4) and (5)] for E_M with α = 1 (full NP melting) and E_EV with α = 1 and β = 0.1 for λ = 532 nm; it was supposed that α = 1 is connected with full NP melting and β = 0.1 is correlated with the detection of NP initial evaporation in experiments with the formation of visible evaporated NP mass around the NP.^2,13,16,71 The dependence of K_abs(r₀) for λ = 532 nm and gold NPs^122,123 was calculated and is presented in Fig. 15. As can be seen, the experimental and theoretical results agree well with each other.

The change in r₀ and the subsequent change in K_abs(r₀) determine the dependence of E_M and E_EV. This dependence leads to the formation of a minimum of E_M and E_EV at r₀ ∼ 30–50 nm, which is associated with a maximum of K_abs at these values of r₀. An increase in E_M and E_EV at r₀ ∼ 60–70 nm due to a sharp decrease in K_abs(r₀) is replaced by a subsequent slight decrease in E_M and E_EV with an increase in r₀ > 70 nm of Au NPs and a laser wavelength of 532 nm.

The changes of r₀ and K_abs(r₀) lead to the formation of a minimum of E_M, E_EV associated with the K_abs maximum for gold NPs at r₀ ∼ 30 nm for laser wavelength 355 nm and at r₀ ∼ 40 nm for 550 nm. Thereafter, increasing E_M, E_EV with increasing r₀ > 40 nm is associated with a decrease in K_abs(r₀) and an increase in r₀. The insignificant influence of t_P in the range 1 × 10⁻¹⁴ s < t_P < 1 × 10⁻¹¹ s on E_M, E_EV is due to the virtual absence of heat losses of the NPs into the surrounding water in this range.

The results of analytical calculations of E_M and E_EV for silver NPs under the action of laser pulses with λ = 400 nm or 355 nm and comparison with experimental data are presented in Fig. 17, which shows how the threshold fluences E_M and E_EV depend on the silver-NP radius r₀ for various values of the laser pulse duration and wavelength λ. The dependences for λ = 400 nm have complicated forms and mirror those of K_abs on r₀.¹²³ Equal values of the threshold fluences E_M and E_EV can be achieved at two or three values of r₀ (and K_abs) for silver NPs. Using the K_abs maximum leads to the implementation of the E_M, E_EV minimum, and vice versa with the K_abs minimum. Reducing t_p from 1 × 10⁻⁸ s to 1 × 10⁻¹⁰ s to 1 × 10⁻¹² s decreases E_M and E_EV by ca. one order of magnitude.

The results for experimental threshold fluences of 355-nm laser radiation are shown in Fig. 17(b). Changes in r₀ and K_abs(r₀) lead to the formation of a minimum of E_M, E_EV associated with a maximum of K_abs at t_P = 1 × 10⁻⁸ s and to a much lesser extent at t_P =1 × 10⁻¹⁰ s. Comparison with experimental data^15,51 shows satisfactory agreement. The large difference between the values of E_M and E_EV at t_P = 1 × 10⁻⁸ s compared to t_p = 1 × 10⁻¹⁰ s and 1 × 10⁻¹² s—especially at small values of r₀ < 40 nm—is explained by the significant influence of the thermal losses of these NPs during laser action, and they weaken as r₀ increases. The values of E_M and E_EV for t_P = 1 × 10⁻¹⁰ s and 1 × 10⁻¹² s and for r₀ > 20 nm practically coincide because of the practical absence of heat exchange between NPs with r₀ = 5–100 nm and the environment at these values of t_P for both wavelengths.

The analytical model of the threshold parameters is limited by the simplicity of the approaches used, and more-correct numerical approaches also have inconsistences as noted earlier (Sec. II A). On the other hand, experimental methods have their limitations, and diagnostics methods have been used with different sensitivities, leading to discrepancies between different experimental results. Moreover, a large number of experimental points makes it possible to neglect some results that fall outside the overall picture or dependence and so contradict numerous prevailing results. In connection with that, Figs. 15(a) and 15(b) show the successful agreement of the theory with numerous experiments for nanosecond laser pulses. For femtosecond pulses, experimental results are insufficient [only one point in Fig. 16(b)] and the discrepancy between theory and experiment is about fourfold.

The processes of melting or evaporation of NPs by laser pulses are presumably thermal and threshold in nature and are initiated when certain fixed threshold temperatures of NPs are reached when heated by pulses with their own threshold parameters. The presented analytical model has been verified by comparison with the results of laser experiments with nanosecond, picosecond, and femtosecond lasers from various scientific groups^{2,13,16,22,24,26,51} and shows fairly good accuracy. This model and results can be used to evaluate the optimal laser threshold fluences for the melting and evaporating of metal (solid) NPs, knowledge of which can be translated into laser and NP parameter requirements, which is very important for the successful application in various fields of nanoscience and nanotechnology.

Experimentally, it has been established that nanosecond, picosecond, and femtosecond laser irradiation of Au NPs leads to a significant reduction in their size up to approximately complete evaporation, depending on the excitation fluence (intensity) and other parameters.^{2,10,13,22,25,41–46,71} Significant reduction in the size of NPs by laser pulses—also called “complete evaporation” or fragmentation of NPs—is a process as a result of which large NPs disappear and ones approximately a tenth of the size appear. These two different terms—significant size reduction^2,11–13,71 or fragmentation^38–46—are used to explain the vanishing of large NPs, which is one of the end results of laser irradiation. In some cases, it is impossible to differentiate between these two definitions. The generation of small NPs can occur via three (or more) possible mechanisms: (i) significant thermal evaporation of large NPs to small sizes; (ii) condensation of vapor of the NP material (metal) into the environment (water) with the formation of small NPs; (iii) fragmentation of NPs via different methods; others mechanisms are possible. Thermal reduction in the size of colloidal gold NPs and determination of the threshold laser fluence were carried out using nanosecond pulses with λ = 532 nm.^13,15,45 An experimental reduction in the size of Au NPs by picosecond pulses with λ = 355 nm or 532 nm was studied.^22,25,38,46 Femtosecond fragmentation of NPs has been studied.^40–45 The thermal mechanism—considered primarily as the evaporation of atoms from the NP surface—can be used to explain experimental results on significant reduction in the size of NPs using nanosecond, picosecond, and (in some cases) femtosecond laser pulses.

The difference between the model presented here and that in Sec. III B is taking into account the significant reduction of the NP radius r₀ [Eq. (10)] and the associated decrease in $KabsM$ [Eq. (11)] for molten metal NPs due to NP evaporation.

The value of Q_TEV is determined by the relation $QTEV=QT−QT∞+QM+QEV=43πr∞3ρ0c0TEV−T∞+LEV+LM$⁠. In Eq. (12), one should take into account the effect of a decrease in NP radius r₀(t) with time [Eq. (10)] and the expression for $KabsMr0$ [Eq. (11)].^123,126,127

The first part of Eq. (13) is the energy spent on heating the NP to the temperature T_EV and its complete melting and evaporation. The second part determines the effect of thermal losses of the NP due to thermal conductivity from its surface. The value of E_FR decreases with decreasing t_P (e.g., from t_P = 1 × 10⁻⁸ s to 1 × 10⁻¹⁰ s) and increasing r_∞ because of a significant reduction in thermal conductivity losses.

Experimental data^2,13,15,45 agree satisfactorily with the theoretical dependences in Fig. 18, which confirms the thermal evaporation of NPs and the accuracy of the theoretical model. There are no experimental determinations of laser fluence in Refs. 25, 38, and 53, which makes it impossible to compare them with other results.

The point of intersection of the dependences (lines) Q_abs(r_∞) and Q_C(r_∞) is marked in Fig. 19 by vertical line 4. The condition Q_abs(r_∞) < Q_C means that the NP is heated below T_EV by radiation with a fluence below the threshold E_FR, and thermal conductivity carries away the absorbed NP energy.

Reducing t_P under constant E from t_P = 1 × 10⁻⁸ s to 1 × 10⁻⁹ s leads to a proportional decrease in Q_C. In turn, this leads to a decrease in Q_TEV+Q_C and the fulfillment of the condition Q_abs(r_∞) > Q_TEV + Q_C for the entire range of r_∞ [see Fig. 19(a)].

The calculated dependences of Q_abs, Q_C, and Q_TEV on r_∞ for experimental parameters⁴⁵ are shown in Fig. 19(b). The condition Q_abs(r_∞) > Q_TEV + Q_C for r_∞ > 17 nm in Fig. 19(b) means complete evaporation of NPs of the given r_∞. The condition Q_abs(r_∞) > Q_C for 17 nm > r_∞ > 14 nm means partial evaporation of NPs of the given r_∞. The case of Q_abs(r_∞) < Q_C at r_∞ < 14 nm means that the evaporation of NPs of the presented sizes does not occur because of intense thermal conductivity from the NP surface. These results are consistent with experimental data⁴⁵ when the radiation E = 1.44 J·cm⁻² does not evaporate colloidal NPs with radii r_∞ ∼ 10–12 nm.

Significant evaporation (fragmentation) of gold and silver NPs by picosecond and femtosecond laser pulses has been achieved.^11,46,48 The laser energy of femtosecond pulses with t_P ∼ 1 × 10⁻¹³ s is absorbed by electrons in metal NPs when T_e ≫ T_i, and it is transferred to a lattice with a characteristic electron–phonon coupling time of τ_eph ∼ 1 × 10⁻¹² s.⁶⁹ If the laser fluence (or intensity) is below the threshold of Coulomb fragmentation and the electron temperature T_e is insufficient to provide the thermal emission of electrons, then the following processes are realized in the thermal regime. For t > τ_eph, the lattice and electron temperatures are equal to each other. The subsequent NP melting and evaporation begin after the end of the femtosecond laser pulse because of the NP absorbed laser energy at t > 1 × 10⁻¹² s ≫ t_P.

The action of picosecond pulses with t_P ≥ 1 × 10⁻¹¹ s is accompanied by simultaneous absorption of laser energy by electrons, electron–phonon relaxation, and lattice heating. The electrons and the lattice without much difference in temperature reach maximum values at t = t_P. Heat loss due to thermal conductivity from the NP occurs with a characteristic time t ≥ 10⁻¹⁰ s⁹⁵ depending on r_∞, which significantly exceeds t_P for picosecond and femtosecond pulses and does not operate during t_P.

The thermal energy stored in the NPs is spent on their complete melting and evaporation with E_FR [Eq. (13)] for picosecond pulses with Q_C = 0 and α = β = 1. Figure 20 shows the dependences of E_FR [Eq. (13)] and K_abs for solid gold NPs with λ = 532 nm on r_∞ and the experimental data for E_FR.⁴⁶ The theoretical value of E_FR is proportional to $EFR∼r∞Kabsr∞$⁠. The location of the K_abs maximum determines the position of the E_FR minimum in the same place. An increase in r_∞ and a decrease in K_abs ultimately lead to an increase in E_FR for r_∞ > 30 nm. Experimental results⁴⁶ confirm the photothermal mechanism of picosecond laser-induced reduction in the size of gold NPs at the used fluence value.

The thermal mechanism plays a crucial role in laser-induced size reduction of metallic NPs using nanosecond, picosecond, and in many cases femtosecond pulses, and a corresponding analytical thermal model has been analyzed. Various possible mechanisms—such as Coulomb fragmentation, surface stress mediation, phase explosion, etc.—must use much higher laser intensities compared to the thermal mechanism. Based on the law of conservation of energy, the threshold laser fluences are estimated, and their dependences on the pulse duration and NP radius are established and discussed. The predicted threshold fluences that realize significant evaporation of spherical gold NPs in water are compared with experimental data for nanosecond and picoseconds pulses, and satisfactory agreement between these results confirms the validity of the developed model. The presented model and results can be used to evaluate the characteristic parameters of laser thermal processing of NPs and their application in various high-temperature laser technologies.

