Physics of picosecond pulse laser ablation

Short pulse laser-matter interaction (LMI) has gained considerable interest over the last two decades as pulsed laser technologies have extended the capabilities of laser-driven material processing and research, including advancements in high precision cutting and drilling,^1–3 surface nanostructuring,^4–6 nanoparticle synthesis,^7–10 and laser-induced plasma spectroscopy.¹¹ Much of the research to date has been focused on material processing for the ultrashort sub-picosecond pulse regime (∼100-500 fs FWHM pulse width), particularly for fluences below ∼10 J/cm² (attainable by high rep-rate, low pulse energy lasers) and near-infrared (NIR) wavelengths of ∼800–1100 nm that are easily produced by common short-pulse gain media. Removal rates in this regime are on the order of hundreds of nanometers per pulse. The near isochoric nature of the energy deposition process also simplifies physical interpretation and modeling by alleviating the need for a spatially and temporally resolved treatment of laser interaction with the ablative plume. Previous studies have identified evaporation, near critical point phase separation (explosive boiling), cavitation, and mechanical fracture as the physical processes driving material ablation within this regime.^12–19 

Far less attention has been given to pulsed laser ablation in the ∼1-100 ps pulse width regime, which offers advantages in terms of reduced laser system complexity and cost compared to current femtosecond technology,²⁰ while still providing similar capability in terms of low collateral damage, limited heat affected zones, and the ability to process both metals and dielectrics. Commercial picosecond lasers also offer options for efficient frequency conversion from the NIR (e.g., 1064 nm) to shorter wavelengths into the visible (e.g., 532 nm) and ultraviolet (UV) (e.g., 355 nm) ranges. As we will demonstrate here for pulse widths of 20 ps and 28 ps (FWHM), the most efficient removal (in terms of energy per volume of removed material) occurs in the UV. In fact, ablation rates and material removal efficiency at 355 nm, 20 ps can exceed those at 800 nm, 100 fs in some metals for the same fluence.¹⁹ 

To better understand the physical processes governing picosecond pulse laser ablation, we coupled experimentation with the multi-physics radiation hydrodynamic simulation to study material removal in two metals [Al 6061 and 316L stainless steel (SS)] and a semiconductor material [undoped crystalline Si (〈100〉)] at laser wavelengths of 355 nm and 1064 nm over a fluence range of 0.1-40 J/cm². These materials were selected to cover a range of ablation-relevant material properties related to melt and critical point conditions, thermal diffusivity, shock heating, and absorptivity. Material absorptivity and ablation rate were measured experimentally as a function of laser wavelength and fluence, and ejecta was collected under controlled conditions to evaluate its structure. High-speed pump-probe imaging was used to measure the timing and velocity of the ejecta, and the ablation craters were imaged using high resolution scanning electron microscopy. The experimental data together with the multi-physics simulations were used to evaluate the relevant physical processes, including laser-plasma interaction (LPI) within the ablative plume, electron-ion thermal conduction, laser-driven shock heating, and the imposition of tensile stress during release. Explosive boiling and melt cavitation are identified as the dominant removal mechanisms in different regions of the fluence spectrum. While previous studies (e.g., Refs. 16–18) have provided much insight into the aforementioned physical processes, this work, to our knowledge, is the first to present a comprehensive study of the picosecond pulse laser ablation process. Some findings here are also likely relevant to the ultrashort pulse regime. Our results suggest approaches leveraging hydrodynamic ejection mechanisms to improve both ablation rate and efficiency.

A schematic of the experimental setup utilized for the ablation measurements is shown in Fig. 1. The pulsed laser (EKSPLA PL2231) was operated at 50 Hz with either a 20 ps or 28 ps (FWHM) pulse width for wavelengths of 355 nm and 1064 nm, respectively. A meter focal length lens was used to produce a 130 μm (1/e²) diameter spot on the target. Laser pulse energy was modulated to vary the on-target fluence, and a synchronized shutter was used to control the pulse number for each multi-pulse sequence. The laser ablation measurements were performed in air, and the ablation rate was determined by measuring the ablation crater depth using a 3D laser scanning confocal microscope (Keyence VK-X100). High resolution images of crater surface morphology were captured using a 3D scanning electron microscope (FEI DualBeam Quanta) operating at 5 kV with the sample at a 52° tilt.

