Perspectives on ultraintense laser-driven terahertz radiation from plasmas

Terahertz (THz) radiation, broadly defined as the electromagnetic waves in the frequency range from 0.1 to ∼30 THz, has attracted considerable attention and interest over the past decades,^1–4 primarily due to its promising applications in a wide range of fields, such as materials, chemistry, biomedicine, communications, and security. High-power THz radiation sources are essential for these applications. Compared to the neighboring microwaves and infrared waves, it is relatively difficult to produce strong THz radiation that lies well in the transitional spectral region between electronics and photonics, resulting in the ever so-called “THz gap.” Until the end of the last century, along with the rapid progress of ultrafast laser technology, the THz sources have witnessed tremendous development particularly in recent two decades. The existing high-power THz sources can be assigned broadly into two categories, high-average-power and high-peak-power ones. Iconic examples of the former include the optically pumped THz gas lasers,⁵ free-electron lasers (FEL),⁶ electrically pumped photonic-crystal lasers,⁷ and quantum cascade lasers.^8–10 While for the latter, currently, the most prominent approaches are based on conventional electron accelerators and ultrafast laser-pumped crystals.¹¹ At the best, large-scale accelerators, such as SLAC,¹² can deliver THz pulses with sub-mJ pulse energies, GW-level peak powers, GV/m-scale focused field strengths, and broadband spectral contents. The THz free-electron lasers can produce tunable narrowband radiation with a high spectral energy density at a MW-scale peak power.⁶ Due to the inherent huge Coulomb repulsion among electrons, the compromise between the electron bunch charge and bunch duration makes it difficult to enhance the THz energy.¹³ In many university-scale laboratories, the most utilized strong-field THz sources are produced via nonlinear frequency conversion in ultrafast laser-pumped crystals. Since the crystals will be damaged at high pump laser intensities of ∼10¹⁰ W/cm², the most direct way to enhance the THz yield is to increase the crystal size while increasing the pump laser energy.^14,15 0.9 mJ THz pulses in the spectral range of <5 THz are generated from a 400 mm² partitioned organic crystal pumped by an infrared laser pulse at the 1.2–1.6 μm wavelength.^16,17 Generation of pulses tunable over the high-frequency THz and mid-infrared range from 15 to 75 THz is demonstrated by difference-frequency generation in the GaSe crystal.¹⁸ Very recently, over 10 mJ, low-frequency (<1 THz) THz pulses have been produced by a joule-scale femtosecond laser pulse pumping a cryogenically cooled lithium niobate (LiNbO₃) crystal with a size of ∼70 × 80 mm² in the tilted pulse front geometry.¹⁹ Multicycle THz pulses with an energy of 1.3 mJ have been demonstrated from centimeter-sized periodically poled crystals pumped with a joule-class laser system.²⁰ Hurdles in the growth of large-size high-quality crystals and the inherent multiphoton absorption effect of crystals limit the potential of higher energy output in crystal-based THz sources.

Under the laser irradiance at intensities of >10¹² W/cm², matter whether in the solid, liquid, or gaseous phases will be ionized into plasmas. Laser-produced plasmas offer a damage-free medium for THz generation. The most extensively studied scheme is the THz generation from two-color laser-induced plasma filaments in air, where the THz radiation is attributed to the transient photocurrent produced during the laser ionization of air.^21–24 Usually it delivers a THz pulse energy in the few-μJ range at a laser-to-THz conversion efficiency of the order of 10⁻⁴.²⁵ The fundamental pump wavelength significantly influences the THz generation efficiency, which tends to scale strongly with the wavelength.^26,27 Using a pump laser system at 3.9 μm and its second harmonic about 0.2 mJ THz pulse energy has been demonstrated.²⁸ The THz yield tends to get saturated at the pump laser intensities around 10¹⁵ W/cm².^21–28 At present, the available focused laser peak intensity can be far above 10¹⁸ W/cm², i.e., at a so-called relativistic level,²⁹ where the electron quiver velocity in the laser field approaches the speed of light. Such ultraintense laser systems have grown rapidly in prevalence and availability in recent years and even become available in many university-scale laboratories.³⁰ Ultraintense laser−matter interactions have been applied routinely as a compact particle accelerator and an ultrafast radiation source, where short-duration high-energy electrons, ions, and neutrons can be accelerated,^31–34 and the accelerated electrons can radiate ultrafast electromagnetic waves at short wavelengths, such as high harmonics and x rays via various processes.^35–37 Recently, the generation of long-wavelength THz radiation from ultraintense laser–plasma interactions has gained a growing interest,³⁸ due to the high scalability of THz pulse energies without saturation as the driving laser intensity increases.