The presented numerical and analytical calculation methods have their advantages and disadvantages, flaws and limitations. Computer calculations based of the continuum and atomistic (molecular) approaches are much more difficult to implement, use, and compare with experiments, and only experts can verify or repeat these results. The analytical approach is less precise, but it is much simpler and more convenient to use even for nonspecialists for evaluations and applications, as well as for comparison with experimental results. However, the presented theoretical results in a number cases do not agree very well with experimental results. Improvement of the presented numerical and analytical calculation methods should be achieved by developing more-complex models and choosing suitable initial conditions, and mainly by comparison with experimental data to achieve a better match or disentangle the causes of deviations between experiment and theory.

Manufacturing spherical nanostructures with well-defined radii from many different nanomaterials helps greatly to accelerate NP applications in various nanotechnologies. One of the first laser nanotechnologies was the melting and subsequent spheroidization of NPs. The melting of NPs in liquid by laser pulses implements selective heating of raw particles to produce spherical particles of various sizes including SMPs.

Continuous Ag films with a thickness of 6 ± 0.5 nm deposited on quartz substrates and placed under a fluid layer were transformed into discontinuous irregular nanostructures (DINs) by heating with nanosecond laser pulses¹²⁹ (Fig. 21). During the first stage, the Ag films were rasterized with a laser with a wavelength of 266 nm, t_P = 9 ns, a frequency of 50 Hz, and E = 35 ± 0.41 mJ·cm⁻², which converted a film into a DIN array. For the second stage of experiments on laser irradiation, samples with similar DIN morphology and optical properties were selected. During the second stage, the Ag DIN was irradiated from the substrate side with laser light with E = 40–80 mJ·cm⁻² for a total exposure time of 4–10 s. The results in Fig. 21 show the possibility of producing a dense collection of Ag NPs of monomodal size, starting from raw nanomaterials. This method may be useful for converting metallic films into spherical NPs, which are relevant for biosensors.

The melting of nanomaterials placed in liquid with laser pulses has become a simple method to synthesize submicron spheres. When melting is carried out with pulses of nanosecond and longer duration, the energy absorbed by the particle is transferred to the environment during the pulse and after its end, leading to a decrease in temperature, which limits the efficiency of the process. The possibility of using a picosecond laser for synthesizing Ge SMPs (200–1000 nm in diameter) was realized by irradiating Ge powder in water;¹³⁰ the Ge powder was converted into SMPs by laser-melting shape change, and larger particles (1000–2000 nm) were separated into two fractions of liquid droplets. The Koshizaki group has studied the formation of SMPs under laser irradiation,^131–133 with the pulsed laser irradiation of ZnO NPs in water leading to the formation of single-crystalline particles.¹³¹ The analysis revealed an increase in the size of the crystals and a shape transformation to spherical (Fig. 22). Laser irradiation leads to the fusion of NPs and the growth of NPs into spherical particles; they retain their optical and electrical properties after rapid cooling, and thanks to their structure and smooth surfaces, selective UV light perception is observed.

Pulsed laser melting of a mixture of Au and iron oxide NPs dispersed in ethanol leads to the formation of spherical SMPs of Au and Fe.¹³² The formation mechanism of spherical composite Au–Fe SMPs was studied using laser pulses of 355 nm and an untreated colloidal solution of Fe₂O₃ NPs as a source of iron to implement the fusion of Au and Fe₂O₃ NPs. At a high Au content exceeding 70% by mass, crystalline alloys with a high Au content were formed without phase separation. Aggregation control is necessary for the selective formation of homogeneous or separated phases of larger SMPs by laser melting.

The selective methods of laser irradiation of nanostructures lead to unexpected and important NPs, such as hollow NPs or nanocrystals with an inner cavity, rod-shaped NPs or nanoprisms, NPs from bimetallic alloys, etc. The use of a slit nozzle makes it possible to control the number of pulses irradiated onto particles flowing through the slit nozzle and to elucidate the mechanism of laser melting by irradiating a certain number of pulses.¹³³ The analysis showed that particles passing through the slit nozzle in one pass were irradiated by two, three, or more pulses. Hollow submicron particles were formed using a small number of pulses, and the voids therein disappeared as the number of laser pulses increased. Using a slit nozzle, the initial formation process and morphological transition of spherical particles were observed.

The transformation of individual gold nanospheres into rod-shaped nanostructures by laser heating with a CW near-infrared diode laser with an output power of 1 W has been studied.¹³⁴ TEM was used to observe various sets of transformations into elongated rod-like nanostructures, and the photothermal properties during laser-induced melting were studied using the extinction efficiency calculated using the discrete dipole approximation. The effect on the nonradiative properties expands the possibilities for developing nanoscale thermometry required for photothermal applications in living cells based on the relationship among NP parameters, temperature distribution, and tissues.

The formation mechanism of Au-based composite particles by laser-pulse irradiation of a mixture of two different types of Au/M_xO_y (M = Fe, Co, Ni) NPs dispersed in liquids has been studied.¹³⁵ This method can be used to synthesize and manipulate various composite particles with differing morphology and bimetallic structure (AuFe, AuCo, and AuNi). A similar method of forming composite particles was used to study the chemical interactions of a solvent with particles during laser melting of α-Fe₂O₃.¹¹⁸ The morphology and transformation of the internal structure of α-Fe₂O₃ during pulsed laser melting were studied, and the existence of Fe, FeO, and Fe₃O₄ was shown by means of phase equilibrium diagrams.

Irradiation of spherical Au NPs with nanosecond laser pulses causes a shape transformation leading to the formation of NPs with an inner cavity, obtained by capturing molecules from the environment (e.g., water and surfactant) during irradiation.¹³⁶ The sensitive ratio between the heating and cooling of the NPs causes them to expand and subsequently recrystallize, keeping the exogenous matter inside. This method of forming hollow plasmonic NPs has potential applications for nanoscale gas and liquid storage.

The synthesis of copper oxide (Cu₂O) and copper NPs from copper oxide (CuO) NPs dispersed in ethanol was carried out by heating the NPs with a nanosecond pulsed Nd:YAG laser.¹³⁷ It was shown that for a successful phase transition in the initial process of laser melting in liquid, a minimum particle size of 23–29 nm is required.

Laser nanowelding and the formation of nanostructures and networks from NPs have been carried out in many studies.^138–144 A characteristic picture of this process shows the formation of a certain nanonetwork due to laser welding of NPs (Fig. 23). Laser irradiation-induced nanowelding of metal NPs is of particular interest because it provides a convenient method for transforming the morphology of NPs and fabricating special nanostructures.

One of the first studies of the laser formation of gold nanonetworks was that by Mafune et al.¹³⁹ Gold NPs with a diameter of ca. 20 nm were irradiated with a pulsed laser with a wavelength of 532 nm at 10 Hz and 5 J·cm⁻² per pulse, and nanonetworks were produced by using an exposure time of 20 min (12 000 laser shots) and a pulse duration of t_P = 5 × 10⁻⁹ s. Gold nanonetworks and much-smaller gold NPs were obtained selectively by choosing the correct laser and NP parameters. Optical absorption microscopy showed optical absorption by the gold nanonetworks at wavelengths greater than 600 nm. This work demonstrated for the first time a possible strategy for nanowelding metallic NPs for widespread applications. Illumination prompted the Ag NPs to form branched structures or higher-order assemblies, and fluorescence microscopy was used to image the nanowelding kinetics of Ag NPs under CW irradiation.¹³⁸ 

Gold nanorods were welded with femtosecond laser pulses, resulting in the formation of dimers and trimers due to the formation of necks between individual nanorods, and electron tomography was used to study the structure of these necks.¹⁴⁰ A direct comparison of the plasmonic linewidth of single-crystal and welded Au nanorods was carried out, which made it possible for the structural and plasmonic properties to be related. The appearance of single defects leads to significant plasmon broadening.

For the nanojoining of Ag/Au NPs, three methods have been studied comparatively.¹⁴¹ Laser annealing with millisecond pulses exhibits thermally activated diffusion in the solid state, and femtosecond irradiation with various intensities exhibits two effects: photofragmentation at fairly high intensity (10¹⁴ W·cm⁻²) and nanojoining at low intensity (10¹⁰ W·cm⁻²). This nanojoining is achieved by melting and fusion welding. The coupled Au NPs are expected to find potential applications, such as probes for surface-enhanced Raman spectroscopy.

Ultrashort laser pulses have been used for the nanowelding of Ag nanowires in order to minimize the thermal damage to the polymer substrate.¹⁴² Rather than using nanosecond pulses, femtosecond laser irradiation made it possible to join the Ag nanowires while retaining their crystalline structure. Femtosecond laser nanowelding led to neither destruction of the nanowire junction nor thermal fusion. The proposed process could become a useful nanowelding technique for fabricating integrated plasmonic devices as well as transparent conductive electrodes on flexible substrates.

Carefully chosen laser parameters make it possible to selectively excite different substructures and control the morphology of Au-nanosphere aggregates melted by a femtosecond laser to obtain larger spherical NPs, nanostructures such as nanorods and nanoprisms, and necklace-shaped 1D nanostructures.³⁰ The self-assembled aggregates formed by 10-nm spherical Au NPs in water were subjected to melting (Fig. 24) using a beam with a wavelength of 400 nm, 570 nm, or 620 nm, t_P = 50 fs, and an intensity of 2.5 × 10¹⁰ W·cm⁻² to 7.0 × 10¹⁰ W·cm⁻², irradiated for the same time of 5–10 s at 2500–5000 pulses and a repetition rate of 500 Hz.

The spatial extent of ensembles of heated NPs can be selected by adjusting the laser wavelength λ, which allows control of the morphology of the nanostructures formed during melting. Changing λ causes different melting mechanisms, corresponding to either interband or plasmonic excitation. The results of photoinduced melting show that it is possible to produce larger spherical NPs (λ = 400 nm), nanorods and nanoprisms (λ = 570 nm), and larger 1D bridged structures connected by necklaces (λ = 620 nm). Several interesting methods have been proposed for the photoinduced conversion of the architectures of self-assembled Cu nanoclusters from ribbons to spheres¹⁴³ and the bending of gold nanorods with light.¹⁴⁴ 

Laser irradiation-induced melting, reshaping, and nanowelding of metal NPs are of significant interest because they provide useful and controllable methods for transforming the shape, structure, and size of NPs. As a growing area of research, nanojoining is becoming a key area for producing complex nanostructures and nanonetworks with functional prefabricated components for new nanodevices, and various nanojoining methods are being developed.

To fix the spatial locations of NPs, they are often placed on various substrates. Moreover, these new methods leading to the formation of new nanostructures after laser exposure can be used in material science and technology, nanoelectronics, plasmonics, and nanocatalysis. This subsection presents analysis of the results of laser irradiation of NPs located on substrates and the possible applications of the obtained results in various laser nanotechnologies.