Time-resolved pump probe imaging²¹ was used to capture material ejection. Two linearly polarized lasers (Litron, 532 nm, 3 ns FWHM) were used as probes to illuminate the target from the side. The polarization directions of the lasers were orthogonal, allowing them to be spatially combined using a polarizing beam splitter. The time delay of the probe pulses relative to the primary laser ablation pulse, and to each other, was electronically controlled to capture selected timepoints during the ablation process. In order to evaluate the state of the ejecta, collection experiments were performed using the setup shown in the inset of Fig. 1. The target sample was oriented at an angle of 30° relative to laser beam and a clean Si wafer positioned below the target was used to collect ejecta close to the ablation site (standoff distance of 3 mm) in order to reduce the probability of vapor condensation in transit. The ejecta collection experiments were performed in vacuum (1 mTorr) to promote direct transport from the substrate to the collection site and to eliminate issues with the induced shock in the air interacting with the collection plate.

In order to characterize the wavelength and fluence dependence of laser absorptivity, measurements were obtained using the experimental configuration shown in Fig. 2. For consistency, focusing conditions were identical to those utilized for the ablation measurement study. The incident pulse energy was monitored by splitting a small amount of light into an energy meter. Reflected light from the sample was collected with a 0.45 NA lens to minimize loss due to scattering, and the sensitivity of reflection to NA was evaluated. A new site was used for each measurement. Reflectivity was calculated as the measured reflected energy, using a laser line filter to reject non-laser light, relative to the measured incident pulse energy.

The three target materials [Al 6061, undoped crystalline Si (〈100〉), and 316L SS] were selected to study the effect of thermophysical properties related to energy deposition, thermal transport, phase transformation, and material decomposition on picosecond pulse laser ablation. A summary of the relevant properties, including thermal diffusivity (D), enthalpy of fusion (H_m), melt temperature (T_m), boiling temperature (T_b), critical temperature (T_cr), and room temperature, low fluence reflectivity (R), is provided in Table I. All properties are derived from an experimental LLNL material database. Previous research on femtosecond pulse laser ablation identified critical point phase separation (explosive boiling),¹⁶ cavitation,¹⁷ and fracture¹⁸ as the primary mechanisms of material removal in the ultrashort pulse regime. A discussion of these mechanisms is provided in the following sections, but they are introduced here in terms of the relevant material properties.

Explosive boiling involves unstable void nucleation, growth, and coalescence at material states near the critical point. The process is driven by superheating on a time scale much shorter than thermal diffusion and is primarily influenced by energy deposition, rate of heating, and phase stability. The critical temperature for Si (∼4973 K) is lower than that of Al (∼6473 K) and SS (∼6473 K), and as a result Si has a lower threshold for reaching explosive boiling conditions. Si also has lower low-fluence reflectivity at 355 nm and 1064 nm. To our knowledge, however, the fluence dependent absorptivity during picosecond pulse laser ablation is not well known for Al, Si, or SS. To address this knowledge gap, experimental reflectivity measurements are presented in the following section.

Cavitation driven removal is governed by melt production rate and the tensile capacity of the melt. The former is influenced by absorbed energy, rate of heating, and thermal transport. Of the three materials considered, Al has the lowest melting point (∼933 K) and the highest thermal diffusivity (∼0.4 cm²/s). As a result, Al is sensitive to deep melt penetration when heating rates are high relative to diffusive transport. In this work, we have also considered hot electron thermal diffusion, and its contribution will be discussed later in the text. Si has a moderate melting point (∼1683 K) with a relatively high enthalpy of fusion (∼1814 J/g) but experiences enhanced shock heating due to a near 20% solid state volume collapse from the diamond to β-Sn phase.^22,23 SS has both the highest melting point (∼2063 K) and the lowest thermal diffusivity (∼0.05 cm²/s) making it more resistant to deep melt penetration. The temperature and strain-rate dependence of the melt tensile capacity has been investigated in recent years by Refs. 24 and 25 for Al and Fe, respectively, considering strain rates in the 10⁹-10¹⁰ s⁻¹ range associated with femtosecond laser pulses. In addition, Mayer and Mayer²⁶ used molecular dynamics simulation to study dynamic fracture of homogeneous melts of Al, Cu, Ni, Fe, Ti, and Pb over a wider range of imposed strain rate (10⁷-10¹¹ s⁻¹). The studies showed a strong dependence of tensile capacity on temperature, but found that tensile capacity decreases relatively slowly with a decrease in the strain rate.

