Ultrafast imaging for uncovering laser–material interaction dynamics

The physical mechanism of the dynamics in laser–material interaction has been an important research area. In addition to theoretical analysis, direct imaging‐based observation of ultrafast dynamic processes is an important approach to understand many fundamental issues in laser–material interaction such as inertial confinement fusion (ICF), laser accelerator construction, and advanced laser production. In this review, the principles and applications of three types of commonly used ultrafast imaging methods are introduced, including the pump–probe, X‐ray diagnosis, and single‐shot optical burst imaging. We focus on the technical features such as the spatial and temporal resolution for each technique, and present several conventional applications.


| INTRODUCTION
The laser is one of the most important inventions in the 20th century, which has markedly improved scientific research capabilities and the quality of human life. With the continuous advancements in laser technology, the output power of lasers has significantly increased.
For example, the widely used fiber laser has achieved 10 kW-level continuous wave (CW) output with a single fiber, 1,2 and over 100 kW laser power through beam combining technology for broad applications in laser processing, laser weapons, and other fields. 3 The highenergy pulsed laser has been developed with a maximum pulse energy of 1 MJ for the nanosecond laser 4 and approximately 10 PW peak power for the femtosecond laser. 5 The introduction of the highpower laser source allowed creating a series of new frontier interdisciplinary subjects, including inertial confinement fusion (ICF), 6 laser-driven particle accelerator, 7,8 strong field quantum electrodynamics, 9 and laser material processing. 10 For many of those topics, the interaction between the high-power laser and materials has resulted in new interesting and fundamental physics problems. When the laser intensity exceeds 10 18 W/cm 2 , the majority of the irradiated materials are ionized instantaneously and form high-energy-density states of material. This highly nonlinear process is accompanied by new physical phenomena and effects, which are commonly analyzed by large-scale numerical simulations to understand their internal mechanisms. 11 Similarly, in the field of laser material processing, the fundamental scientific problems behind the material removal process remain unanswered. 12 In ICF facilities, understanding the damage mechanism of optical elements under intense laser irradiation has been a persisting challenge for scientists. 13 Therefore, direct imaging observations of the ultrafast dynamic process of laser-material interaction are of great scientific value for revealing its physical mechanism and validating the dynamics modeling.
In the process of laser-material interaction, the thermalization time of optically excited electrons is of the order of femtoseconds. Phonon relaxation occurs within several picoseconds, thermal diffusion and shock wave generation range from tens of picoseconds to nanoseconds, and the dynamics of molten pool occurs of a microsecond order. The frame rate of conventional array sensors, such as charge-coupled devices (CCDs) and complementary metal oxide semiconductors (CMOSs), is not large enough for temporally resolved photography of the aforementioned ultrafast dynamics. The pump-probe technique is a commonly used optical detection method to observe the ultrafast events initiated by a laser beam, through imaging or non-imaging strategies. Due to its relatively simple and flexible optical setup, adaptive temporal and spatial resolution can be achieved when observing repeatable phenomena (Section 2). In high-energy-density physics, the laser plasma radiation spectrum is in the energy range of 10 −2 -100 keV. Accordingly, several time-resolved X-ray imaging techniques have been developed as diagnostic methods (Section 3). A series of all-optical ultrafast imaging techniques based on single-shot laser pulses have been developed to capture these nonrepeatable ultrafast phenomena. Due to these advancements, the temporal resolution has been improved to hundreds or even tens of femtoseconds (Section 4). In the following three sections of this review, we introduce the applications of these imaging techniques involved in the dynamics of laser-material interaction. The scope of discussion mainly focuses on the application of high-power laser, including strong-field physics, damage mechanisms, and laser processing methods.