An overview of the achieved highest pulse energies and peak powers and the approximate spectral energy density and coverage for various types of state-of-the-art pulsed THz sources is given in Fig. 1. These accelerator-, crystal-, and plasma-based technologies have their respective characteristics. There exists a number of reviews on accelerator-based or laser-driven THz sources.^39–44 The emphasis in this article is on the ultraintense laser-driven terahertz radiation from plasmas, as an extension of laser plasma-based ultrafast radiation sources in the long-wavelength end. This Perspective is written to deliver a brief introduction to the current status and the likely challenges of this field and provide some suggestions for future development orientations. This paper is organized as follows. In Sec. II, the generation of THz radiation from different media pumped by ultraintense laser pulses is outlined, with emphasis on the underlying physical mechanisms, radiation characteristics, and further optimization strategies. The metrology techniques applicable to the low-repetition-rate ultraintense laser-driven THz pulses are presented in Sec. III. In Sec. IV, potential applications utilizing the resulting extreme THz pulses are briefly discussed, and in particular, some selected illustrative examples of newly emerging applications in materials, chemistry, and biology. Finally, the concluding remarks are presented in Sec. V.

THz radiation can be generated during ultraintense laser interactions with solids, gases, or liquids. According to the initial states of laser-irradiated targets, here, the THz generation is classified broadly into three scenarios, as shown in Fig. 2. The laser–matter interaction processes and accordingly the THz generation physics as well as the resulting THz characteristics could be much different in each scenario. In the following, we provide our perspectives for future ultraintense laser-driven THz sources based on the current status and expected advances.

The first demonstration of the THz generation from relativistic laser-produced plasmas was reported by Hamster et al. in the 1990s.^45,46 The THz radiation from solid targets was found to be much stronger than that from gaseous targets by around two orders of magnitude. This can be understood qualitatively by the fact that solids have a much higher electron density and thereby a stronger laser absorption than gases. In light of this, experiments on ultraintense laser-driven THz radiation mostly employ solid targets.^47–63 These experimental studies aim to understand the underlying THz generation mechanisms and also to enhance the THz yield. It is found that the THz radiation from laser-irradiated solid targets stems primarily from the transient electron currents in various forms, depending on the laser and target parameters used.

During ultraintense laser interactions with solids, the solid target within the laser skin depth at the front surface is ionized into dense plasmas by the laser pulse leading edge or the amplified spontaneous emission (ASE) prior to the arrival of the main pulse. Through various processes, such as resonant absorption, vacuum heating, and transverse or longitudinal laser ponderomotive force,⁶⁴ a large number of electrons at the target front surface will be accelerated to high energies over a tiny spatiotemporal scale, launching a huge-current of energetic electrons (usually termed as “fast electrons”). For bulk targets, a part of the resulting fast electrons transport along the front target surface, forming a lateral transient current [Fig. 2(a)]. Under appropriate laser-plasma parameters, a directed fast-electron current can be formed by the drifting of a fast electron beam along the target surface due to the confinement of the spontaneous quasistatic magnetic and electrostatic fields at the surface.⁶⁵ The time-varying lateral transient electron currents emit electromagnetic radiation outward from the front side of solid targets,^51,56 and accordingly, the radiation duration or spectrum is mainly dependent on the temporal evolution (pulse duration and relaxation time) of the transient current.⁴⁷ Usually, the fast electron current driven by femtosecond laser pulses has a characteristic timescale ranging from tens of femtoseconds to picoseconds, and hence the coherent radiation could fall in the THz domain. The ejection of fast electrons from the target front surface to vacuum will also contribute to the THz radiation via the process of coherent transition radiation (CTR).⁶⁶ 

For thin foil targets, intense THz radiation can also be emitted from the rear side of solid targets. Compared to the target-front radiation, the target-rear THz radiation circumvents the direct interference from ultraintense laser components and hence making it more convenient to collect the divergent THz radiation in a large solid angle. Since the ultraintense laser pulse cannot penetrate the dense solid targets, the source of target-rear THz radiation is the transient currents induced by the emission of laser-accelerated forward fast electrons that transport through the target^63,67 [see Fig. 2(a)]. When the ultrashort fast electron bunches of femtosecond-to-picosecond durations traverse the interface between the target surface and vacuum, strong coherent radiation in the THz band is produced via CTR due to the sharp dielectric discontinuity.⁵⁸ The THz pulse width is comparable to the electron bunch duration. As a result of fast electrons escaping the target, a strong electrostatic sheath field will also be established at the target surface, leading to the so-called target normal sheath acceleration of ions.³³ During ion acceleration, the spatiotemporal charge distribution in the sheath is similar to a transient dipole. Because the characteristic timescale of the sheath evolution is usually of the order of picoseconds, the resulting dipole-like radiation is expected to be in the THz frequency band.⁵² For those electrons confined in the sheath, the deceleration and acceleration of electrons by the sheath field will produce bremsstrahlung emission, and the electrons leaving and reentering the target will also induce transition radiation. Such bremsstrahlung-like transition radiation under appropriate laser and sheath parameters could also fall in the THz regime.⁶³ The resulting THz pulse duration is dependent on the sheath relaxation dynamics.