Reflectivity measurements for Al, SS, and Si are presented in Fig. 3 for laser wavelengths of 355 nm and 1064 nm over a fluence range of 0.1 to 40 W/cm². The measurements for Al and SS show a strong dependence of reflectivity on the wavelength, with two to three times lower reflectivity at 355 nm for fluences between 2 and 15 J/cm². In the lower fluence range (<2 J/cm²), all three materials exhibit a sharp nonlinear increase in reflectivity with decreasing fluence at 355 nm. Figures 4(a) and 4(b) present the corresponding ablation rate measurements, based on multi-pulse average removal, as a function of incident and absorbed fluence, respectively. Between 5 and 10 J/cm², the measured material removal rate in Al and Si is comparable and increases from approximately 200 to 300 nm. The ablation rate in SS over the corresponding range is approximately 100 nm. In the higher fluence range above ∼10 J/cm², the ablation rate in Al and Si increases sharply to micrometer-scale removal and varies approximately linearly over the fluence range from 1 to 2 μm. A similar trend is observed for SS but the measured ablation rate above 10 J/cm² is two to four times lower than the corresponding measurements for Al and Si.

The observed wavelength dependence of reflectivity can be primarily explained by LPI effects within the ablative plume. Research has shown that the excitation wavelength plays a critical role in laser-target and laser-plasma coupling, which in turn affects absorptivity and plasma plume morphology.²⁷ More specifically, laser wavelength impacts the relative contribution of the three primary mechanisms of laser energy absorption (within the energy regime of interest): electron-neutral atom inverse Bremsstrahlung (IB), electron-ion IB, and photoionization. The relative contribution of electron-neutral atom IB is generally negligible compared to electron-ion IB, with the exception of the very early stages of the laser evaporation process. For electron-ion IB, the critical electron density, defined as the free electron density at which the plasma oscillation frequency equals the laser frequency, plays an important role in establishing the energy partition between laser-target and laser-plasma coupling. Near the critical density, laser-driven plasma becomes opaque (i.e., more reflective) to the incident laser light. Since the critical electron density is proportional to the inverse of the wavelength squared, the critical electron density is approximately nine times lower for 1064 nm than 355 nm. As a result, the threshold for achieving critical density is significantly lower for plasma interacting with 1064 nm laser light, leading to higher reflectivity and lower penetration into the expanding plume. While electron-ion collisions are the dominant mechanism for laser absorption in the picosecond pulse regime, as will be shown later in this text through radiation hydrodynamic simulation, it is noted that the higher photon energy of UV wavelengths also enhances photoionization during the early stages of plume evolution. In addition, since IB provides the leading mechanism for plasma ignition in metals, UV wavelengths generally have higher ignition thresholds.²⁸ Both of these characteristics enhance laser-target coupling for UV wavelengths during the early stages of the ablation process. Therefore, in the context of the aforementioned absorption theory, the reflectivity measurements presented in Fig. 3 are consistent with established trends for UV and IR absorption.