Nedyalkov et al.¹⁴⁵ obtained experimental results for gold NPs with radii of 20 nm, 40 nm, and 100 nm deposited on various substrates and irradiated with femtosecond laser pulses with a wavelength of 800 nm. Gold NPs with a radius of 100 nm deposited on a Pt substrate were irradiated with a single laser pulse with a fluence of 20 mJ·cm⁻² or 40 mJ·cm⁻². At the lower fluence, the NP cluster formed during deposition remained on the substrate after the laser pulse, while single NPs were ejected and nanoholes were formed in the substrate; at the higher fluence, clusters were ejected and traces of substrate melting appeared. The characteristic time for temperature equilibration inside the NP volume due to electron thermal conductivity is $tT0∼r024χ0$⁠, where χ₀ is the coefficient of thermal diffusivity; for gold NPs with r₀ = 100 nm, we have t_T0 ∼ 2 × 10⁻¹¹ s, which is longer than the pulse duration (t_P ∼ 1 × 10⁻¹³ s) and the electron–phonon coupling time (τ_eph ∼ 1 × 10⁻¹² s). A nonhomogeneous intensity distribution with a wavelength of 800 nm inside a gold NP with r₀ = 100 nm (see Sec. III A⁹⁹) leads to laser-pulse heating in only a thin layer near the NP surface up to a time shorter than t_T0. Also, the near-field intensity distribution shows strong localization of the zone with maximum enhancement near the contact point between the particle and the substrate. Thus, NP heating is expected to be the main mechanism for substrate damage. The next study¹⁴⁶ is connected with the previous results, where the possibility of using a KrF excimer laser to clean layers of tungsten NPs from the surfaces of solids with unavoidable impurities was studied. A visualization setup was used to image the speed and direction of the ejected material and the dynamics of fragment mobilization, and a very good level of surface cleaning was achieved, even after eight pulses at the lowest laser fluence (0.5 J·cm⁻²). The movement of material from laser-irradiated surfaces occurs in two modes on very different time scales: (i) a fast plume moving at a speed of 40 000 km/h, followed by (ii) the ejection of large conglomerates of particles at speeds of several hundred kilometers per hour.

References 147–152 discussed recent progress in the laser nanoprinting of colloidal particles of arbitrary configuration with nanometric precision, as well as related applications. Various methods based on various physical mechanisms have been discussed, including optical forces, light-driven electric fields, optothermal effects, laser-directed thermocapillary flows, and photochemical reactions.¹⁴⁷ Picolitre volumes of Ag NP droplets were printed on photopaper, polyimide, and transparent polyimide substrates using a femtosecond pulsed laser with a minimum heat-affected zone for sintering printed Ag patterns. They revealed a highly organized bonded nanostructure that cannot be obtained by heating in an oven. The surface morphology achieved in laser sintering is interpreted as being the result of the joining of NPs due to solid-state diffusion. Because of their flexible and versatile capabilities, the fabrication of inexpensive and mechanically robust flexible electronic patterns using metal NPs is gaining more attention because of their growing applications in nanophotonics, microelectronics, flexible displays, touch-screen panels, and medical devices.

A spatially defined arrangement of metallic NPs in a stable glass matrix was obtained by irradiation with a nanosecond excimer laser at a wavelength of 193 nm.¹⁴⁸ Using several laser pulses, the original gold film disintegrates into particles of that material previously deposited on the glass surface, then further pulses lead to an increase in the temperature of the glass surface, its melting, and the subsequent implanting of the gold NPs into the softened glass matrix. It is assumed that the implanting of gold particles occurs according to the scheme shown in Fig. 25.

Laser reduction in the size of Au NPs located on a Ti substrate has been carried out.¹⁴⁹ Attempts to reduce the deposited gold coating to the size of Au NPs and melt it into the titanium matrix using a laser beam were completed successfully. A new method was proposed for fabricating a substrate attached to Au NPs using a femtosecond laser with a spatiotemporal shape,¹⁵⁰ which results in a spatial reshaping that allows Au NPs to be deposited by laser reduction onto silicon substrates. A new combined approach to the fabrication of spherical porous gold NPs on glass substrates under ambient conditions was presented, using a UV laser-induced particle preparation process followed by wet chemical selective etching.¹⁵¹ The laser process applied to a silver/gold bilayer system with different thicknesses of the individual layers generates spherical mixed particles and influences their properties. NPs consisting of a mixture of several metals, as well as porous NPs, exhibit properties that can be used in spectroscopic detection, surfaced-enhanced Raman scattering (SERS) applications, molecular detection, and biosensors.

Traditional top-down nanofabrication approaches such as photolithography and electron beam lithography offer reliable and robust production of high-resolution nanostructures. Recently, bottom-up techniques have enabled the on-demand creation of superstructures with controllable configurations with single-particle resolution by assembling individual building blocks.¹⁵² Compared to methods such as self-assembly and DNA nanotechnology, optically controlled assembly has the advantages of remote control, manipulation of individual components according to location, applicability to a wide range of building blocks, and arbitrary configurations of assembled structures.

The performance of silver is often hampered greatly by the presence on its surface of a tarnish layer that is several nanometers thick. An experimental study using in situ optical spectroscopy was reported of the temperature dependence of the plasmon response of Ag NPs, both pure and tarnished.¹⁵³ For tarnished NPs, the temperature dynamics of the thermal decomposition of the contamination layer were observed in real time and compared with the corresponding behavior of spatially extended flat surfaces.

Another area where plasmonics can be of great benefit is 3D printing, also known as additive manufacturing,^154,155 which has been developed over the past decade and is becoming the preferred production approach for many objects. There are many 3D printing approaches, one of which is based on powder sintering, which involves scanning with a high-power CO₂ laser to directly heat the particles for sintering;¹⁵⁴ that approach can be regarded as being a technology of laser additive manufacturing for future-oriented applications.¹⁵⁵ Targeted NPs are deposited onto a substrate, and laser sintering of the NPs allows direct and quick formation of a functional layer even on heat-sensitive flexible and stretchable substrates. Selective laser sintering has been studied to create a conductive layer, with particular attention paid to layers based on a laser-sintered Ni electrode, i.e., electrical interconnections on various substrates, which is critical for both passive and active electronics. Laser sintering has been applied successfully to a wide range of metal and metal-oxide NPs, as well as corresponding experimental designs, with new functionalities that were unattainable using conventional fabrication technologies.

Another type of additive manufacturing is laser cladding of microsized or nanosized powder from various materials on substrates in order to form novel nanostructures, layers, etc.⁴ Laser cladding is an additive manufacturing technology that allows the creation of coatings from a wide range of materials that can be deposited because of the surface fusion of metals. The main properties of the process involve obtaining perfect metallurgically bonded and fully dense functional coatings with a minimum heat-affected zone, low dilution between the substrate and filler material at reduced thickness, and a homogeneous microstructure resulting from the rapid solidification rate, which contributes to the wear resistance of the coatings. The powder is fed into a moving molten substrate pool created by a laser beam for melting, and is deposited as a coating to suit local application requirements. Additive manufacturing technology of direct powder laser deposition has had an impact on surface modification, rapid prototyping, tooling, and low-volume manufacturing and is opening new perspectives for surface engineered materials.

Note that there are a number of publications devoted to various areas of laser cladding. One of the first theoretical models of the laser cladding of metallic microparticles on a metal surface was that in Ref. 156. The absorption of laser energy, heating, and melting during gas–metal-powder laser cladding onto a substrate have been studied theoretically, and the spatiotemporal dependences of the process parameters for pulsed and continuous cladding modes have been studied. The melting of metallic particles by a laser beam before they reach the surface has been demonstrated, which can lead to increased cladding efficiency and upgrading of the cladding layer. These results are still used to construct theoretical models and implement various modes of laser cladding.^157–159 

Modeling the temperature distribution in the additive manufacturing process is one of the main tasks.¹⁵⁹ To model the temperature profile of the laser cladding of nickel on stainless-steel substrates, a 3D model has been developed. The numerical temperature–time profile confirmed that the maximum temperature was above the melting point of the pre-clad layer, resulting in the formation of a molten pool that later solidified to form a clad layer with better metallurgical bonds and low dilution rates. Laser-directed energy deposition of high-density stainless-steel samples was carried out by varying the laser and powder parameters,¹⁵⁸ etc., and the deposition efficiency was found to be highly dependent on the scanning strategy and laser parameters. Extensive analysis of the resulting dimensions, surface roughness, microstructure, and hardness was carried out, providing new information affecting the quality and efficiency of the process.

Instead of laser cladding of metal powder, particles from various materials are used. Ti-based ceramic coatings on substrates have been produced successfully by laser cladding to improve the wear resistance of the alloy for biological applications, taking into account the influence of process parameters and NPs on the quality characteristics of coatings.^160,161 The developed laser cladding system was used as shown in Fig. 26.

The laser system comprised a fiber laser with a maximum power of 500 W, connected with a connector. The pre-deposited powder and thin substrate layer were melted rapidly by the laser beam, and the sample was transported in a zigzag manner together with a moving table containing argon to protect the melt pool during the laser cladding process. Adding CeO₂ NPs led to improvements in the microstructure, mechanical properties, crystalline granularity, and wear resistance of the coatings. Laser cladding is one of the main processes for producing and manufacturing 3D structures with dimensional restoration and wear and corrosion protection.

The properties of the laser and powder, the processing parameters, and the properties of the processed material determine the success of the laser nanoprinting of colloidal particles of arbitrary configuration with nanometric precision and the laser additive manufacturing and cladding in industrial and future new applications. The application of laser nanoprinting and powder cladding should be based on the excellent process stability and reproducibility to be achieved in the future developments of these methods.

Many manufacturers of automotive car engines and aircraft turbines are interested in improving combustion efficiency while reducing unwanted environmental pollutants. The efficiency of fuel-burning engines must be as high as possible and pollutant emissions as low as possible, and such requirements cannot be achieved using traditional engine ignition methods. Spark plugs reach their limit when the required high ignition pressure requires too much voltage. Several alternative approaches to laser and plasma ignition can improve overall efficiency. The main advantage of laser ignition (LI) is the use of high combustion-chamber pressures and very lean mixtures to reduce unwanted emissions. LI is considered one of the most promising concepts and combines the necessary reduction in pollutant emissions with higher efficiency of the combustion engine.

Strong interaction of laser radiation with plasmonic NPs opens up opportunities for various technologies that use photophysical processes enhanced by this light–matter interaction. The first studies of the ignition of aerosol particles in air by laser radiation were carried out experimentally¹⁶² and theoretically.¹⁶³ The threshold values of the laser radiation intensity for initiating the ignition of aerosol particles in air were calculated and then compared with the experimental results,¹⁶² and adequate agreement was obtained. Next, the heating, evaporation, and combustion of a solid aerosol particle in a gas under irradiation were studied theoretically by solving the equations of quasi-stationary diffusive–convective heat and mass transfer between the particle and the surrounding gas.¹⁶⁴ 

The possibilities for using particles for laser engine ignition include the heating, evaporation, ignition, combustion, and plasma breakdown of liquid or solid microparticles and nanoparticles under laser action. Heating and evaporation of fuel particles and ignition of plasma can be carried out by a pulsed laser, and the LI mechanism is based on gas breakdown. LI has become an active topic of research and analysis regarding laser-induced spark ignition in engines containing microparticles and nanoparticles of special fuel, including consideration of fundamental processes and possible applications.¹⁶⁵ 

Laser-induced spark ignition can replace traditional electric spark plugs in engines that must operate at much higher compression ratios and at temperatures of up to 4000 K for engine ignition. Recent LI results from single-cylinder engine testing and the development of a compact laser suitable for engine ignition are encouraging and will continue to further the practical adoption of laser spark plugs. Recent advances have led to many subsequent developments and the first applications of LI in aerospace.¹⁶⁶ Aluminum, boron, and magnesium as used in explosives and rocket fuel have been ignited successfully using a CO₂ laser. Advances in science and technology have led to the development of novel laser devices, but problems such as integration, stability, and vibration resistance have yet to be solved.