Within the framework of the above-mentioned physical models, the THz radiation from bulk or foil solid targets has been studied systematically. The resulting coherent THz radiation usually possesses an ultrabroadband spectrum even up to tens of THz,^56,58 which can be manipulated effectively by tuning the laser and target parameters.⁶² Since the THz radiation is emitted from a point-like source with a rather large divergence angle (even >90°), collection optics with large acceptance angles are required to collimate the THz radiation and deliver more available THz energy for practical applications. In terms of THz yields, intense THz pulses with energies up to a few millijoules have been produced under the TW-scale femtosecond laser pump,^59,68 corresponding to a laser-to-THz energy conversion efficiency of ∼1%, which is already comparable to the highest reported efficiencies of crystal-based THz sources.^16,19 With the picosecond laser pump, it has been demonstrated that the THz pulse energy is as high as ∼200 mJ and the THz peak power reaches the unprecedented TW level,⁶² which has already exceeded accelerator- and crystal-based THz sources^12,16,19 by more than one order of magnitude [see Fig. 1(a)]. Despite the broadband spectra, the average spectral energy density can reach over 100 mJ/THz, even far beyond the free electron lasers that are recognized to be the state-of-the-art narrowband THz sources [see Fig. 1(b)]. Moreover, at the laser intensity less than 10²⁰ W/cm², the THz yield has not shown any tendency of saturation with the pump laser energy, and hence the THz energy is expected to be further scalable with the increase in pump laser energy.⁶⁹ The THz yield and generation efficiency could be further enhanced by taking some optimization measures, e.g., employing appropriate laser wavelengths and polarization,⁷⁰ tuning the target-front or target-rear plasma scale length with a controlled laser prepulse,^54,57,71 nanostructuring the target front surface,^72,73 which have been demonstrated experimentally.

In addition to the common planar solid targets, some specially structured solid targets could help to optimize the directionality, spectral tunability and generation efficiency of the emitted THz radiation, such as microplasma foils⁷⁴ or waveguides,⁷⁵ grating targets,⁷⁶ annular targets,⁷⁷ cone targets,⁷⁸ and T-shaped targets,⁷⁹ to name just a few. Some simulations or experiments on these structured targets have been performed. In particular, the THz generation from laser-irradiated wires has aroused increasing attention in recent years, as the metal wire can not only guide and collimate fast electrons⁸⁰ but also serve as a low-loss THz waveguide.⁸¹ ∼3 mJ low-frequency (<1 THz) THz radiation within a half divergence angle of ∼30° has been demonstrated from metallic wires irradiated by a 100 TW-scale laser.⁸² Nevertheless, the underlying THz generation processes have not been well understood, and different physical models have been proposed, such as the current-carrying line antenna,^83,84 the electron-excited surface wave,⁸⁵ and the electromagnetic field-guided electron helical undulator.⁸⁶ 

Unlike the dense solid targets, the laser pulse can propagate through the underdense gas targets over a long distance [see Fig. 2(b)]. The most distinct scenario in gas targets is the excitation of large-amplitude electron plasma waves and the well-known laser wakefield electron acceleration. In the first report of the THz emission from ultraintense laser-irradiated gas targets,⁴⁵ the THz radiation was attributed to be originated from the longitudinal transient current induced by the laser ponderomotive force in the laser propagation direction. In 2003, Leemans et al. observed the radiation in the spectral range of 0.3–3 THz with energies around 3–5 nJ over a collection angle of 30 mrad and explained it as the coherent transition radiation induced by laser wakefield-accelerated electrons crossing the plasma-vacuum boundary.⁸⁷ Very recently, ∼4 mJ broadband THz radiation has been measured from 100 TW-scale laser–gas interaction experiments, and its origin is inferred to be from plasma electrons accelerated by the laser ponderomotive force and the plasma wakefields.⁸⁸ 

While only a handful of experiments are reported on the THz generation from ultraintense laser–gas interactions, there have already been a large number of physical models proposed and numerical simulations performed. One of the most striking models is linked to the mode conversion from laser-excited plasma waves. When the intense laser pulse is incident obliquely to an inhomogeneous underdense plasma of a positive density gradient, laser-excited wakefields could emit broadband, positively chirped THz radiation around the specular reflection direction through electrostatic–electromagnetic mode conversion.⁸⁹ The divergence angle of the resulting THz radiation is comparable to that of the laser driver, and the THz duration depends mainly on the plasma density profile in the propagation path of the driving laser pulse. If the plasma has a positive density gradient along the laser propagation direction, the plasma wakefields could efficiently radiate at harmonics of the plasma frequency for appropriate plasma and driver parameters, resulting in narrow-band THz radiation.⁹⁰ In the magnetized plasmas, the laser wakefields become partially electromagnetic with a nonzero group velocity and could emit electromagnetic pulses with a frequency close to the plasma frequency.^91,92 Particle-in-cell simulations show that the THz to far-infrared radiation with a multi-millijoule energy can be generated during two-color or mid-infrared relativistic laser–gas interactions,^93,94 where the transient photocurrents by ionization and the transition radiation of laser-accelerated electrons are recognized to be the major sources of THz radiation. It has been proposed recently that intense single-cycle high-frequency THz pulses could also be generated via photon deceleration or frequency downshifting of high-power drive lasers in low-density plasmas.^95,96 Depending on the laser–gas interaction parameters, the aforesaid models could contribute independently or jointly to the resultant THz radiation. Elaborated experimental configurations and comprehensive characterization of the THz radiation, plasma profiles as well as the accelerated electrons are required to clarify the THz generation physics.⁵⁵ 