As expected, based on the findings from the reflectivity study, the ablation rate in all three materials is also strongly dependent on wavelength. The discrepancy in ablation rate between wavelengths of 355 nm and 1064 nm can, therefore, largely be explained by the wavelength dependence of absorptivity. Figure 4(b), however, which presents ablation rate as a function of absorbed fluence, highlights a regime for Si between 10 and 15 J/cm², where reflectivity alone cannot explain the difference in the material removal rate. For this regime, the spatial distribution of energy absorption within the expanding ablative plume must also influence the extent of material removal. This topic is addressed explicitly later in the text but is introduced here in terms of the relevant physical processes. As previously discussed, laser energy is primarily absorbed within the plasma near critical density and is transported to the ablation surface via thermal diffusion. The inward (toward the ablation surface) diffusive transport is counteracted by the supersonic outflow of the ablation plume. These counteracting processes affect the recoil pressure imparted on the target and the resulting laser-driven shock. As will be discussed, the magnitude and timing of this laser-driven shock influences both heat penetration and hydrodynamic processes that drive material ejection. While Fig. 4(b) provides the evidence of this effect in Si, over a range where there is similar absorbed energy but dissimilar material removal, it is believed that the ablation rate curves at 355 nm and 1064 nm also diverge for Al and SS at higher absorbed fluence.

The corresponding removal efficiency for all three materials, in terms of incident energy per volume of removed material, is presented in Fig. 5. For fluences above ∼10 J/cm², 355 nm pulsed laser ablation is close to an order of magnitude more efficient for all three materials than the measured removal at 1064 nm. This enhancement in ablation efficiency at 355 nm, however, drops to roughly a factor of two below 10 J/cm², indicative of a fluence threshold in the dominant removal process. The sharp increase in ablation rate that was noted earlier between 10 and 20 J/cm² is also observable in the efficiency data, where the energy per volume of removal drops by a factor of two to three. In other words, picosecond pulse laser ablation is two to three times more efficient above ∼10 J/cm² than at lower fluence. An enhancement in removal efficiency at 355 nm is also observed for SS, including a near 50% increase in removal efficiency between 10 and 20 J/cm². Both of these observations are consistent with the previously discussed trends for the other two materials. It is noteworthy, however, that the ablation rate for SS is approximately three times lower than that observed for Al and Si. As will be discussed later in this text, the lower removal rate can be attributed to shallower melt production during laser driven shock heating.

Previous studies on modeling ultrashort pulse laser ablation have provided insight into the physical processes that drive material ablation in the femtosecond pulse regime. Driven by the aforementioned developments in short pulse laser technology, these studies have primarily focused on NIR laser wavelengths and fluences less than 10 J/cm² that produce sub-micrometer ablative removal per pulse. Here, we present a brief review of selected studies relevant to the current work.

Early studies on short pulse laser ablation treated removal as an evaporative process.¹² It was later recognized that at higher fluence, the material can be driven to near critical point conditions where it becomes thermodynamically unstable, resulting in deeper removal through explosive melt ejection (explosive boiling).^14,16 Vidal et al.,¹⁶ for example, investigated femtosecond pulse laser ablation of Al at 1 μm wavelength, using a one-dimensional hydrodynamic code with a quotidian equation of state (QEOS), considering a fluence range of 1-100 J/cm². The simulations pointed to spinodal decomposition, or the development of inhomogeneous structures, as a result of thermodynamic instabilities near the critical point, as the driving mechanism for separation of the ablated matter. It is noted that laser energy deposition for the simulations was obtained by solving the Helmholtz equation using the common Drude approximation for the complex electrical conductivity. This model assumes that laser energy is absorbed in the solid state and does not address LPI within the ablative plume.

Povarnitsyn et al.¹⁷ extended this work by explicitly studying the effects of laser driven shock and release wave propagation on ablative removal in Al, Au, Cu, and Ni for 100 fs, 800 nm pulses ranging in fluence from 0.1 to 10 J/cm². One of the most significant findings of the study was the role of tension on material decomposition, which is imposed during release wave propagation from the ablation surface. Following the initial laser-driven shock, the material undergoes release, and this rarefaction causes the liquid phase to become metastable, where its lifetime can be estimated using the theory of homogeneous nucleation. An EOS model capable of describing the metastable liquid state, as well as the kinetics of gas bubble formation, was developed to capture melt cavitation during the simulation. Leveraging the near isochoric nature of femtosecond pulsed laser absorption, hydrocode simulations were performed using an initialized approximate energy distribution based on Beer's law, in lieu of an explicit LPI model. The study concluded that the dominant mechanism for material removal during femtosecond pulse laser ablation originates from mechanical decomposition of the metastable liquid state.