On the other hand, pyrotechnics and explosives are energetic materials that can store and quickly release enormous amounts of chemical energy. Aluminum (Al) is a particularly important fuel in many applications because of its high energy density, which can be released via exothermic oxidation. The plasmon resonance of Al NPs with well-defined parameters can be used to heat and ignite them with a laser.¹⁶⁷ An Al NP array was illuminated and heated by a green laser (532 nm) in the range of 1–200 mW. The ignition threshold is critically dependent on the absorbed power, which provides control over the ignition of Al NPs. The LI of explosives and pyrotechnics is safer and more environmentally friendly than electric ignition. A review was presented of the laser-induced performance of photothermal ignition materials.¹⁶⁸ A study and application were carried out of various photothermal materials including metal and metallic oxide NPs, nanocomposites, and laser-ignitable explosives, and these photothermal nanomaterials were compared. Reducing the ignition energy requires the use of ideal photothermal energetic nanomaterials with good light absorption. This suggests that energetic nanocomposites can be used in various thermal applications as potential igniters, reactions in which are initiated by selective light irradiation. These studies are still continuing intensively^169–171 with the aim of implementing the LI concept.

NPs heated by laser radiation emit thermal radiation from their surfaces, and the measurement technique based on determining their temperatures from the emission of thermal radiation by the NPs and their concentration and size is called laser-induced incandescence (LII). LII depends on the thermophysical properties of NPs and the heat and mass transfer between them and the surrounding gas. Determining the temperature and heat of evaporation of the processes that contribute to nonequilibrium nanoscale balances of heat and mass of laser-heated particles is very important for the applications of LII methods. The evaporation process of carbon and iron NPs induced by a nanosecond pulse was investigated by LII.¹⁷² It was shown that the equilibrium description of NP evaporation in the model of LII is invalid at high laser fluence, and an essential correction of the description of the evaporation process is necessary. Using nanosecond heating of NPs produces high NP temperatures, which facilitates the observation of their thermal radiation in order to study the function of the refractive index and NP evaporation temperature using LII.¹⁷³ 

The formation of laser-induced plasma and its effect on time-resolved spectral intensity were studied under the conditions of LII measurement.¹⁷⁴ Under laser radiation with fluence exceeding 0.8 J·cm⁻², the NP absorption cross-section increases because of inverse bremsstrahlung absorption, and bremsstrahlung emission results in an overestimation of the NP temperature. Analytical expressions for the fluence curves provide insights into the mechanisms underlying the nanosecond pulsed heating of aerosol NPs.¹⁷⁵ The first analytical model of the relationship between laser fluence and NP peak temperature allowed comparison of published experimental fluence curves by analyzing three fluence regimes: low, moderate/transition, and high. A new time-resolved method for measuring LII on iron NPs combined with the likelihood of a Bayesian model was proposed.¹⁷⁶ This approach evaluates parameter uncertainties and model complexity and has been used to understand experimental data for iron NPs in argon.

The ignition system in a gasoline engine creates high-voltage (ca. 40 kV) electric sparks to ignite the fuel–air mixture, and the combustion of this mixture in the cylinders produces the motive force. Known ignition systems have the disadvantage that they cause high-frequency interference and also require relatively careful maintenance with incomplete combustion of the mixture. Therefore, the focus of scientific research is alternative LII systems to improve the efficiency and reduce pollutant emissions, which seems to be an attractive method for improving the combustion process.

The optical ignition system of internal combustion engines uses a semiconductor laser and optical fiber to transmit radiation into the engine cylinders to ignite the fuel mixture for combustion. The mechanism of optical breakdown is provided by electron avalanche and multiphoton photoionization. Compared to conventional ignition by electric spark plugs, LI offers potential benefits such as the ability to deliver laser energy simultaneously to different positions and the temporal control of ignition, more-complete burning of the mixture, feasible savings in fuel consumption, and a smaller amount of car emissions of harmful exhaust gases. The technology could also support increased use of vehicle electrification through its application to ignite hybrid (electric–gas) engines.

Advances in the LI of natural-gas engines with a compact laser and the implementation of laser devices with multiple (up to four) beam outputs in a single-cylinder engine led to the successful development of a prototype laser spark plug.¹⁶⁵ The innovation was tested in a four-cylinder gasoline passenger-car engine with multipoint fuel injection, which ran on lean air–gasoline mixtures with a higher-than-usual concentration of air in the fuel. It has been shown that the LI of such lean air–fuel mixtures can actually lead to improved engine performance compared to electric plug ignition. Results could also lead to the development of a compact LI system for stationary gas engines, such as those used in factories and power plants. LI can be used in engines that run on ultra-lean mixtures of hydrogen and air at high pressure, resulting in efficient and energetic combustion with lower emissions. LI will find its way into specific applications, such as propulsion systems of future space transportation, or the operation of large natural-gas engines. Additional tests are needed to ensure that these devices can compete with the simple and inexpensive electrical spark plug.

It is possible to use methods of the LI of nanostructures by using particles of various metal nanoenergetic materials for explosives and solid fuels, rapid initiation in LI systems of engines, and material synthesis and processing. Associated with the combustion of particles in various engines, the incandescence signal carries information about their properties, and this method was applied to investigate the properties of metal, carbon, and other NPs.

One of the main goals of laser evaporation (fragmentation) of NPs is to obtain small NPs of less than 5–10 nm in size for various possible applications. This size range is very difficult to synthesize chemically, thereby making laser fragmentation very interesting.

The physical processes of laser fragmentation of gold and silver NPs were analyzed in Secs. II and III. This section analyzes the use of laser fragmentation—taking into account laser and other parameters—to generate various NPs less than 5–10 nm in size (nanoclusters). One of the first studies in this area was that reported in Ref. 177. Amorphous metal NPs have attracted interest because of their structurally disordered nature and combined nanoscale strategies, both of which provide superior properties compared to their crystalline counterparts.¹⁷⁸ Laser fragmentation in liquid was used to amorphize metal oxide NPs from crystalline materials to form amorphous Fe₂O₃ NPs and CoO and NiO nanoflakes of the initial particles in water. The use of amorphous metal NPs was also discussed in catalysis and magnetism.

Instead of fragmentation of gold and silver NPs, it is interesting to obtain small NPs from special materials. The synthesis of PbTe NPs with an average diameter of ca. 6 nm and a sharp size distribution was studied by pulsed laser fragmentation of micron-sized PbTe powder in distilled water under careful control using X-ray diffraction and TEM.¹⁷⁹ 

Fragmented NPs resulting from laser exposure can enhance the efficiency of various fuels and contribute to the purification of exhaust gases in both industrial applications and transportation vehicles, which is an important modern task given the fight against environmental pollutants. The laser synthesis of Pt and PtPd NPs in a g/h liquid flow was studied.¹⁸⁰ Laser catalyst preparation facilitates the synthesis of alloys of NPs, unlike the traditional wet chemical preparation of PtPd NPs.

By optimizing the fragmentation conditions, laser-generated NP catalysts with a corresponding fraction of less than 10 nm and a productivity of greater than 1 g/h meet the requirements of oxidation of CO and NO in the purification of industrial and vehicle exhaust gases. After the laser fragmentation of Al NPs, their size decreased from 100 nm to 10 nm, and their surface increased tenfold,¹⁸¹ which was characterized by TEM and disk-centrifuge measurements. Fragmented Al NPs can be considered as additives for liquid composite fuels.

Various new properties of laser-fragmented NPs can be used in new and existing nanoelectronic and photonic devices to improve their parameters and achieve new capabilities. The physicochemical properties of semiconductor CdSe NPs obtained by laser fragmentation of CdSe powder dispersed in acetone were studied.¹⁸² An important difference was observed in the sizes of NPs synthesized at infrared wavelength 1064 nm (size: 12 nm) and green wavelength 532 nm (size: 3 nm). CdSe NPs with small sizes are of interest for nanoelectronics. UV-sensitive and solar-blind optoelectronic devices were demonstrated by combining photosensitive ZnO NPs synthesized by pulsed laser fragmentation with the transport characteristics of graphene.¹⁸³ 

The original ZnO NPs were concentrated around ca. 140 nm [shown in orange in Fig. 27(c)]. Pulsed laser fragmentation of ZnO NPs in liquids led to a significant reduction in size and the formation of a bimodal size distribution [Fig. 27(c)] with sizes of ca. 18 nm and 46 nm [shown in blue in Fig. 27(c)]. For bimodal spheres, ideal packing can be achieved effectively when smaller particles are simply placed in the spaces between larger particles, potentially benefiting the final device design. These sensitive ZnO NPs offer high UV wavelength selectivity, making them well suited for photodetector applications.

Numerous studies have been devoted to fragmenting various NPs for possible applications in biomedical investigations and nano and photoacoustic imaging, cancer treatment, and photothermal therapy. The femtosecond laser fragmentation of non-aggregated, low-dispersed, crystalline Si-based NPs for the rapid preparation of concentrated aqueous solutions of ultrapure Si-based colloidal NPs was studied.¹⁸⁴ With significantly higher purity than their chemically synthesized counterparts and photoluminescent response, NPs open up opportunities for in vivo biological applications such as drug vectoring, imaging, and therapy.

Femtosecond laser fragmentation has been used to synthesize TiN NPs up to 4 nm in size with a very narrow size distribution as a promising plasmonic alternative for biomedical applications.¹⁸⁵ Based on the absorption band in the region of tissue transparency and acceptable biocompatibility, TiN NPs demonstrated a strong effect of photothermal therapy on cancer cells as sensitizers of local hyperthermia under near-infrared laser excitation. Laser-synthesized TiN NPs can be used to develop biomedical methods using plasmonic effects for photothermal therapy, photoacoustic imaging, and SERS.

Laser fragmentation of colloidal particles of bismuth ferrite (BiFeO₃) of submicron size was carried out by irradiation with a circular and elliptical fluid jet to synthesize sharply reduced NPs.¹⁸⁶ This treatment achieved a size reduction from 450 nm to less than 10 nm and a narrow size distribution, which were analyzed using UV-vis, XRD, and TEM to provide information on the morphology and composition of the material. Bismuth ferrite NPs can be viewed as a base material used for cancer treatment. The use of amplification of the laser–NP interaction by means of diffuse laser beams to effectively reduce the NP size was studied.¹⁸⁷ It is important to control the parameters of NPs obtained by laser reduction in liquid for selecting the size and composition of materials suitable for catalysis.

The opposite results are obtained by processing metal colloids with laser pulses, leading to the formation and increase in the size of NPs. Silver spheres were prepared by laser heating in a solution of a silver-containing precursor,¹⁸⁸ and the nucleation and growth of gold NPs initiated by nanosecond and femtosecond laser irradiation of an aqueous colloid was also carried out.¹⁸⁹ In particular, irradiation of an aqueous solution of AuCl₄ with nanosecond laser pulses at a wavelength of 532 nm leads to the formation of monodisperse Au NPs with a size of 5 nm with no additives or capping agents via a plasmon-enhanced photothermal autocatalytic mechanism.¹⁸⁹ The heating of Au NPs during nanosecond irradiation forms almost-uniform particles with a size of 5 nm, while the absence of heating of particles under femtosecond laser irradiation leads to the growth of particles up to 40 nm in size.