Compared to the THz generation from solids, the THz radiation from gas targets could be inferior in the THz yield and generation efficiency due to the low electron density and laser absorption but be superior in terms of spectral tunability and operational repetition rates. The THz spectrum could be tunable by tailoring the gas density profiles. For gas targets, the problems of target debris and electromagnetic pulse disruptions are significantly mitigated, enabling high-repetition-rate tunable THz radiation sources. On the other hand, the resulting THz radiation is expected to have a small divergence angle and usually overlap with the residual ultraintense laser pulses and high-energy electron beams in space, making it unfavorable to collect and diagnose the THz radiation.

Gas clusters are a promising alternative target medium for the THz generation, because cluster targets combine advantages of both gas and solid targets: the laser absorption is much higher than for gases, resulting in stronger THz radiation, and there is much less debris generation than for solid targets, enabling high operational repetition rates. Usually by injecting gases with high pressure into a vacuum chamber, the cooling associated with adiabatic expansion of gases can cause the atoms nucleate to form nanosize clusters. The local electron density in individual clusters may reach solid-density while the average density of cluster targets is similar to that of gas targets. Experiments using a mildly relativistic laser pulse show that the THz radiation from clusters is much stronger than that from gas targets by two orders of magnitude.⁹⁷ Measurements reveal a conical angular distribution in both forward and backward directions with radial polarization and also a significant emission in the forward direction with elliptical polarization.⁹⁸ The observed THz properties suggest the time-varying electric quadrupoles produced by the laser ponderomotive force as the major mechanism of THz generation.⁹⁹ The use of multiple incident laser pulses has been proposed to improve the radiation properties. It is demonstrated that the interaction between argon clusters and intense double laser pulses with appropriate intervals in time and space produces THz radiation with high forward directivity, power enhancement, and linear polarization with a variable direction.¹⁰⁰ 

Beyond solids and gases, laser-pumped liquids have also been proposed in recent years as an exotic THz generation medium,¹⁰¹ as most liquids like water strongly absorb THz radiation. Various liquid materials (e.g., water,¹⁰² organic solvent,¹⁰³ cryogenic liquid nitrogen,¹⁰⁴ and liquid metal¹⁰⁵) and configurations (e.g., lines,¹⁰⁶ films,^102,107 droplets,¹⁰⁸ and cells¹⁰³) have been tried, as shown in Fig. 2(c). THz pulse energies up to ∼80 μJ are obtained when pumping with ∼30 mJ 0.8 μm laser pulses, and the THz spectral bandwidth reaches up to ∼100 THz.¹⁰³ The generation mechanism is attributed mainly to the strong photocurrents created during laser propagation. It is found that there exists an optimal laser incidence angle, pulse duration, and an appropriate preplasma profile to boost the THz generation.¹⁰¹ It should be pointed out that currently almost all experiments are constrained at low laser intensities and thereby in ambient environment. One reason is the difficulty in the implementation of liquid targets in high vacuum conditions to be compatible with ultrashort ultraintense laser systems. Benefited from the advances in liquid targetry,¹⁰⁹ several experiments on the relativistic laser–liquid interactions have been reported recently for the proton acceleration^110,111 and high-harmonic generation.¹¹² 

Compared with gas targets, liquid targets with a comparable material density to solids are expected to be superior in terms of THz yield and generation efficiencies. Compared with solid targets, the fluidity of liquids allows self-refreshing and debris-free (or little) features, making it suitable for high-repetition-rate operation. Hence, ultraintense laser-irradiated liquids are recognized as promising candidates for bright THz sources that meet the high-repetition-rate and high-average-power needs of some applications. With ongoing development efforts in liquid-based, vacuum-compatible targets, and optics, relativistic laser-driven THz liquid photonics can be envisaged in the near future.

Emphasis of previous studies was laid on the understanding of THz generation mechanisms and the upscaling of THz yield. Currently, the THz generation from ultraintense laser interactions with planar metallic solid targets has been studied systematically and the underlying physical processes have been well understood.³⁸ Extreme THz pulses with unprecedented energies of ∼200 mJ have been demonstrated experimentally.⁶² In order to further improve the THz performance and fit more intriguing applications, continued efforts in optimizing the THz generation need to be made in the following aspects.

1. THz generation scenarios. Compared to those from solid targets, studies on the THz generation from ultraintense laser-pumped gas or liquid targets are much meager and inadequate. Although there have been a number of proposals from theoretical calculations or numerical simulations in which the resulting THz radiation shows exciting properties, relevant experimental verification is still very lacking. It is quite necessary to purposefully create specific laser-target parameters to clarify the THz generation physics that is still unclear, for example, the role of laser-excited plasma waves played in the THz generation from gases. Extending THz generation over wider laser and target parameters will further expand the domain and push the limits of achievable THz parameters.