| TIME-RESOLVED PUMP-PROBE IMAGING
The origin of time-resolved pump-probe (TRPP) detection technology can be traced back to over a century ago. 14 Common optical pump-probe techniques include time-resolved reflection/ transmission spectroscopy, 15 transient absorption spectroscopy, 16 time-resolved Raman scattering spectroscopy, 17 time-resolved photoluminescence spectroscopy, 18 surface/bulk second harmonic generation, 19 time-resolved four-wave mixing, 20 and timeresolved infrared/THz time-domain/X-ray spectroscopy. [21][22][23] In addition to spectroscopy, pump-probe imaging is the most reported transient method used within the imaging technology field. 24 Currently, researchers mainly from two fields are focusing on the use of pump-probe imaging technology to study the mechanism of laser-material interaction: laser material processing and ultraprecision machining of optical elements. The former focuses on the mechanism of material modification or ablation under laser irradiation with the goal of contributing to enhancing the optimization of processing parameters through the study of dynamic processes. The latter mainly provides high-quality and high damage threshold optical elements for high-energy laser devices (especially ICF), and ensures that the optical elements will not be damaged or fail under high-power laser irradiation. In this section, we introduce the principle of ultrafast pump-probe imaging and provide a detailed discussion on the two above-mentioned areas.

| Principle
A pump-probe imaging system consists of a delay stage, a laser source, a beam splitter, and an imaging detector, as shown in Figure 1. The pump and the probe branch are separated from the laser pulse by the beam splitter and irradiated to the area of the sample. In some applications, there are two lasers for pumping and probing, respectively, for example, a nanosecond (pump) and a picosecond (probe) laser. The pump beam is used to excite the sample and induce a dynamic phenomenon. Simultaneously, the probe pulse passes this interaction area and irradiates the detector, carrying the transient information. The delay between the pump and probe pulses can be adjusted by mechanical and electronic devices, which determine the various time slices of the total dynamic process. [25][26][27] Taking into account the transmission situation, the non-excited medium has an absorption coefficient ɑ 0 . Excited states mostly decay exponentially, and the absorption rate decreases to Δɑ 0 immediately after excitation according to where τ is the time delay after excitation and τ ex is the excited state lifetime. The light intensity I t (τ) that changes with time satisfies the following equation: where L is the light transmission length of the sample.
The relative change of the transmitted light intensity ΔT(τ) and the time delay τ can be related by the following equation: F I G U R E 1 Schematic diagram of the pump-probe imaging system The above calculation shows that the time-resolved information of the sample can be obtained by detecting the delayed intensity of laser beam at different times.
The pump-probe technique includes two core concepts. First, it maps the demand of temporal resolution to the demand of spatial resolution through spatio-temporal transformation technology, which ensures the femtosecond precision positioning capability at the beginning of the detection. Since the speed of light is constant, the propagation distance of light is proportional to the time, and the light path corresponding to a time of 1 fs is 0.3 μm. Although the existing mechanical and electronic equipment cannot distinguish the time of 1 fs, the spatial resolution of 1 μm is easy to achieve. The current capability of spatial resolution can be at the nanosecond level, corresponding to attosecond-level temporal resolution. 28

| Dynamics in ultra-short laser processing
While many theoretical studies have been focused on the simulation of the ablation dynamics of metals, 29-33 the experimental study of ablation dynamics over a long time period remains essential for the in-depth understanding of the ablation mechanism. Pump-probe microscopy was widely used to observe the surface ablation dynamics on various metals such as Ti, Au, 34 and aluminum. 35 In addition, the lattice dynamics of Au and Ni irradiated by laser in the first ten picoseconds can also be captured by pump detection technology. 36 The complete dynamics process By choosing a high-power laser to ablate Al, the dynamic process of femtosecond laser ablation with an energy injection much higher than the ablation threshold was achieved, and a time-delay image from 50 fs to 10 ns was recorded. The shadowgraphs recorded by Zhang et al.
provide for the first time a direct dynamic picture of the interesting hybrid ablation process in an intuitional way. 38 The Coulombic explosion is one possible mechanism when a highenergy laser pulse irradiates a dielectric or semiconductor material. In addition, after the target is heated by an ultrafast laser pulse, a strong thermoelastic wave may be formed due to the sudden thermal expansion of the ablated area. This thermoelastic shock wave may cause spalling or chipping inside the target material and eventually lead to matter ejection. 39 Currently, research on the dynamic process of femtosecond laserinduced plasma generation is mainly carried out by single-pulse irradiation. However, the plasma dynamics during multi-pulse laser ablation remains unclear. Therefore, by studying the laser-induced plasma dynamics during the ablation process of multiple femtosecond laser pulses of silicon, the structure-matter excitation of air plasma was directly observed in the femtosecond time scale, revealing the mechanism of the plasma and shock wave expansion. 40 The two basic mechanisms are of great significance for gaining an in-depth understanding of the nature of the interaction between ultrafast lasers and matter. Figure 3  show that the ablation process in air and liquid is similar in the first 10 ps of laser ablation; however, after 10 ps, there is a significant difference from the ablation process usually observed in air. 41 At the same time, due to the influence of the liquid environment, the reflectivity does not decrease due to scattering and absorption, but increases, and strongly depends on the liquid used. The above results indicate the influence of the liquid environment on the ablation process during laser ablation.
F I G U R E 2 Time-resolved measurements of the surface dynamics from pump-probe reflectometry (PPR) of Al in (A) and stainless steel in (B). Reproduced with permission. 37 Copyright 2020, Elsevier