Laser fragmentation and processing of NPs is emerging as a unique and scalable technique for producing small NPs and nanomaterials for catalysis, imaging, biomedicine, and energy in a scalable and clean manner. Because of the need for functionality for the applications of fragmented NPs in optics, biology, and energy, their structure, size, etc. have become topics of intensive research interest, highlighting the advantages of laser-fragmented NPs for these types of applications. Small NPs are ideal nanomaterials for nanointegration applications, including high throughput, convenient nanomaterial preparation, and continuous operation. Precise control of NP size in the 1–10-nm range and recent advances toward the multi-gram scale are helping to expand the applicability of such nanomaterials in various fields. The intrinsic properties of NPs open up various applications in which miniaturization is followed by self-assembly and nanostructure formation (e.g., of electronic equipment), weight reduction (as a result of increased material efficiency), and/or improved material functionality (e.g., higher durability, thermal stability, solubility). The remarkable properties of tunable dimensions of nanomaterials produced by laser–matter interaction (e.g., size distribution, agglomeration state/dispersion, crystal structure, surface area, shape/morphology) make them a topic of intense research interest in material science, with far-reaching applications.

The laser fragmentation of colloids using high-power femtosecond lasers appears to be a key process that is attractive for the industrial manufacturing of nanomaterials. Laser fragmentation of NPs faces serious challenges in producing NPs of a specific size and shape, reducing polydispersity and increasing production profitability, and developing nanodelivery systems. Although there are some unsolved physical, chemical, and technical problems, several methods have been proposed for overcoming the above issues, including the selection of an appropriate liquid or stabilizing agent, optimization of laser parameters, and irradiation methods. The unique properties of femtosecond lasers constitute another topic of intense research interest with the aim of increasing the production and stability of NPs. Laser fragmentation in liquids offers alternative ways to obtain colloids with controlled NP sizes, but despite their great potential, certain aspects of the fragmentation processes and their products need to be optimized. Along with the development of new strategies to improve NP productivity, future efforts should be aimed at further improving the properties of the resulting NPs and the degree of process control required for industrial applications. This is an important opportunity of which it is worth taking advantage.

Laser induced breakdown spectroscopy (LIBS) has been used to determine the properties of metal samples by initiating laser breakdown at their surfaces. Recently, De Giacomo and colleagues^190–194 proposed a new method based on the deposition of NPs onto the surface of a sample before laser irradiation and the interaction of the NP plasmons with the laser pulse, which leads to a significant increase in the emission signal from the breakdown, i.e., NP-enhanced LIBS (NELIBS). Compared to LIBS, NELIBS exhibits a signal enhancement of one to two orders of magnitude, including analysis of metallic samples and aqueous solutions. The laser pulse induces plasmon resonance of conduction electrons in metallic NPs, which enhances the incident electromagnetic field near the NP surface, allowing efficient generation of seed electrons via field emission. When the surface of a metal sample is coated with NPs during laser irradiation, the electromagnetic field of the incident light increases significantly (by one to two orders of magnitude), and direct emission of electrons from the surface can be obtained. Solutions can be analyzed using LIBS by depositing and drying droplets of the solution onto a substrate.¹⁹¹ If only a few microliters of solution are available for analysis, then using NPs can greatly improve the sensitivity of LIBS, enabling sub-ppb measurement levels. The field-enhancement effect can influence the types of analytes deposited on the NP layer, allowing almost all of the precipitated analyte to be transferred into the plasma in a single laser shot.

The mechanisms responsible for NELIBS have been discussed, with particular emphasis on the effect of plasmonic enhancement of the laser electromagnetic field.¹⁹² NELIBS’s greatest strength is its ability to perform elemental analysis with selected laser parameters and on a very small amount of sample with better sensitivity than that achieved with the traditional LIBS approach. The importance of NELIBS is in reducing the number of laser shots to one, decreasing the energy of the laser pulse to protect the sample from laser-induced damage, and its use to scan a sample with NPs deposited over a large area. A study was conducted on the influences of the size and surface concentration of Au NPs and the laser parameters on signal amplification during NELIBS.¹⁹³ Improvement in the emission signal with NELIBS occurs at a certain surface concentration of NPs depending on their size. The dependence of the enhancement of the NELIBS emission signal on nanoparticle–protein solutions (human serum albumin, cytochrome C) applied dropwise and dried on a titanium substrate was studied.¹⁹⁴ The experimental signal shows that NELIBS can be used to determine the number of protein units required to form a nanoparticle–protein corona. A comparative study provided a more general understanding of NELIBS and assessed its prospects.¹⁹⁵ The sensitivity of spectral methods should be increased for determining small amounts or concentrations of various metals or gases. NELIBS uses Au NPs for enhanced sensitivity in the analysis of aluminum alloys.¹⁹⁶ 

Gold NPs were spread onto analyzed samples with 99.9% Al alloy and pulses of different energies were used for an eightfold increase in the intensity of spectral lines of the Al alloy, which can be useful for detecting many trace elements in alloys. The influences of the laser pulse energy and the properties of the gold NPs with a diameter of 10–20 nm placed in argon on the argon LIBS signal were studied experimentally.¹⁹⁷ The breakdown threshold of argon was reduced significantly because of the presence of the gold NPs, which facilitates the detection of Ar emission at laser fluences that do not allow plasma formation without the presence of NPs. This effect was used to determine the low mass concentration of nanoaerosol, which cannot be detected by other direct methods, and it can be used to analyze trace gas. The effect of different nanoparticle shapes (e.g., nanosphere, nanorod) has been studied for NELIBS.¹⁹⁸ NELIBS experiments were carried out with spherical Au NPs and nanorods deposited on the surface of a Ti target, and the difference was established in their emission spectra.

Recent advances in NELIBS have been reviewed.^199,200 These reviews give a comprehensive overview of the progress in analytical laser and plasma spectroscopy research related to NPs, focusing on the results of the past decade and describing the operations underlying all existing technologies. The concepts, equipment, data analysis, and various applications of NELIBS are discussed in detail, as are the monitoring of NP synthesis, the characterization of NPs, and plasmonic signal enhancement achieved by NPs. Based on ongoing research, NELIBS should be expanded to detect metals, transparent and fresh samples, and biological fluids to detect labeled proteins and phospholipids.

In conclusion, we briefly note two nonlinear processes—i.e., multiphoton luminescence and photothermal effects—that influence and determine the achievement of conditions for changing the morphology of NPs and their reduction. The placement and morphology of NPs were detected based on nonlinear optical luminescence.^201,202 The two-photon absorption and two-photon excited photoluminescence of gold NPs and their small nanoclusters can be used in biological applications for multiphoton microscopy, allowing deeper material imaging and causing less damage to biological samples compared to one-photon microscopy. This is a technique for the label-free imaging of gold (metallic) NPs with single-particle sensitivity using multiphoton luminescence that is easily detected and recorded in real-time without the need for exogenous fluorophore labeling. Metal NPs can be used as a viable alternative to fluorophores or semiconductor NPs for biological labeling and imaging. The multi-resonant plasmonic geometry of NPs has been studied by two-photon photoluminescence to detect their possible melting or evaporation under intense irradiation.^203,204 Multi-resonant plasmonic geometries such as nanocylinders and nanorods have been studied for applications in quantum dots and second-harmonic generation because of their easy fabrication and reproducibility compared to complex multi-resonant systems such as dimers or nanoclusters.²⁰³ Luminescence induced by multiphoton absorption was studied in triangular silver nanoprisms fabricated with nanosphere lithography, when excitation pulses from an infrared femtosecond laser overlap the main surface plasmon resonance of the particles. At higher illumination intensities, some particles experienced a sudden multiple increase in signal brightness, which was associated with deformation or melting of the particles into spheroids. Brightening may be due to either the reduction of surface silver oxide into highly luminescent silver nanoclusters or the formation or activation of emission centers on the particle surface caused by particle melting.²⁰⁴ 

The photothermal processes of laser heating of metal NPs, the dependences of temperatures of NPs and the environment, their time scales, and other parameters describing these processes were discussed and analyzed.²⁰⁵ The influence of NP temperature on optical properties when gold NPs are heated by laser radiation and the subsequent influence on the dynamics of NP heating were discussed, including the assessment of laser thresholds for the initiation of subsequent photothermal processes of NP transformations. Photothermal methods of nanothermometry of NPs during laser heating were considered, including changes in the refractive indices of metals and spectral thermometry of optical scattering by NPs, Raman spectroscopy, and thermal distortion of the refractive index in an environment heated by an NP. The fields of application of photothermal processes during laser heating of NPs were considered, including thermochemical reactions and optothermal chemical catalysis initiated by laser-heated NPs, LII, etc. Considerable attention was paid to taking into account the temperature dependences of NP parameters in the various fields of application of laser heating of NPs.

It is interesting to note the possibilities of using nanostructures to create innovative devices for optical and laser diagnostics of various diseases. Ultrasensitive detection of clinically relevant biomarkers that exhibit different levels of expression in cancer and affect cellular transformation, carcinogenesis, and metastasis using surface plasmon resonance (SPR) sensors is very important for the successful treatment of this disease. An SPR sensor was developed based on nanomaterial for specific label-free detection of biomarkers.²⁰⁶ Highly sensitive SPR platforms have been developed for rapid and specific diagnosis of different variants of SARS-CoV²⁰⁷ based on the developed “Methodologies of Photonic Sensing,” which combines optical sensing technology (SPR) with “gene scissors.” A highly sensitive SPR sensor has been proposed for the diagnosis of inherited diseases at femtomolar level with real-time quantification and has been used successfully for the analysis of recombinant plasmids.²⁰⁸ These innovative SPR platforms will facilitate fast, sensitive, and accurate target detection.

In the previous parts of this review, studies and applications of individual processes and mechanisms of laser heating, melting, evaporation, reduction, and fragmentation of NPs were discussed. In this part, various recent studies and subsequent applications of the complex processes of laser irradiation of NPs in liquids and their processing are analyzed. The complexity of the multiscale intertwined processes involved in the laser processing of NPs—which is contingent upon laser fluence and transitions from low to high levels—necessitates meticulous consideration of the operational regimes, the mechanisms governing these complex processes, and their potential applications.

Pulsed laser ablation in liquid (PLAL) is a versatile technique for synthesizing a wide variety of colloidal metal, semiconductor, oxide, carbide, and alloy NPs by ablation with a high-intensity pulsed laser of a bulk target immersed in the desired liquid.^9,11,12 When a high-intensity laser pulse reaches the target material, its surface is evaporated, forming a hot plasma plume and cavitation bubble containing ions, atoms, droplets, and molecules of both the target and the liquid, followed by vapor condensation. Subsequently, primary NPs are formed in the liquid medium via condensation nucleation, growth, and coalescence. The production of NPs by PLAL offers several advantages, including the synthesis of surfactant-free NPs, the versatility of the process, and the ability to produce NPs with complex structures and compositions that challenge standard chemical methods. However, despite its advantages, the industrial uses of PLAL remain limited because of its low production rate compared to chemical synthesis. The productivity of PLAL ranges from several milligrams to several grams per hour depending on the experimental parameters associated with using a high-power laser at high scanning speed, which requires a huge initial investment. To address the limitations of PLAL, cost-effective alternatives have been proposed, such as double-beam and parallel diffractive multi-beam laser ablation in liquids, in conjunction with static diffractive optical elements to facilitate parallel processing via multi-beam techniques.^209,210 Furthermore, to enhance NP size control and mitigate the characteristic bimodality observed in NPs synthesized by ablation, double-pulse laser ablation in liquids has been suggested for the reduction of NP bimodality, leveraging subnanosecond optimization of the delay between pulses.²¹¹ 

Recently, specific nanomaterials with unique chemical and physical properties were generated by PLAL in some special liquids. Uncapped Ag NPs with a diameter of 1.66 ± 0.37 nm were obtained by laser ablation of an Ag target in deionized water and exhibited excellent dispersion stability over 70 d with no external disturbance because of their negatively charged surfaces.²¹² The formation of nanostructures during laser processing, including reaction pathways and mechanisms in organic liquids, is influenced by various factors such as chemical reactions, solvent choice, synthesis methods, and laser parameters, which may lead to the generation of doped, compounded, and encapsulated nanoparticles.²¹³ As possible components of electronic devices, spherical TeO₂ NPs with an ultrawide band gap (>3.2 eV) and sizes of ca. 39 ± 12 nm and 29 ± 6 nm were synthesized successfully.²¹⁴ Nanosecond PLAL of synthetic graphite was used for synthesizing spherical graphene in ethanol and distilled water without the use of toxic chemicals.²¹⁵ The ablation behavior and modification mechanism of silicon carbide (SiC)—a typical hard-to-machine material—were studied under different laser energies.²¹⁶ The dynamics of laser ablation in liquid environments were investigated via atomistic simulations conducted on FeNi targets irradiated by picosecond laser pulses across a wide fluence range.²¹⁷ Three fluence regimes were investigated: (i) low fluence, at which atomic clusters and small NPs form via the evaporation of metal atoms followed by condensation in a low-density region at the front of the ablation plume; (ii) medium fluence, characterized by the roughening and decomposition of the upper portion of a transient spongy structure composed of interconnected liquid regions, resulting in the formation of larger NPs; (iii) high fluence, at which the NPs form primarily at the phase separation front propagating through the ablation plume, cool from the supercritical state, and solidify under conditions of deep undercooling, yielding a population of defect-rich NPs.