2. Tunability of THz properties. Besides THz pulse energies and generation efficiencies, there are many other important parameters for a THz source like waveform, spectrum, polarization, wavefront, and divergence. More studies are required to find effective means to manipulate these THz properties. Take, for example, the THz spectral tunability. Currently, the experimentally demonstrated THz pulses from laser plasmas are usually of a single-cycle waveform and an ultrabroadband spectrum. How to produce multi-cycle narrowband strong-field THz radiation from laser plasmas remains an open question. Very recently, several schemes have been proposed via simulations but still await experimental verification, for example, by irradiating a nano-dimensional overdense plasma sheet with two counter-propagating detuned laser pulses,¹¹³ from an intense femtosecond laser-irradiated extended undulatory wire,¹¹⁴ or via the laser-excited wakefield radiation.⁹⁰ 

3. Repetition rates and stability. This is a crucial factor that determines its availability for specific practical applications. The repetition rates and stability of THz radiation from ultraintense laser–plasma interactions are limited ultimately by those of the laser drivers and the target delivery systems. At present, the available commercial petawatt, 100 TW and TW-scale laser systems³⁰ are capable of operating at a highest repetition rate of 1 Hz, 10 Hz, and 1 kHz, respectively. To enable the stable output of THz radiation at laser repetition rates, various target delivery systems have been developed,¹¹⁵ including foil-type or wire-type tape targets, rotating disks, nozzle-based gas or cluster targets, and free-flowing film-, line-, or droplet-type liquid targets. Implementation of these target systems has been examined before in some ultraintense laser–plasma experiments. Recently, high-repetition-rate ultraintense laser-driven THz sources have been demonstrated by using foil-type,¹¹⁶ wire-type¹¹⁷ tape targets, and droplet-type liquid targets,¹⁰⁸ respectively. It is suggested that intense THz sources with a watt-scale average power and a-few-percent energy fluctuation are achievable.

Comprehensive and precise characterization of THz properties, in particular, the THz waveform and spectrum is the important basis for the full understanding of THz generation physics and the rational application of THz radiation. Due to the low repetition rates (mostly in the single-shot mode) and the considerable shot-to-shot fluctuations of ultraintense laser experiments, conventional scanning-based techniques for THz detection, such as electro-optic (EO) sampling¹¹⁸ and Fourier transform spectrometer, are no longer applicable, and hence, it is required to develop single-shot approaches for measuring the properties of ultraintense laser-driven THz radiation. In the following, we discuss two categories of dedicated methodologies for characterizing the THz waveform and spectrum, respectively. Their basic principles and characteristics are briefly outlined, and the challenges and future development orientations are evaluated.

The most commonly used fundamental principle behind the THz waveform detection methods is the EO or Pockels effect, which describes the change of refractive indices of a material in the presence of an external electric field. When the THz and ultrashort probe pulses are superimposed in the EO crystal, the THz field induces transient birefringence in the crystal, leading to a change in polarization of the ultrashort probe pulse.¹¹⁹ To sample the THz field waveform in a single shot with sufficient resolution, the kernel is to establish a strategy of mapping a temporal series onto an observable physical quantity of the laser probe. Based on this idea, three kinds of single-shot methodologies have been proposed and demonstrated [see Fig. 3(a)]: (1) frequency-to-time encoding with a chirped probe pulse,¹²⁰ (2) space-to-time or angle-to-time encoding with echelon mirrors,¹²¹ and (3) space-to-time encoding with noncollinear geometry¹²² or collinear tilted optical probe intensity front.¹²³ These coherent detection methods have been applied successfully in the characterization of ultraintense laser-driven THz radiation.^51,52,59 More details on the technical implementation and performance characteristics can be found, e.g., in Refs. 120–125.

Given the fact that the ultraintense laser-driven THz radiation from plasmas usually has complicated spatiotemporal and polarization distributions,^98,126,127 it is necessary to develop single-shot THz time-domain metrology with spatial or polarization resolution. By incorporating imaging with cylindrical lens into the single-shot methodologies, it is achievable two-dimensional temporal-spatial resolution.¹²⁸ The introduction of polarization sensitive devices¹²⁹ or appropriate geometries into the EO sampling¹³⁰ has opened the possibility to real-time measure the polarization of THz beams. Emerging techniques, such as deep learning and compressed sensing, could help achieve the single-shot multi-dimensional THz characterization. For example, three-dimensional terahertz photography has been demonstrated very recently by multiplexing the optical probe in both the time and spatial-frequency domains.¹³¹ It should be pointed out that, for the above-mentioned single-shot methods, an additional reference shot without THz radiation is required. The shot-to-shot fluctuations in energy, spatial, and spectral distributions of the laser probe at ultraintense laser systems will degrade the signal-to-noise ratio of practical measurements.