| Damage mechanism of optical elements for intense laser
The ICF devices, such as the National Ignition Facility (NIF), have a large demand for ultraprecision optical elements, which are vulnerable to damage and failure in high-power laser systems. 42 Research on the damage mechanism of transparent optical elements continues to be an important scientific subject. [43][44][45][46][47] The small cracks that form in the optical element during the manufacturing process are generally considered to be the cause of damage. TRPP with a time resolution of nanosecond can capture the temporal changes in laser-induced damage. [48][49][50] The defects absorb the laser energy and cause localized temperature rise, leading to the formation of plasma at the interface between the material and air.
The plasma expands and produces shock waves and stress waves.
Eventually, the shock waves and stress waves cause circumferential and radial cracks, thus leading to the appearance of a pit structure. The dynamic process was recorded by TRPP, as shown in Over a longer temporal scale, particle spray is formed from the damaged area. The particles are ejected during the laser-induced breakdown process. These particles cause adverse effects on the entire system, including a reduction in the mechanical properties of the deposited coating and aggravating laser-induced damage to optical components. Therefore, observing the path of ejected particles has become the focus of research. TRPP technology is the main method used to observe the behaviors of ejected particles with a time resolution of <0.5 μs. [52][53][54][55][56] The ejection process of particles is irrelevant to the laser parameters and mainly depends on the volume of the particles excited by the laser. The particle residues forming the initial pulse could be excited by subsequent pulses. With an increase in the number of pulses, the distance of particle ejection increases. 57 These results provide sufficient valuable information about the trajectory of the ejected particles that can give insights to prevent damage to adjacent optical components.

| ULTRAFAST X-RAY IMAGING
Since the 1970s, X-ray imaging technology has developed rapidly and has been widely used in industrial flaw detection, weld inspection, and medical inspection. With the development of image digitization technology, higher spatial resolution, wider dynamic range, and realtime imaging capabilities without geometric distortion can be guaranteed under a large field of view. [58][59][60] The X-ray high-speed imaging system is commonly used now in the imaging inspection field because it can quickly and dynamically monitor the internal structure, size, position, and dynamic changes of the detected object. Currently, X-ray high-speed imaging is also widely applied for the high-speed monitoring of the dynamic process of laser-material interactions, including molten pool monitoring, plasma evolution, and myoglobin structural dynamics. This section reviews the research on the use of X-rays to monitor dynamic processes at high speeds when lasers interact with materials.

| Principle
The X-ray is widely used in the imaging research of the internal structure of metal and high-density plasma due to its penetration characteristics. Observation of the dynamic process requires timeresolved X-ray imaging technology. Generally, continuous photography with a μs-ms frame interval can be realized using a long pulse or continuous X-ray light source, such as a synchrotron radiation light source, combined with shutter exposure time control of the X-ray camera, as shown in Figure 5A. 61 Alternatively, a plasma source excited by an intense laser pulse can generate an X-ray pulse with an ultrafast temporal resolution based on mechanisms such as Compton scattering, 63 wakefield acceleration, 64 and bremsstrahlung, 62 which can achieve fs-ps temporal resolution. Figure 5B shows a typical laser wakefield accelerator-driven bremsstrahlung X-ray source for advanced radiographic imaging.
The former method has been widely used in the field of laser processing, [65][66][67][68][69][70][71][72][73] and the latter ones are often used to capture ultrafast dynamic processes in the strong field physics of laser-matter interaction because it is convenient to realize time synchronization through the trigger of a laser pulse. 74 However, due to the low repetition rate of intense laser pulses, it is often only possible to capture a single frame image.