More recently, Wu and Zhigilei¹⁸ investigated femtosecond pulse laser ablation of Al in the 0.6-1.1 J/cm² range using a hybrid continuum-level, two-temperature electron-phonon thermal diffusion model coupled to a classical molecular dynamics kinetics model. Similar to Ref. 17, isochoric heating was assumed and the laser energy deposition was approximated with an initial boundary condition. This study identified a spallation regime where laser-induced stress drives separation through the growth, coalescence, and eventual percolation of multiple voids within the melt layer. An apparent threshold for material separation was found to be dependent on temperature and imposed tension.

One of the challenges in modeling picosecond pulse laser ablation is that laser energy deposition can no longer be regarded as isochoric, as is commonly exploited for studies of the femtosecond pulse regime. As will be shown, the temporal and spatial evolution of energy deposition within the ablative plume can have a significant effect on the ensuing hydrodynamics and the resulting material removal. This necessitates the coupling of thermal diffusion, hydrodynamics, and LPI solvers. Modeling of the picosecond pulse laser ablation process can also present computational challenges compared to the ultrashort pulse regime in terms of the relevant physical length scales. The broader pulse width allows for hydrodynamic processes that evolve on the order of nanoseconds to microseconds, and at sub-surface distances on the order of micrometers, which must be resolved in concert with the sub-femtosecond and sub-nanometer processes governing laser energy absorption and evolution during the pulse.

In order to investigate the relevant physical processes associated with picosecond pulse laser ablation, a numerical model was developed in the multi-physics radiation hydrodynamic code HYDRA.²⁹ HYDRA is a 2D/3D arbitrary Lagrangian-Eulerian (ALE) finite element code with coupled LPI, hydrodynamics, thermal diffusion, and radiation transport solvers for modeling laser energy deposition, heat transport, shock and acoustic wave propagation, and the fluid and elastic-plastic response of materials. HYDRA includes a laser ray tracing deposition package based on an inverse Bremsstrahlung model. For the present study, a multi-phase EOS model derived from the experimentally anchored Livermore Equation of State (LEOS) database was coupled with a Thomas-Fermi electronic EOS. Elasticity and plasticity were modeled using an experimentally calibrated Steinburg-Guinan model,³⁰ and thermal diffusion was treated with a two-temperature electron-ion conduction model based on Ref. 31. HYDRA is a parallel code employing POSIX thread, OpenMP, and MPI strategies that have been developed to run on LLNL's high performance computing clusters.

An axisymmetric depiction of the model showing ablative plume expansion and laser-driven shock propagation during picosecond pulse laser ablation of an Al target is presented in Fig. 6(a). For the studies presented in this paper, a 1D version of the model along the central axis of the beam, as illustrated in Fig. 6(b), was utilized to study material response in isolation from 3D geometric effects. The 1D approximation was considered reasonable given the relative length scales involved (laser spot diameter of 130 μm and ablation depth less than 3 μm). The model included a special non-reflecting, outflow boundary at the leading edge to eliminate artificial reflection of the ablative plume. A spatial resolution of 10 nm was utilized, based on convergence studies, and the simulations were run with a sub-femtosecond time step.

Research on short pulse laser ablation has pointed to critical point phase separation as one of the driving mechanisms of material removal (e.g., Refs. 16 and 32). When a material is heated rapidly, a superheated liquid state can be reached where the temperature exceeds the boiling point for a given pressure. Near the critical temperature, thermodynamic properties fluctuate rapidly, and large density fluctuations within a small volume lead to bubble nucleation. As the temperature further converges on the critical point, bubble nucleation and growth rate increase exponentially until the confluence of bubbles results in unstable growth and, ultimately, the explosive ejection of liquid droplets (explosive boiling).