Due to the inherent phonon absorption in crystals, the EO-based measurements have a limited spectral response range, for example, the detectable upper limit for the most utilized zinc telluride (ZnTe) and gallium phosphide (GaP) crystals is at ∼5 and ∼10 THz,¹¹⁹ respectively. Note that the THz radiation from laser plasmas is usually of ultrabroadband spectra covering even up to tens of THz.^58,88,103 In addition, the EO measurements require an ultrashort laser probe, which is not always available especially at picosecond high-power laser systems. Hence, probe-free single-shot ultrabroadband detection techniques are much desiderated to characterize the ultraintense laser-driven THz radiation from plasmas.

Two single-shot ultrabroadband THz spectrometers have been developed recently, a filter-based multichannel calorimeter¹³² and a noncollinear autocorrelator.¹³³ The former can be used to evaluate the THz spectral energy density distribution at discretized frequency points, and the latter is capable of measuring continuous THz spectra with sufficient spectral resolution and detection window. Figure 3(b) sketches the detection principle of the newly developed noncollinear autocorrelator. The noncollinear autocorrelator is a modified beam-division Mach–Zehnder interferometer (enabling ultrabroadband detection), in which the two replica beams are recombined noncollinearly onto a THz camera. The non-collinear geometry provides the space-to-time mapping between the spatial position in the THz spot and the relative time delay of the two beams, enabling single-shot measurements of the THz autocorrelation signal. The THz spectrum is retrieved by taking Fourier transform of the measured THz autocorrelation interferogram. In combination with efficient THz nonlinear crystals, it could be possible to realize the THz frequency-resolved optical gating system (THz-FROG), capable of simultaneously gaining information on the pulse duration as well as the spectral amplitude and phase. Alternatively, one may use the ultra-thin EO crystals to mitigate the phonon absorption effect,¹³⁴ enabling ultrabroadband THz spectral detection if a-few-femtoseconds ultrashort laser probes are available and the rather weak EO signal is detectable.

Now that intense THz radiation can be produced from ultraintense laser–plasma interactions, next one of the most important topics for this field is to fully exploit such THz radiation into practical applications. In our opinion, ultraintense laser-driven THz radiation from plasmas has enabled prospective applications that can roughly be classified in terms of laser-plasmas diagnosis, materials control, and THz strong-field physics, as shown in Fig. 4. These concepts are briefly outlined by highlighting only a few illustrative examples in the following so as to stimulate more ideas.

Since the THz radiation is a kind of self-radiation induced during ultraintense laser–plasma interactions, laser plasmas imprint their information on the THz radiation. With the single-shot THz metrology, the most immediate application of ultraintense laser-driven THz radiation is to serve as a novel, real-time, noninvasive, in situ diagnosis of laser plasmas [see Fig. 4(a)]. Specifically, in the scenarios where the transient electron currents dominate the THz generation, the THz intensity and waveform or spectrum directly reflect the amplitude and temporal evolution of transient currents, respectively. In the cases where the plasma waves dominate the THz generation, the amplitude of laser-excited plasma waves or the plasma density profile in the laser propagation path could be inferred intuitively from the THz radiation characteristics. The THz transition radiation has commonly been used for temporal characterization of the electron bunches in laser wakefield accelerators.^135,136 For the THz radiation from the rear side of laser-irradiated foils, the THz transition radiation can been utilized to diagnose the fast-electron bunch charge,^62,63 temporal structure, and divergence,⁵⁸ while the THz sheath radiation components can be applied to infer the sheath charge, field strength, and relaxation dynamics.⁶³ The ultrabroadband THz pulse can also be exploited as an ultrafast probe to measure the plasma density in the range of 10¹⁶–10²⁰ cm⁻³ (Refs. 137 and 138) and near-DC conductivity¹³⁹ and their temporal evolution. Along with deeper insight into THz generation physics and advances in multidimensional THz metrology, it is anticipated that the THz radiation will play an increasingly significant and unique role in the laser-plasma diagnostics. The THz diagnosis of plasmas could provide micrometer-scale, femtosecond-level spatiotemporal resolutions at the best.

One of the most unique features for THz radiation is that the THz band or photon energy overlaps well with the inherent frequencies or characteristic energies of numerous fundamental motions of ions, electrons, and electron spins in all phases of matter. For example, THz radiation can couple resonantly to many collective motion modes or quasi-particles in matter, such as the vibration of lattice (phonons) and the precession of spin waves (magnons) in solids, the rotation of macromolecules in gases, and the stretching of hydrogen bonding in liquids. The relatively weak THz radiation is often used as a probe for characterization of elementary processes in complex materials. A considerably different scenario occurs at strong THz fields. Intense THz radiation allows one to actively drive the selected degrees of freedom in matter, such as phonons or magnons into regimes far beyond the small-perturbation limit, thereby triggering nonlinear THz responses of materials and potentially resulting in novel states of matter. Hence, it has been well recognized that strong-field THz radiation can serve as a powerful tool for selective, coherent, nonthermal, and ultrafast control over matter, and consequently, have tremendous application prospects in many fields ranging from materials science to accelerators. Currently, most of relevant studies employ the crystal-based THz sources, and remarkable application examples include nonlinear THz spectroscopy,¹⁴⁰ the coherent control of low-frequency spin waves,¹⁴¹ and the manipulation of free charged particles for compact accelerator.¹⁴² In contrast, the ultraintense laser-driven THz radiation from plasmas possesses higher field strengths and broader frequency spectra, enabling more extreme application scenarios especially those the common THz sources cannot reach. It should be emphasized that nonlinear THz light–matter interaction is a newly emerging and increasingly active research field, and the discussion here cannot cover all relevant ideas. Just as illustrative and representative examples, a selection of specific application scenarios of ultraintense laser-driven THz radiation in fields of materials, chemistry, and biology are briefly outlined in the following. More detailed reviews and prospects on THz control over matter in different subjects can be found, e.g., in Refs. 143–148.