| Molten pool monitoring for laser processing
A critical research topic in high-power laser material processing is the evolution of the molten pool. The morphology and temperature of the molten pool have a huge influence on the dimensional accuracy, residual stress, and structural performance of the final component or system, and it is an object of significant monitoring importance. X-ray high-speed imaging plays a crucial role in molten pool monitoring. phenomenon is always an attractive problem in the metal laser melting processing field, and can be quantified using the ultra-high-speed synchrotron radiation X-ray imaging. 75 Two different molten pool shapes were observed using this method, as shown in Figure 6.  Figure 7. This study reveals the mechanism controlling the formation of a highly nonequilibrium microstructure during DED-AM. During the whole period, the spatial resolution was approximately 100 µm, and the temporal resolution was less than 1 ms. 80 The

| Ultrafast imaging for intense laser plasma physics
In laser-driven ICF, asymmetry of the hot spot and the mixing effect of the shell material into the hot spot remain important factors restricting the further increase of the ICF implosion yield. Capturing an image of the hot spot with high temporal and spatial resolution is essential to analyze the asymmetry of the hot spot shape and the physical design of the symmetry control of the hot spot. The X-ray high-speed microscopic imaging system based on Kirkpatrick-Baez (KB) is widely applied in the field of ICF. [85][86][87][88] A variety of KB microscopic imaging systems have been developed, [89][90][91] such as the total reflection wide-band KB microscopic imaging system, the multilayer film quasi-single energy response KB microscopic imaging system, and the multi-mirror structure advanced KB microscopic imaging system. The above imaging system has a large field of view (~1 mm), high spatial resolution (<5 μm), and high temporal resolution (<100 ps), as shown in Figure 12A,B. It has been used to observe the process of CH shell implosion, black cavity implosion at both ends, and glass shell implosion to push the target. Some of the experimental results are shown in Figure 12C.
X-ray framing recording technology is equipped with simple pinhole imaging or KB microscopic imaging or curved crystal imaging with high spatial resolution imaging capabilities, which can achieve a two-dimensional high-spatial resolution imaging diagnosis to observe the evolution of the hot spot morphology during implosion. To further improve the imaging time-resolution capability, an X-ray drift imaging system was developed by combining electronic drift technology. By accelerating the photoelectron in the drift zone, the "velocity dispersion" of the electron group was accomplished. The principle and schematic of the system are shown in Figure 13. The pulse was broadened by 20 times the width, and the time resolution was increased from 60 to 20 ps, with high spatiotemporal resolution diagnostic capabilities. 92,93 This technique has been used in the research of hot spot morphology evolution, fuel movement state, and the degree of hot spot mixing. As the charge effect increases, the noise increases, and the signal-to-noise ratio of the acquired image is lower, which affects the spatial resolution of the image.  94 In this review, we mainly focus on the active illumination ultrafast imaging techniques that require an ultra-shot illuminating

| Single-shot frequency-domain holography
Single-shot frequency-domain holography (FDH) is a single-shot ultrafast phase measurement technique with one-dimensional spatial resolution, developed based on an extension of the frequencydomain interferometry (FDI) method. 103,104 The advantage of FDH is the single-shot measurement of optical phase shifts with femtosecond temporal resolution over a picosecond time scale. Figure 15 shows In their experiments, two chirped and frequency-doubled pulses took holographic snapshots of the ionization front and wake, which was created by an energy of 1 J and a 30 fs pump pulse. The spatial resolution was close to 3 μm, and the temporal resolution was 30 fs.