The pulsed laser processing of nanomaterials in liquids can produce uniform, multi-component, nonequilibrium nanostructures with independently and precisely controlled sizes, compositions, morphologies, and defect densities. Four main laser methods are used and often act together, i.e., laser melting, reduction (evaporation), fragmentation, and defect-engineering of NPs in liquid. To create massive NPs (i.e., SMPs), a straightforward procedure called laser melting in liquids (LML) is used. The heating of NPs above their melting temperature and their subsequent melting and coalescence lead to the formation of large SMPs with a particle size of ca. 100 nm to 1 μm.

Pulsed LML (PLML) can produce crystalline spherical SMPs by irradiating lasers onto raw particles dispersed in liquid, these being impossible to obtain by traditional particle fabrication methods. This method is based on the photothermal processing of particles dispersed in liquid, using appropriate laser fluences to reshape or melt agglomerated or aggregated particles from raw particles of different materials (semiconductors, oxides, carbides, etc.) to form large SMPs. This method essentially refers to the nanoscale thermal processes, such as thermal spheroidization of particles from particles of other shapes, substrate modification, etc.

Koshizaki and colleagues conducted a series of studies on the laser melting of NPs, and their results for previous years up to 2022 are presented in Refs. 132, 133, 135, and 218–220. Laser melting was used to produce Pt SMPs by 266-nm laser irradiation; compared to gold, platinum is arguably a better material for fabricating electronic wires using ink-jet printing, and it can be used in high-temperature conditions because of its higher melting point.²¹⁸ The formation of spherical SMPs is regulated by rapid heating above the melting point, with the instantaneously formed vapor layers (thermally induced nanobubbles) playing a critical role in facilitating rapid temperature increases, as well as in achieving maximally attained temperatures.²¹⁹ To enhance the mass production of SMPs, which is essential for various promising applications, a guided slit nozzle designed for a continuous flow system suitable for high-energy, low-frequency (several tens of hertz) laser pulses was developed. Through single flow passage irradiation of a sufficiently slow continuous suspension flow via the slit nozzle, the formation rate of spherical SMPs exceeded 90%.²²⁰ Some basic mechanisms and features have been studied for improved particle melting processes. The particle size distribution results from the aggregation and merging of the raw particles in the liquid during transient heating, and the SMP size can be controlled via the viscosity of the liquid, with high viscosity inhibiting rapid aggregation and increasing the particle size.²²¹ Analyses have been done of the basic mechanism behind PLML to produce crystalline spherical SMPs, the parameters affecting the particle size and morphology, possible application examples, and future directions of this method.²²² Thiol-stabilized NPs, dodecanethiol or toluenethiol derivatized gold, silver, and copper NPs, and films formed on a plate were prepared to observe photoinduced reactions.²⁰ Pulsed laser irradiation at 1064 nm (at which the NPs do not exhibit plasmonic absorption) caused the fusion and size growth of metal particles, initiated by photothermal conversion, which was observed and analyzed using diagnostic methods.

The bond breaking/formation of copper or copper (II) interfaces with ethanol during the absorption of pulses for Cu-CuO-Cu₂O formation applicable as an electrocatalyst in ethanol oxidation fuel cells was analyzed.²²³ The observed exponential and logarithmic changes in the content of heterostructures for the CuO-ethanol and Cu-ethanol samples irradiated with different fluences are interpreted as the dominant role of physical and chemical reactions. The formation of composite particles by pulsed laser melting of α-Fe₂O₃ in ethyl acetate and ethanol was studied.¹¹⁸ 

Laser fragmentation and evaporation in liquids is a technique for generating Au NPs with narrow size distributions and is also the only useful method for creating small NPs. The laser fragmentation is realized by the laser absorption and heating of NPs and subsequent different mechanisms. Defect-rich NPs can be formed via the fast cooling of laser-heated NPs. In general, combinations of laser-processing methods are used successfully to produce NPs, and they allow precise control over the size of colloidal nanomaterials, ranging from a few micrometers to as small as 1–3 nm. High-power nanosecond, picosecond, and femtosecond lasers are typically used because high instantaneous fluence is required for intense particle–laser interactions. The ability to synthesize novel materials has also been studied, including high‐purity nonequilibrium amorphous or defect-rich phases.

The fragmentation mechanisms discussed in the literature can be separated into photothermal,^1,2,6 laser phase explosion,^12,117 photomechanical,^65,102,103 and electrostatic^7,72–74 ones. The thermal fragmentation mechanism was first discussed in terms of quasi-equilibrium processes, during which the particle thermodynamic state adheres to the binodal curve of solid–gas to liquid–gas coexistence, and the NP size reduction proceeds via evaporation and condensation of the evaporated atoms into small NPs. Fragmentation was supported by experimental observations of bimodal size distributions of fragmentation products.²² Rapid heating of colloidal NPs excited by femtosecond to picosecond laser pulses leads to the situation in which the energy losses due to the evaporation rates are insufficient to compensate for the energy release due to laser absorption. At sufficiently high levels of excitation, the NPs can be superheated to the thermodynamic stability limit of the molten material, leading to so-called phase explosion, i.e., spontaneous decomposition into vapor and liquid droplets.^12,117 The laser-induced explosion of strongly absorbing NPs is realized via rapid overheating during the ultrashort laser pulse when the influence of heat diffusion is minimal. Coulomb explosion of NPs can be achieved in the case of electrostatic repulsion of their charged parts as a result of ultrashort laser pulse action.^7,72–74 Local near-field enhancement of the laser intensity can also contribute to femtosecond laser-induced NP fragmentation via nonthermal (and directional) electron and ion emission, strong NP charging, and subsequent Coulomb instability. The explosion of NPs can be accompanied by the generation of plasma and shock waves with the supersonic expansion of particle fragments of high kinetic energy, etc. Photomechanical processes can also contribute to NP fragmentation, particularly under conditions of stress confinement, when the NP heating time is shorter than the time required for thermoelastic relaxation, i.e., NP expansion.^65,102,103

The laser fragmentation of Au NPs by picosecond pulses in water and the transient melting and fragmentation processes were investigated with a combination of time-resolved X-ray probing and atomistic simulations.^224,225 Chemically synthesized Au NPs [via cetyltrimethylammonium bromide (CTAB)] with a narrow size distribution and an average size of ca. 44 nm and laser-ablated Au NPs with a broader size distribution were used in the experimental study. A pump–probe experiment was conducted with a picosecond laser (pulse duration:1 ps; wavelength: 400 nm) as the excitation (pump) source and a time-resolved X-ray probe (pulse duration: 60 ps), both at a repetition rate of 1 kHz. A sequence of several nonequilibrium processes and the cascade of thermal and nonthermal fragmentation regimes triggered by the laser irradiation were revealed. At low laser fluence below the threshold for NP melting, the laser-induced processes are limited to NP heating by laser excitation and cooling due to the heat transfer to the surrounding water. The lattice expansion measured at a time delay of 60 ps yields information on the maximum NP lattice temperature reached by the time that the excited electrons have equilibrated with the lattice, but the heat dissipation into the water environment is still negligible and needs to be confirmed. The lattice expansion corresponding to the melting temperature of the CTAB NPs is subsequently used to define the fluence threshold F₀ for reaching the melting temperature. The melting and re-solidification regime continues until ca. 3F₀, and the initially faceted CTAB NPs are transformed into spherical shapes. At fluences of approximately three times the melting threshold, the results indicate a transient overheating of crystalline NPs above the melting temperature. Above this fluence level, fragmentation begins with the evaporation of Au atoms from the surfaces of the irradiated NPs and condensation of the metal vapor into small NPs surrounding the remaining core NPs. Further increase of the laser fluence in experiments above the 5F₀ melting threshold results in the transition from the evaporation−condensation regime to the regime of so-called phase explosion, i.e., a rapid (explosive) phase decomposition of the superheated Au NPs into small vapor and atomic clusters, and NPs with sizes that do not exceed several nanometers.

Using an example of 44-nm CTAB Au NPs in water, the effective absorption cross section evaluated from the experimental fluence dependence of the lattice expansion deviates from the theoretical value predicted by Mie theory by more than 50%. Given this uncertainty and taking into account the use of approaches that assume a constant (fluence-independent) value of the NP absorption cross section and that the heat dissipation into the water environment is negligible, establishing quantitative links between experiment and theory is hindered. It is necessary to account for the temperature dependence and the effect of the increase in NP size as a result of NP melting on the absorption properties of the NPs when heated to the melting (or evaporation) temperature.^123,127 Also, for 1-ps laser pulses, heat losses due to thermal conduction from the NP surface are negligible, but for a probe pulse duration of 60 ps, the thermal losses from NPs with a radius of r₀ = 22 nm can reach 10% of the total absorbed energy, which must be accounted for in the NP energy balance.²⁰⁵ 

For femtosecond and picosecond pulses, a complex interplay between photomechanical and photothermal processes determines the final particle size distribution. The contribution of stress‐mediated processes to the photomechanical fragmentation of single IrO₂ NPs in water by a single 1040-nm, 10-ps laser pulse has been studied for the synthesis of active and redox-sensitive colloidal nanoclusters.²²⁶ The fragmentation was carried out in a continuously running liquid jet, and 2-nm nanoclusters were generated accompanied by larger fragments with sizes ranging from a few tens of nanometers to several micrometers. An efficiency of up to 18 µg J⁻¹ was reached for the nanosized product, exceeding comparable values reported for high‐power LAL by one order of magnitude. Photomechanically assisted laser fragmentation of NPs is a versatile and scalable technology for the design and development of nanomaterials. The obtained nanoclusters exhibit high catalytic activity and stability in oxygen evolution reactions, which makes them promising candidates for electrocatalytic sensing.