1. In crystalline materials, strong THz fields of sufficient strengths are able to induce significant displacive distortion of lattice by moving ions far away from their equilibrium positions¹⁴⁵ and thereby potentially triggering structural transition of materials into a new stable or metastable crystalline phase, such as the light-induced transient superconductivity-like states¹⁴⁹ and the paraelectric–ferroelectric phase transition.¹⁵⁰ The ferroelectricity-associated lattice vibrational modes usually lie at THz frequencies. For example, the ferroelectric phonons for the PbTiO₃ and LiNbO₃ at room temperature are at 4.5 and 7.5 THz, respectively, which are beyond the spectral range of common crystal-based THz sources. Strong-field THz pulses with an appropriate spectrum can directly and coherently excite the ferroelectric mode and thereby manipulate the ferroelectric polarization on ultrafast time scales. An extreme THz pulse with GV/m-scale field strengths and frequencies resonant with ferroelectric modes can even flip the ferroelectric orientation completely within the shortest 100 fs time scale,¹⁵¹ hopefully enabling an ultrafast ferroelectric switch at the unprecedented THz speed.

2. In chemistry, a large part of the vibrational–rotational modes of highly relevant chemical bonds (e.g., C–O, C=O, C–H_x) fall in the THz band. Strong-field THz pulses of appropriate spectra is capable of resonantly driving molecular motions and collective displacement of polar species, opening the opportunity to achieve THz mode-selective chemistry in molecules and clusters.¹⁵² For example, for the dissociative absorption of carbon monoxide (CO) on a Rh surface, the V/Å-level THz field at 2.5 THz which is resonant with the hindered-translation mode of CO can excite the angular excursions and C–O bond stretching, leading to a dramatic lowering of the dissociation barrier, and finally, enabling THz-initiated surface catalytic reactions.¹⁵³ 

3. In biology, hydrogen bonds play a central role in the structure and function of biomolecules. Given the fact the characteristic spectrum of hydrogen-bonding dynamics lies mostly at the THz frequencies (e.g., hydrogen-bond stretch vibration at ∼6 THz), it is believed that strong-field THz radiation can modulate the intermolecular or intramolecular interactions and thereby the conformation of biomolecules. The irradiation of intense terahertz waves with proper spectra could dissociate fibrous conformation of peptide with little influences of thermal effect.^154,155 Note that most neurodegenerative diseases are characterized by the intracellular or extracellular aggregation of misfolded proteins,¹⁵⁶ such as amyloid-β and tau in Alzheimer disease and α-synuclein in Parkinson disease. The intense THz-induced impact on biomolecular conformation suggests that strong-field THz waves could be applicable as a processing tool in engineering medical materials and even for invasive manipulation of biomolecular structures or cellular functions, possibly offering a new perspective for the disease therapy.

The illustrative application scenarios above have not yet been realized experimentally due to the lacking of appropriate intense THz drivers. Note that the THz parameters required by the above-mentioned scenarios particularly the THz field strengths and spectra are entirely accessible by ultraintense laser-driven THz sources,^61,63,68 and hopefully, these THz applications in the fields of material phase transition, ultrafast catalysis, and biomedicine will come true in near future. In addition to THz radiation, ultraintense laser–plasma interactions also produce concomitantly energetic electrons,³² ions,³³ and ultrafast radiation at other wavelengths.^157,158 These high-flux charged particles and photons are intrinsically synchronized with THz radiation and consequently can serve as multifunctional ultrafast probes,^159,160 enabling a unique pump-probe platform for the study of extreme THz wave–matter interactions, as shown in Fig. 4(b). Specifically, laser–plasma-based THz sources hold considerable promise for experiments like THz pump—ultrafast x-ray probe¹⁶¹ or THz pump—ultrafast electron diffraction probe,¹⁶² which were previously accessible only at the large-scale accelerator-based THz sources.