In order to investigate the applicability of the critical point phase separation model to picosecond pulse laser ablation, a HYDRA model was developed to capture LPI, two-temperature electron-ion thermal diffusion, and laser-driven shock effects. To ensure compatible input energy with the experiments, total surface reflection was based on the experimental measurements. Laser absorption within the ablative plume was calculated using an inverse Bremsstrahlung plasma absorption mode, as previously noted. Figure 7 presents the lattice temperature distribution in an Al specimen at selected time points during and after a 20 ps, 355 nm, 20 J/cm² pulse. The temperature profiles show that heat penetration is dominated by laser-driven shock heating, which occurs over approximately the first 100 ps, and that evaporative heat loss is considerable. The simulations also indicate that the shock heated melt is placed in tension during release, as noted by exaggerated adiabatic cooling due to the artificially high tensile strain capacity of the melt (i.e., the melt in this model is prevented from cavitating due to unphysical van der Waals loops in the EOS).

As a rudimentary, but insightful, exercise, removal based on the critical point phase separation model was estimated as the depth of material where the lattice temperature exceeded 90% of the critical temperature,¹⁶ as indicated in Fig. 7. This approach does not attempt to resolve the kinetics associated with bubble nucleation and growth, but rather provides an estimate for the depth of material where explosive boiling conditions are approached. In other words, is there sufficient heat penetration through thermal diffusion or through laser driven shock heating to support the experimentally observed removal based on a criterion of reaching 90% of the critical point temperature? This question is addressed in Fig. 8, which provides a comparison of the measured ablation rate in Al for 355 nm, 20 ps pulses ranging from 5 to 40 J/cm². In the lower fluence range (<10 J/cm²), the predicted ablation rate compares reasonably well with the measured value of approximately 100-200 nm. Heat penetration above 10 J/cm², however, is not sufficient to explain the observed transition to micrometer-scale ablation, and the estimated ablation rate based on the critical point phase separation model is three to four times lower than the measured rate. Hence, critical point phase separation may explain removal in Al at 355 nm below 10 J/cm², but it cannot explain the more efficient micrometer-scale removal at higher fluence.

The inadequacy of established thermal transport mechanisms (shock heating and thermal diffusion) to support a critical point phase separation model for the observed transition in Al to micrometer-scale removal above ∼10 J/cm², along with indications from the radiation hydrocode simulations of tensile strain demand in the melt during release, provide the motivation for investigating an alternative EOS model that accounts for melt cavitation. To this end, a mixed-phase LEOS model that simulates a stable mixed liquid-gas phase in tension was utilized. The mixed-phase LEOS model uses a Maxwell construction to remove the metastable states of overheated liquid and undercooled gas within the phase transition to approximate cavitation over the relevant time scale associated with release wave propagation through a melt.

Figures 9(a) and 9(b) show the evolution of fluid pressure during release wave propagation (shown above) and the density distribution at 3 ns (shown below) for the conventional multi-phase and mixed-phase LEOS models, respectively. For the conventional multi-phase LEOS model, the release wave propagates through the melt as a rarefaction wave, achieving a relatively large negative pressure of 4 kbar. The artificially high tensile capacity preserves the integrity of the melt and only material driven well into vapor phase (beyond the influence of the unphysical van der Waals phase transition) is removed. For the mixed-phase LEOS model, tension in the liquid is modeled as a mixed liquid-gas phase that expands toward the equilibrium vapor pressure under persistent tensile loading, confined dynamically by inertial effects. As shown in the density plot in Fig. 9(b), separation occurs near the melt-solid interface and evolves as an expanding internal vapor cavity that drives the adjacent melt layer outward with an instantaneous velocity at 3 ns of ∼100 m/s.