Ultrafast photoinduced melting is related to the nonequilibrium phase transitions and kinetics of electron and ion dynamics under femtosecond photoexcitation.⁶⁹ Femtosecond laser pulses trigger nonequilibrium photoinduced ultrafast melting processes, which remain incompletely understood because of problems in resolving the accompanying kinetics at an appropriate spatiotemporal resolution. Ultrafast energy transfer processes in confined Au nanospheres were revealed by a multiplexed femtosecond X-ray probe.⁶⁷ Real-time images of electron density distributions with the corresponding lattice structures show that energy transfer begins with subpicosecond melting at the specimen boundary before lattice thermalization and continues via void formation. Two-temperature MD simulations revealed the presence of both heterogeneous melting with the melting front propagating from the surface and grain boundaries and homogeneous melting with random melting seeds and nanoscale voids. Photoinduced nonequilibrium phase transitions accompanying ultrafast melting before lattice thermalization motivated the study of dynamic interactions between electrons and crystalline ions and ultrafast photoinduced weakening of the crystal bonding. The ultrafast dynamics of Ge bonding orbitals to drive photoinduced melting were directly controlled.²²⁷ Increased photoexcitation of bonding electrons amplifies the orbital disturbance to expedite the lattice by performing time-resolved resonant X-ray scattering with an X-ray free-electron laser. The lattice disorder time exhibits a strong nonlinear dependence on the laser fluence, with transient behavior from thermally driven to nonthermally dominated kinetics. The impact of bonding orbitals on lattice stability was revealed with a unifying interpretation on photoinduced melting. The transient ion pressure induced by photoexcited electrons controls the overall melting kinetics, which directly reveals the fluctuating density distributions and evaluates the ion pressure and Gibbs free energy from two-temperature MD.²²⁸ The ultrafast nonequilibrium melting can be described by the inverse nucleation process with voids as the nucleation seeds. The strongly driven solid-to-liquid transition of metallic gold is explained by the nucleation of voids, which is facilitated by photoexcited electron-initiated ion pressure, to understand the ultrafast nonequilibrium kinetics.

The large number of interactions in nanosystems results in their complex behavior, which requires observations with atomic and high temporal resolutions to match the characteristic timescale of nanosystems. Time-resolved electron microscopy has been used to develop novel methods and instruments for high-speed, atomic-resolution observations.⁷³ 

The pulsed laser irradiation of NPs in liquids and the subsequent processes lead to the transformation of NP properties due to possible laser generation and engineering of nanostructured defects, formation of hybrid NPs, their amorphization, and other changes in various NPs. The synthesis of partially amorphous Fe-based oxide NPs by pulsed laser ablation in water was studied.²²⁹ High-fluence nanosecond pulses provide melting and quenching of NPs and a significantly higher amorphization rate, while picosecond fragmentation always presents minor fractions of crystalline a-Fe even at higher laser intensity. Amorphization can also be associated with an apparent size reduction effect, while complete amorphization of metal oxides can be associated with stronger oxidation effects. Visible-light-absorbing modified ZnO (black) NPs with spherical morphology and free of impurity phases were prepared by creating surface defects and vacancies (confirmed by XPS analysis and Raman and photoluminescence spectra) using pulsed laser irradiation in liquid.²³⁰ Thin films were prepared using these materials by the doctor blade method, and their photocatalytic studies showed enhanced activity in decomposition of organic dyes under visible light irradiation. High-temperature gradients in laser synthesis and processing lead to kinetically controlled particle nucleation and freezing of crystallographic high-temperature crystalline phases, which can now be gradually written into NPs using pulsed laser defect engineering in liquid (PUDEL) under continuous flow conditions in a flat liquid jet setup.²³¹ Thus, the PUDEL approach combines several important features of laser post-processing, i.e., a high level of process control, portability, and scalability, which are useful for developing scarce NPs and can be used for further studying the role of structural disorder and defects in materials science, spectroscopy, and heterogeneous catalysis. MoSe₂ powder was irradiated with different laser powers and ablation times in isopropyl alcohol, and the types of formed NPs with a bimodal size distribution were studied.²³² The first type is onion-structured NPs, which are formed by nucleation on the surface of melted droplets. The second type is polycrystalline NPs, which are formed by coalescence of crystalline nanoclusters fragmented from the powder during laser ablation. The high photothermal conversion is attributed primarily to defects and structural disorder in the laser-synthesized NPs, which allow absorption of photons with energies smaller than the bandgap energy and facilitate nonradiative recombination of photoexcited carriers. Synthesis of low-dimensional nanomaterials including zero-dimensional quantum dots, one-dimensional nanowires and nanotubes, and 2D nanosheets and nanobelts was realized.²³³ 

MD was recently introduced for studying the processes of ultra-short and short laser interaction with NPs in liquids—including describing the plasma and vapor formation processes—and the pulsed laser synthesis of nanocolloids. MD operates at the atomic level, facilitating dynamic simulations of materials and providing physical interpretations for its quantities, rendering it an efficient approach for examining the characteristics and behaviors of materials during material processing.^102–109 As a powerful method for studying the characteristics and behaviors of the laser processing of metal NPs, the two-temperature model integrated with MD effectively emulates the impacts of the liquid medium and laser energy and energy transfer and relaxation processes on the NP size, morphology, and microscopic dynamics of gold NPs, and recent advancements in MD simulations of laser-processed metal NPs have been presented.²³⁴ Large-scale MD modeling of the process of laser-assisted fragmentation of colloidal solution of silicon (Si) NPs was carried out. They can be used as contrast agents for visualization, as effective sensitizers of radiofrequency hyperthermia for cancer theranostics, in photodynamic therapy, and as carriers of therapeutic radionuclides.²³⁵ 

Photoacoustic detection of the transient phase transformation of NPs (Ag, TiO₂, CeO₂, ZrO₂) during laser synthesis and processing of colloids in ethanol, water, and glycerin was studied for controllable preparation and measurement of the thermal and spectroscopic properties of spherical micro/nano-particles of various materials.²³⁶ The relationships between the intensity of photoacoustic signals and the thermal expansion, crystal transformation, material phase transitions, spheroidization, and evaporation of NPs were studied systematically, and such transient processes and changes of state can be excited separately by adjusting the laser fluence.

The pulsed laser processing of NPs in liquids is very interesting for fabricating nanomaterials for catalysis. Laser irradiation of colloids allows the fabrication of clean metal oxide NPs (spheres, rods, flowers, metal-oxide core–shells and doped oxide NPs, heterostructures, etc.), which are used widely as catalysts for oxidation catalysis, electrocatalysis, and photocatalytic pollutant degradation because of their physical and chemical properties at the nanoscale.²³⁷ The advantages, challenges, and experimental solutions for the pulsed laser synthesis in liquids of nanoelectrocatalysts were discussed, including catalysts for water oxidation and oxygen reduction, hydrogen evolution, nitrogen reduction, carbon dioxide reduction, and organic oxidation, as well as light-energy-utilizing nanophotocatalysts (incorporating solar energy) to drive chemical reactions.²³⁸ Laser-fabricated photocatalysts such as monometallic and bimetallic systems, metal oxides, hydroxides, carbides, and nitrides have been used for solar water splitting, the photodegradation of dyes, water pollutants, and bacteria, and the light-induced oxidation of organic molecules. Pulsed laser heating and fabrication of functional nanomaterials in liquids with multi-controllable sizes, morphology, crystal structure, and photocatalytic characteristics and their applications in the catalytic degradation of various organic pollutants in the water system and the catalytic reduction of toxic nitrophenol and nitrobenzene have been summarized.²³⁹ 

A comprehensive overview of the development of nanostructured materials via pulsed laser techniques and their use as energy and environment-related applications was presented.²⁴⁰ Pulsed laser-induced generation of nanostructures in liquids for energy and environmental applications was analyzed qualitatively, including the mechanisms of pulsed laser techniques by considering various experimental conditions for producing efficient nanomaterials and NPs. The formation mechanism of pulsed laser-induced nanostructures in liquids and the influence of laser parameters and experimental conditions on the formation of nanostructured materials were analyzed. Metal NPs, alloys, and semiconductor metal oxides and non-oxide materials produced by pulsed laser ablation and irradiation in liquids were used for photocatalytic remediation and water splitting. Recent progress on several kinds of both photo and electroactive nanomaterials was reviewed, and catalytic behaviors and the corresponding stability were discussed. The latest advances related to the application of these nanostructured materials produced via pulsed-laser-in-liquid techniques in various energy (supercapacitor, batteries, and hydrogen production) and environmental remediation (wastewater treatment and conversion of waste into value-added product) processes and for sensors and biomedical applications were reviewed.

The fabrication of defect-rich induced black titanium dioxide TiO₂ (B TiO₂) NPs with enhanced visible light absorption by pulsed laser irradiation in de-ionized water was studied²⁴¹ with the transformation of the major anatase phase to rutile. Photocatalytic degradation studies showed excellent visible-light-induced degradation activity of organic dyes, and the photoelectrocatalytic activity of B TiO₂ films at neutral pH demonstrated selectivity in oxygen evolution reaction. Spherical silver (Ag) and tin dioxide (SnO₂) nanocomposites were prepared successfully using laser-induced deposition in a liquid medium, and pristine Ag nanoparticles were melted into spheres with reduced average particle size and welded with the SnO₂ nanomaterials by laser heating and liquid quenching.²⁴² The photocatalytic performance of the spherical Ag and SnO₂ nanocomposites revealed enhancement of the reaction rate constant of the degradation of methylene blue aqueous solution. Laser fragmentation in a liquid jet produces monodisperse, sub-5-nm NPs in a fully continuous operation, but the NP yield and laser power productivity are still below the established gram-scale laser ablation method. Using iridium NPs as an important catalyst for the acidic oxygen evolution reaction, a significant improvement in NP laser power productivity was observed with increase of the initial particle concentration to several grams per liter.²⁴³ The number of applied laser pulses controls the degree of NP surface oxidation, while the diameter of the monodisperse product particles of ca. 2 nm was independent of the initial particle size and concentration.

The fabrication and processing of alloy NPs by pulsed laser application in liquids is a very interesting and useful method for possible application in the biomedical field. The laser ablation of equiatomic FeCoNi medium-entropy alloy (MEA) in water using a high-repetition-rate UV picosecond laser was carried out, and the extinction spectra of the colloidal solution, morphology, elemental composition, and size distribution of MEA NPs were investigated as a function of laser irradiation time and power.²⁴⁴ Au-Fe, Au-B, Fe-B, and Fe-Ag alloy NPs have been obtained by laser synthesis and showed attractive properties for cancer imaging and treatment with higher efficiency, feasibility, and tolerability, endowed with medium-term biodegradability and synergy of theranostic functions.²⁴⁵ The accurate calculation of the optical properties of metastable alloys of both plasmonic (Au) and magnetic (Co) elements, and also for other magnetic-plasmonic (Au-Fe) and typical plasmonic (Au-Ag) nanoalloys, obtained through a tailored laser synthesis procedure was reported for applications in quantum optics, magneto-plasmonics, metamaterials, and plasmon-enhanced catalysis.²⁴⁶ 

Polymer-coated Au-B NPs were obtained by laser ablation in liquid, where the two elements coexist by short-range boron segregation in the gold lattice for the investigation of combined boron neutron capture therapy and X-ray radiotherapy for treating normal and resistant hypoxic tumor regions, supported by the localization and quantification with X-ray computed tomography imaging.²⁴⁷ Ultrapure, spherical silver–gold alloy NPs with homogenous elemental distribution were synthesized by laser ablation in liquids and analyzed for their antibacterial activity on different stages of Staphylococcus aureus biofilm formation as well as for different viability parameters.²⁴⁸ AgAu NPs exhibit antibacterial properties against planktonic bacteria, as well as against early-stage and even mature biofilms, while they completely diffuse through the biofilm matrix. NPs primarily target metabolic activity, to a lesser extent membrane integrity, but not biofilm volume. Picosecond-pulsed laser fragmentation in liquids of dispersed drug particles in a liquid-jet passage reactor is used as a wear-free comminution technique using the hydrophobic oral model drugs naproxen, prednisolone, ketoconazole, and megestrol acetate to overcome their low solubility in relevant aqueous media because of reducing the particle size of drug powders to a few hundred nanometers.²⁴⁹ The superior fragmentation efficiency of the liquid-jet passage reactor setup was confirmed, with a 100 times higher fraction of SMPs of the drugs compared to the batch control, which enhances solubility and goes along with minimal chemical degradation (<1%). The application of a pulsed laser ablation technique for the generation of cerium-doped garnet NPs and the formation of spherical particles (partially molten) and agglomerates of NPs via the pulsed laser melting process in chloroform and a sodium citrate and their morphological and optical properties were studied.²⁵⁰ Cerium-doped garnet can be applied as single crystals and used in nuclear medical imaging, astronomy, laser materials, and other R&D applications.