In strong-field physics, the electromagnetic field strength is usually characterized with the normalized vector potential, a₀ = eE_THz/m_ecω₀, where ω₀ is the central angular frequency, e and m_e are the charge and mass of the electron, respectively, and c is the speed of light. For the ultraintense laser-driven THz radiation from plasmas, a₀ is evaluated to approach even exceed unity for the experimentally reported THz parameters [see Fig. 4(c)], and it could be further boosted by optimizing laser or target parameters. This implies that THz wave–matter interactions arrives at the realm of relativistic optics,²⁹ which was accessible only at the near-infrared and mid-infrared wavelengths previously. It will open up a completely new avenue in strong-field physics that is driven by long-wavelength THz pulses. In stark contrast to the common multi-cycle near-infrared drivers, the THz pulse usually has a sub-cycle waveform and a long wavelength, corresponding to a considerably low critical plasma density. It is anticipated that the THz photoionization dynamics and the THz-produced plasmas as well as THz–plasma interactions could exhibit distinct characteristics from the near-infrared cases. Take, for example, the ponderomotive force which arises from the envelope average of the nonlinear term in the oscillatory Lorentz force, the time-averaged envelope concept is no longer applicable to the sub-cycle THz pulses. The elimination of multi-cycle averaging could enhance the efficiency, coherence, and stability of the ponderomotive acceleration.^163,164 Strong-field THz-driven acceleration of high-charge electrons,¹⁶⁵ positrons,¹⁶⁶ or ions¹⁶⁷ from a near-critical-density plasma has been proposed recently. It is predicted that a few tens of MeV energy gain could be obtained through the THz-driven post-acceleration of a laser-generated proton bunch.¹⁶⁸ The strong-field THz radiation can also be applied to assist or induce the generation of secondary radiation, such as high harmonics and attosecond pulses from gases,^169,170 single-cycle attosecond pulses, or sub-cycle relativistic mid-infrared pulses from THz interactions with electron beams.^171,172 Most of THz parameters used in the simulations are within the capacity of the ultraintense laser-driven THz sources. With targeted experimental design and diagnostic setup, it will soon be possible to carry out relevant experiments.

Centered around ultraintense laser-driven THz radiation from plasmas, this paper is intended to give an overview of recent advances within the field, and more importantly, provide some forward outlook at where the field is headed and promising strategies for progress in terms of THz generation, characterization, and applications. It should be pointed out that, since this is an increasingly active research field, it is inevitable that this paper cannot cover all relevant studies and ideas that have been reported.

In terms of THz performance, tremendous strides have been made recently in the pulse energy and conversion efficiency of THz radiation emitted from ultraintense laser-irradiated solid and gas targets. The distinct advantage over other well-established accelerator- and crystal-based THz sources is the high scalability of THz energies without saturation as the drive laser intensity increases. Extreme THz pulses with energies up to ∼200 mJ and peak powers as high as ∼1 TW have been demonstrated experimentally, which exceed other state-of-the-art THz sources by more than one order of magnitude. The optimized THz generation efficiency can reach the level of 1%. Future efforts will continue, on the one hand, to seek more efficient THz generation scenarios with exciting properties over wider laser and target parameters. On the other hand, much attention warrants the tunability of THz properties, for example, the generation of widely tunable narrowband strong-field THz radiation and the improvement of operational repetition rates and stability.

In terms of THz metrology, several single-shot detection methodologies have been developed, enabling high-temporal-resolution waveform measurements and ultrabroadband spectral characterization. In the future, more efforts need to be devoted to improve the signal-to-noise ratios of measurements and develop single-shot multi-dimensional detection schemes. Incorporation with new technologies like compressed sensing and machine learning could provide more potential solutions.

In terms of THz applications, the ultraintense laser-driven THz radiation has been applied successfully to diagnose some ultrafast dynamics involved in laser–plasma interactions. In combination with other high-flux photon and energetic particle beams generated concomitantly, ultraintense laser-driven THz sources offer a unique pump-probe platform of enormous potential, especially for the THz strong-field control over matter. Along with the improvement in the operational repetition rates and stability, experiments on nonlinear THz light–matter interactions in the fields of materials, chemistry, and biology are foreseeable in the near future. Relativistic sub-cycle THz pulses open up a new avenue to strong-field physics, and exotic features could emerge compared to the common multi-cycle near-infrared cases.

In a word, further progress and exciting perspectives in this field can be envisaged in the near future, accompanied by the synergetic advances in the THz genesis, metrology, and applications. Complementary to other well-established intense THz sources, the ultraintense laser-driven THz radiation from plasmas is expected to play an essential role in the study of extreme THz science and cutting-edge applications.

The authors acknowledge their numerous colleagues and students for valuable contributions to the research presented in the paper. This work was supported by the National Natural Science Foundation of China (Grant Nos. 12122415, 12175306, 92050106, 11827807, and 92250307), the National Key Research and Development Program of China (Grant Nos. 2021YFA1400204 and 2021YFA1601700), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA25010000), and the CAS Project for Young Scientists in Basic Research (Grant No. YSBR-059).

The authors have no conflicts to disclose.

Guoqian Liao: Conceptualization (equal); Data curation (lead); Funding acquisition (equal); Visualization (lead); Writing – original draft (lead); Writing – review & editing (equal). Yutong Li: Conceptualization (equal); Data curation (supporting); Funding acquisition (equal); Visualization (supporting); Writing – original draft (supporting); Writing – review & editing (equal).

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