With the incorporation of the mixed-phase LEOS model, HYDRA simulations provide insight into the observed enhancement in removal efficiency at higher fluence. As illustrated in Figs. 10(a) and 10(b), laser-driven shock heating provides the dominant mechanism of heat penetration. Snapshots in time of pressure and lattice temperature during shock propagation show that the melt depth is largely set by the passage of the initial shock wave, as indicated by the instantaneous coincidence of the temperature and shock front. For instance, at 200 ps, the peak of the pressure pulse is located at ∼1600 nm beneath the original surface, which coincides with the depth of material heated to ∼1200 K. Shock heating, therefore, provides a plausible explanation for micrometer-scale heat penetration in the presence of considerable evaporative cooling.

When the ensuing release wave from the ablation surface places the melt in tension [Fig. 11(a)], separation occurs near the melt-solid interface, forming an internal cavity. Figures 11(b)–11(d) show the corresponding fluid pressure, lattice temperature, and density distribution, respectively, at 3 ns. Four distinct regions can be identified (from left to right): (1) a high temperature, low density plasma/vapor plume region, (2) an ejected melt layer, (3) an expanding vapor cavity, and (4) a low temperature, high density solid region. While heat penetration, considering both shock heating and thermal diffusion, is not sufficient for reaching a near critical point condition to support explosive boiling at experimentally observed ablation depths, shock heating does provide a mechanism for micrometer-scale melt penetration, and the ensuing release wave provides tension needed to drive melt ejection through cavitation.

For cavitation-driven removal, laser energy absorption and evolution within the ablative plume critically influence the magnitude of shock heating, the timing of the release wave, and the resulting ablation rate. Figure 12 provides an illustration of these effects for the case of Al considering three 20 J/cm² pulses: (1) 355 nm, 20 ps pulse with an absorptivity of 60%, (2) 1064 nm, 28 ps pulse with an absorptivity of 10%, and (3) 1064 nm 28 ps pulse with the same total absorbed energy as (1). Cases (1) and (2) utilize absorptivity values based on the experimental measurements, while case (3) is an artificial scenario used to evaluate wavelength dependent LPI effects, particularly the spatial distribution of laser energy within the ablative plume. Considering the experimentally based absorptivity cases (1) and (2), the 355 nm pulse, which absorbs 6 times the pulse energy, produces a peak shock pressure that is 3.5 times greater than (2). The larger shock loading drives a deeper melt pool and results in a stronger release wave, both of which promote deeper removal through cavitation.

The comparison between 355 nm and 1064 nm for the same total absorbed energy [cases (1) and (3)] shows that the 355 nm pulse penetrates deeper through the ablative plume and deposits most of the energy near the surface, in contrast to the 1064 nm pulse where the energy is absorbed near the outer region of the ablative plume. This variation in energy distribution results in a 40% reduction in the peak shock pressure for the 1064 nm pulse and also exacerbates wave reverberations in the plume that ultimately degrade the release wave. Of the three cases considered, only the 355 nm pulse is effective in driving melt ejection through cavitation, which highlights the importance of LPI effects.

Figures 13, 14, and 15(a)−15(c) present the evolution of fluid pressure, lattice temperature, and density in Al, Si, and SS, respectively, for a 355 nm, 20 ps–20 J/cm² pulse. The corresponding density distributions during the initiation of cavitation are shown in Figs. 13, 14, and 15(d) over a fluence range of 5-40 J/cm². The resulting ablation rate predictions are plotted with the experimental measurements in Fig. 16. It is noted that 1064 nm pulsed laser ablation of Si was not included in the modeling study due to additional complexities associated with laser absorption in this regime.

In general, the HYDRA ablation rate predictions are in good agreement with the experimental measurements and appear to capture relevant LPI and hydrodynamic effects. The enhanced ablation rate in all three materials above ∼10 J/cm² is attributed with melt ejection through cavitation. The extent of removal is largely controlled by the melt penetration during laser-driven shock heating. For Al, the relatively low melting point of 933 K and high absorptivity at 355 nm contribute to micrometer-scale melt depths. For Si, which has a moderate melting point of 1683 K, micrometer-scale melt penetration is attributed to enhanced shock heating due to the solid state volume collapse between the diamond and β-Sn phases. The observed sub-micrometer removal in SS over this fluence range is attributed to the relatively high melting point of 2063 K and low thermal diffusivity, both of which contribute to shallower melt depths and 2 to 4 times less removal.