The presented research results and their applications indicate a growing interest in laser methods of NP processing and their potential for various applications. In turn, an improved understanding of the complex mechanisms of the laser processing of colloidal NPs will allow for tuning of the sizes, shapes, and internal structures of NPs to meet the needs of practical applications.

This review covers articles published in the past 10 years up to mid-2024, with an emphasis on the results of recent years but also taking into account the results of previously published work. Note that the activity of these studies and publications is increasing every year. This review comprehensively examines the progress of experimental and theoretical studies of the fundamental processes of laser heating, melting, evaporation, fragmentation, and optical breakdown of NPs and their applications.

Experimental results of studying these processes with gold, silver, and other NPs are discussed, including studies of melting and evaporation of the NP surface, as well as the Coulomb fragmentation of NPs by ultrashort laser pulses. These processes are initiated when certain threshold NP temperatures are reached under the action of laser pulses with threshold fluence and other laser parameters. An analytical thermal approach is presented for assessing fluence thresholds depending on the pulse duration, laser wavelength, and nanoparticle radius for nanosecond, picosecond, and femtosecond pulses. Some calculated values of the threshold laser fluences of melting, evaporation, and fragmentation of NPs in water are compared with experimental data for nanosecond and picosecond laser pulses from various scientific groups and satisfactory agreement of these results is presented, which confirms the correctness of the results. On the other hand, these results offer necessary verification of the implementation of the mentioned processes and can also be used to understand, evaluate, and interpret the experimental results. Determining the threshold parameters (fluences) of laser radiation is very important for the successful use of NPs in various laser technologies because of the required accuracy of the results of laser exposure to materials and tissues containing NPs, and they can be transferred into requirements for laser and NP parameters.

The unique advantages of laser-synthesized NPs have been analyzed to promote potential applications in various fields. The areas of application of the laser processing of NPs immersed in a medium (water) or located on substrates are considered, including their reshaping, fragmentation, nanowelding, the formation of nanostructures, and their cladding on surfaces, LI, combustion, and incandescence and applications of laser breakdown initiated by NPs for spectroscopy. These processes are used for the applications of laser interaction with NPs in nanotechnologies, laser processing of NPs, and production of new nanostructures and nanolayers on substrates, nonlinear optical diagnostics, material science, etc.

Further study and development of the field of laser interaction with NPs in media (liquids) will develop in two main directions. The first direction is the continuation of joint experimental and theoretical studies of various processes and mechanisms of laser interaction with NPs and their processing. The second direction is the application of the obtained results and the development of real devices and technologies for industrial use of the obtained NPs, as well as for the design and production of nanostructures on an industrial scale.

The first task is to continue the experimental and theoretical studies of the mechanisms of interaction of lasers with NPs, concerning not only the clarification and understanding of the various experimental results in cases of some discrepancies, but also the exploration of some unexplored gaps for femtosecond and picosecond laser pulses. A better understanding of the processes is necessary in order to achieve a consistent picture of the processes and a reliable use of these results to predict the parameters and properties of nanoproducts, and it is also useful for the transfer of knowledge to other laser nanotechnologies.

Understanding the interaction and processing mechanism can be supported by high-precision experiments with single pulses and pulse trains, assessing the difference between them and selecting possible technologies that can use them together or separately. Another goal of fundamental research is to precisely control the NP size and NP crystalline phase by adjusting the NP properties. New information is needed to better understand the properties of synthesized NPs and the laser–NP–liquid (gas) system and its performance/reproducibility characteristics. Medium-sized monodisperse or selected polydisperse spherical NP systems (which are relevant for many applications), such as 10–50-nm Au NPs, are still difficult to obtain. Because of their wide plasmonic applications, the size control of metal NPs during laser irradiation is still limited and therefore requires further study. The study of photothermal processing to change the shape or melt agglomerated or aggregated particles from precursor particles of various materials (semiconductors, oxides, carbides, etc.) dispersed in liquid to form large submicrometer particles may find its application.

NPs are used as laser-activated light-to-heat converters during laser–NP action, which allows external stimulation of various media. The temperature reached by the irradiated plasmonic nanostructure is often of paramount importance for its application, and this temperature must be determined with sufficient accuracy. Although there are still challenges to overcome before realizing the full potential of laser–NP processes in life sciences and medicine, such as cancer treatment, it is desirable to directly measure and control the temperature experimentally. The vast majority of experiments and theoretical studies have been carried out with gold and silver NPs. At the same time, it is necessary to conduct research with other NPs from different metals, such as nickel, iron, titanium, etc., that have significant absorption in the near-IR range and are likely to be used in industrial applications.

Determining the thresholds of laser melting (E_M), evaporation (E_EV), and fragmentation (E_FR) of NPs is a very significant problem because of the sensitivity, correctness, and reproducibility of the methods used, as well as the influence of various parameters of the laser, NPs, and environment on the final results. At present, these data are insufficient and often contradict each other, sometimes up to an order of magnitude (see Tables I and II). At the same time, they determine the possibility of creating future laser–NP technologies and their application in industry. The study of threshold parameters should be continued for a more correct determination of threshold fluences and elimination of discrepancies in the values of thresholds and other parameters.

The laser fragmentation and processing of NPs is emerging as a unique and scalable technology for producing very small NPs and nanomaterials down to 1–5 nm in size, which can hardly be obtained by laser evaporation or various chemical synthesis methods. Understanding and characterizing the picosecond and femtosecond effects on NPs, leading to significant reduction and fragmentation of NPs, are still far from being fully resolved. Some fundamental mechanisms of melting of solid NPs by femtosecond laser pulses at the nanoscale remain unclear, and their ultrafast phenomena have revived the interest in NP melting and evaporation processes. Direct experimental verification of the various processes is limited by the difficulty of visualizing the internal structures of materials undergoing ultrafast and irreversible transitions. Further systematic experimental and theoretical work is needed to determine for the laser fluences the boundary between thermal and nonthermal (Coulomb, thermomechanical, ablative) mechanisms and to determine the transition regions between them, in which several mechanisms can act simultaneously depending on the laser, NP, and environmental parameters. Precise control of NP size in the range of 5–10 nm and more and progress toward gram-scale NP production can help expand the applicability of such nanomaterials in various fields, in nanobiomedicine as drug carriers, in nanoelectronics for the development of new devices, for nanointegration applications, catalysis, imaging, biomedicine. Further development of picosecond and femtosecond lasers is another hot research topic aimed at increasing the production and stability of NPs. Laser fragmentation of NPs using high-power picosecond and femtosecond lasers can be a key process attractive for the industrial production of NPs of certain small size with improved cost-effectiveness. Some new applications based on the use of high-energy electrons ejected from femtosecond-heated NPs for injection into a semiconductor or various substrates may have possible uses in nanoelectronics.

To understand and describe the processes of heating, melting, evaporation, and fragmentation of NPs, various theoretical methods have been used, including computer and analytical calculations based on the approaches of hydrodynamics, thermal processes in a continuous medium, and MD taking into account the dynamics of individual atoms. First, it is necessary to continue developing theoretical models in order to compare theoretical and experimental data on the threshold densities of laser radiation of NPs. Second, it is necessary to compare the results obtained by different theoretical methods in order to understand the advantages and disadvantages of their approaches and use more-adequate models to describe complex processes. Improvement of the presented numerical and analytical calculation methods should be achieved by developing more-complex models, by choosing suitable initial conditions for the calculation, and mainly by comparing with experimental data to achieve better agreement or disentangle the causes of deviations between experiment and theory.

Laser processing and synthesis of various NPs and nanostructures is becoming an advanced discipline for solving several industrial problems. Applications of these processes include the use of laser melting, shape changing, fragmentation of NPs, formation of nanostructures and nanonetworks, laser processing of NPs located on substrates, their fusion on the surface, and construction of 3D nanostructures in various laser nanotechnologies. Despite the successful results of processing a number of NPs, many problems still need to be solved. The limited productivity of laser processing is a technologically challenging problem that limits the implementation of these methods in real applications and is expected to be improved significantly in the near future. Gram and larger-scale productivity of the obtained NPs with their reproducibility should be achieved at the processing installations, demonstrating competitiveness with other methods of colloidal synthesis. For stationary production of NPs, laser pulses with selected parameters or a laser scanning beam and a hydrodynamic processing chamber should be developed, which will allow for continuous production of NPs.

Differentiation of the properties of the obtained NPs for various applications (electronics, chemistry, biology, industry, etc.) should be accomplished in the future to realize industrial-oriented laser synthesis and processing. Nanojoining, a growing research field, is becoming a key area for producing complex nanostructures with functional ready-made components for new nanodevices, and various nanojoining methods will be developed. Laser cladding is one of the main processes for producing layers on a substrate and fabricating 3D structures with size recovery and wear and corrosion protection. The properties of lasers and material powders, processing parameters, and the properties of the processed material determine the success of laser nanoprinting of colloidal particles with nanometric accuracy, laser additive manufacturing, and cladding in new industrial applications. The application of laser nanojoining and nanoprinting and powder cladding should be based on the excellent process stability and reproducibility, which should be achieved with the future development of these methods.

The use of LI, combustion, and incandescence of NPs as well as the use of laser breakdown initiated by NPs for spectroscopy will be investigated in the future in order to develop new methods and techniques. It is possible to use LI methods of nanostructures of various metallic nanoenergetic materials for explosives and solid propellants, fast initiation in LI systems of engines, etc. The incandescence signal associated with the combustion of particles in various engines carries information about their properties, and this method will be developed further for studying the properties of metallic, carbon, and other NPs. Very low mass concentration of gold NPs causes breakdown of the carrier gas of the aerosol and hence leads to the detection of gas plasma under conditions that would otherwise be insufficient to achieve this for trace-gas analysis purposes. Experiments in this direction demonstrating this analytical application are ongoing.

Although there are still many other problems and new questions will arise as knowledge in this field accumulates rapidly, progress in the study and development of new technologies will make them applicable in various fields. Finally, laser processing of NPs is based on their unique properties, which is of great importance and interest for the largest industrial use, and the properties of the processes are of paramount importance.

Discussing the results of both the fundamentals and applications related to laser heating, melting, evaporation, and fragmentation of NPs in liquids, this article represents a timely and critical review of this important topic. Although many problems remain and new questions will arise as knowledge in this field increases, the progress made since the inception of the proposed methods and the ideas presented will pave the way for NP laser processing to become a versatile, scalable, yet simple method for designing and manufacturing nanomaterials. The high efficiency of NP processing combined with the application potential of the obtained particles demonstrates the viability of the method for large-scale production of new NPs. Solving the mentioned challenges and problems should lead laser-generated nanostructures to real products in the market. The areas of interaction of laser radiation with NPs will continue to develop, and the possible results will undoubtedly find their application in various new laser nanotechnologies.

The author is deeply grateful to his colleagues L. Astafyeva and A. Chumakov for their cooperation.

The author has no conflicts to disclose.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.