| Single-shot FDT
Tomography imaging is based on the measurement of radiation transmitted through an object along different directions, which enables noninvasive imaging of hidden stationary objects from sequentially measured projections. 106 FDT can image a wide range of nonlinear propagation phenomena, including filament formation in gases and the evolution of plasma wakefields. 107,108 Figure 17A shows

| THPM
According to the angular multiplexing holographic technique, THPM can capture ultrafast phenomena that appeared in polarizationsensitive transparent materials. 110,111 Figure 17A shows the schematic representation of the THPM system. A frequency-doubled probe laser pulse was separated into two pulses as a pump and a probe, respectively. The interference holograms between the reference and signal pulses were captured by a CCD camera, which

| STAMP
In the category of single-shot optical imaging, a frequency-timeencoding technique known as STAMP attracts considerable attention due to its advantage of a high frame rate (up to THz), adjustable frame interval (fs-ns), and high image quality (close to optical microscopy). [113][114][115][116][117][118][119] The main principle of STAMP is to map the temporal and spatial information into a linear chirped laser pulse by using a temporal mapping device (TMD) and a spatial mapping device (SMD  Figure 18A shows the 25-frame SF-STAMP configuration, which is the highest frame number of STAMP system. 115 The ultrafast 2D-burst images of the crystalline-to-amorphous  Figure 18B. 115

| FRAME
FRAME is another spatial frequency division technique. The ultrafast 2D videography was first realized by FRAME with spectral compatibility, high temporal, and spatial resolution. In FRAME, the time scale of the frame interval can reach the femtosecond level. 118-120 Figure 19A,B shows the schematic diagram of FRAME. FRAME encodes different carrier frequencies to detect daughter pulses by intensity modulation achieved with a Ronchi grating, and then deciphered in a post-processing step. 121 To demonstrate this capability, the diverging laser pump pulse propagated in the Kerr medium was reconstructed. 122 As shown in Figure 19C, the FRAME imaging system monitored the dynamic process in which the laser beam passes through CS 2 liquid. The frame size was 1002 pixel× 1004 pixel, and the imaging speed was 5 Tfps.
The final spatial resolution of FRAME was approximately 15 lp/mm, and the field of view exceeds 7 × 7 mm. By using 3D interpolation method, Figure 19D shows how the wave front of pump pulse changes when propagating through the CS 2 liquid. 118

| SUMMARY AND OUTLOOK
In summary, the progress of laser technology has led to unprecedented high-energy-density physical phenomena and new theoretical mechanisms, enabled the progress of ICF, laser accelerators, laser advanced manufacturing, and other fields, and made the theoretical research on the interaction between high-power laser and material increasingly attractive. A variety of ultrafast imaging technologies for different applications are being rapidly developed to help researchers observe and understand these transient phenomena. The focus of this paper was on reviewing the principles and applications of three types of commonly used ultrafast imaging methods including pump-probe, X-ray diagnosis technique, and single-shot optical burst imaging. The process above the damage threshold was the main focus. Each ultrafast imaging technique has its own advantages and limitations. For example, the penetrability of X-ray contributes to the internal imaging of plasma, but, for the most part, only one single frame image can be captured for ultrafast photography and the equipment is complex and expensive. Visible light photography based on a femtosecond laser is relatively convenient to implement under ordinary laboratory conditions, but the signal is easily disturbed and distorted in a complex electromagnetic environment.
Several representative ultrafast imaging technologies mentioned in this review are summarized in Table 1 in terms of classification, temporal resolution, and applications.
This review serves as a reference for researchers in related fields, despite difficulties in covering all the ultrafast imaging technologies.
It is expected that the single-shot burst imaging technique, such as FTOP and STAMP, will become a reliable measurement method, while there are still some parameters, such as the number of frames, that need to be improved. It is also noteworthy that the micro-and nano-structures in MEMS (Micro-Electro-Mechanical System) and NEMS (Nano-Electromechanical System), such as high-frequency resonators in 5G and 6G, micro/nano beams, which may be used for biological detection, will also utilize the ultra-fast imaging of both the motion and internal structures and defects, which will be worthy of further study.

CONFLICT OF INTEREST
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study. Abbreviations: FDH, frequency-domain holography; FDT, frequency-domain tomography; FRAME, frequency recognition algorithm for multiple exposures imaging; FTOP, femtosecond time-resolved optical polarimetry; STAMP, sequentially timed all-optical mapping photography; THPM, time-resolved holographic polarization microscopy. a Competent to achieve multi-frame photography in longer durations according to the demands.