SEM images of the ablation craters produced in Si targets by 355 nm, 20 ps pulses with fluences of 1, 14, and 40 J/cm² are shown in Fig. 17. The shallower sub-micrometer depth craters produced by the 1 and 14 J/cm² pulses are relatively smooth, indicative of evaporative removal. In contrast, the crater surface for the 40 J/cm² pulse has well defined contours (ripples) and nanoscale solidification cracks, both of which are characteristics of melt solidification. Both observations are consistent with the proposed transition in ablation mechanism for Si from evaporation and critical point phase separation below ∼10 J/cm² to melt cavitation at higher fluence.

Pump-probe images are shown in Fig. 18 of the ejecta from an Si target irradiated with a 355 nm, 20 ps–20 J/cm² pulse. The images capture micrometer-scale droplets traveling at a velocity of ∼100 m/s, which is consistent with the ejecta velocity predicted in the corresponding HYDRA simulation. The ejecta collection study provided a broader investigation of the ejecta by looking at all three materials over a range of fluence. Figure 19 presents SEM images for the ejecta collected at 3 mm from the target surface in vacuum. The images for 1 and 8 J/cm² show sub-micrometer scale droplets, which are likely attributable to shallow removal due to explosive boiling or possibly to droplets condensed from vapor during transit to the collector. The images at 30 J/cm² and 50 J/cm² show a clear transition for Al and Si to multi-micrometer scale droplets with features characteristic of low Reynolds number droplet impact.^33,34 The rounded profiles (i.e., minimal fingering and satellite droplets) are the result of viscous dissipation during droplet deformation, which prevents kinetic energy from overcoming surface tension to form a corona. In contrast, the particles collected for SS at these higher fluences are considerably smaller, which is consistent with the observed ablation rate (two to four times lower than Al and Si).

In order to study the physical processes governing picosecond pulse laser ablation, we utilized experimentation coupled with multi-physics radiation hydrodynamic simulation to investigate energy deposition, thermal transport, and material removal in Al 6061, 316L SS, and undoped crystalline Si (〈100〉) at laser wavelengths from the NIR to the UV up to 40 J/cm². Experimental measurements of material ablation rate show enhanced removal at the 355 nm wavelength that approaches an order of magnitude increase over the measured removal at 1064 nm. A transition in the ablation rate at 355 nm was identified around ∼10 J/cm² above which the removal efficiency increases by a factor of two to three. Multi-physics radiation hydrodynamic simulations, considering LPI effects and utilizing a novel mixed-phase equation of state model, show that the transition in ablation efficiency is due to the onset of melt ejection through cavitation, where laser-driven shock heating sets the depth of melt penetration and the ensuing release wave from the ablation surface drives cavitation through the imposition of tensile strain within the melt. High-speed pump-probe imaging of the ejecta and ejecta collection studies, as well as scanning electron microscopy of the ablation craters, support the proposed cavitation mechanism in the higher fluence range. The ablation process is critically influenced by LPI effects and the thermophysical properties of the material. In the lower fluence range (<10 J/cm²), heat penetration is consistent with a near critical point phase separation (explosive boiling) removal mechanism.

The ablation process is critically influenced by LPI effects and the thermophysical properties of the ablator material. In all three materials, laser ablation was many times more efficient at the UV wavelength compared to the NIR. This can be explained by wavelength dependent reflectivity and absorption within the evolving ablative plume. UV light, which has a higher critical electron density than NIR wavelengths, penetrates deeper into the plume and absorbs closer to the ablator surface. As a result, UV laser pulses drive higher pressure shocks that promote deeper melt penetration and material removal through cavitation.

Given the energy efficiency of the observed hydrodynamic melt ejection process for picosecond pulse laser ablation, protocols with temporally-shaped pulses that can independently control the creation of the melt and the source of tension are expected to enhance removal rate and efficiency for laser drilling applications.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 with support from Laboratory Directed Research and Development under Grant No. 16-ERD-016.