SignificanceWith the rapid development of free-electron laser (FEL) sources, the ionization of atoms and molecules by strong extreme ultraviolet (XUV) pulses has become a major topic in strong-field physics. A range of novel and even counterintuitive non-perturbative phenomena can be observed in photoionization with strong XUV pulses. This review focuses on non-perturbative effects in photoionization processes, including atomic stabilization, Autler-Townes (AT) splitting, and dynamic interference.ProgressAt high laser intensities, instead of being ionized, the electron tends to remain bound near the nucleus. This suppression of ionization in the high-frequency and high-intensity regime is often referred to as atomic stabilization. In the Kramers-Henneberger (KH) frame, the atomic bound electron wave function evolves into a dichotomous state at high laser intensity, leading to a reduced ionization rate. The formation of dichotomic KH states suggests that stabilization can be monitored by the angle-resolved spectra of the photoelectron. Molecule-like two-center interference fringes are predicted to be observable in a pump-probe scheme. Experimentally, atomic stabilization can be interpreted as the temporal destructive interference of photoelectrons ejected at different times. The angular distribution of the photoelectron in the stabilization regime differs significantly from that in the perturbative regime. Early studies based on reduced-dimensional time-dependent Schrödinger equation (TDSE) calculations suggest that stabilization can be disrupted by nondipole effects or electron-correlation effects. More recent full-dimensional TDSE calculations show that nondipole corrections can even enhance stabilization if a suitable pulse duration is chosen. When the photon energy matches the energy difference between two bound states, Rabi oscillations of the populations can be induced in a two-level system. Rabi oscillations have been identified in a variety of physical systems, including ultracold atoms, Rydberg atoms, molecules, metal nanostructures, Josephson-junction circuits, and quantum dots, across a wide range of frequencies from radio to ultraviolet. Recently, Rabi oscillations have also been observed in the XUV regime. Real-time observation of Rabi oscillations has been achieved by detecting resonance fluorescence, state-dependent refractive index, birefringence, differential reflectivity spectra, and photoelectron spectra. Alongside Rabi oscillations, AT splittings appear in the photoelectron energy spectrum. AT splittings have been observed in various settings, including multiphoton ionization, double ionization, and even in the absence of population oscillation of the initial ground state. The AT doublet is sensitive to many factors, such as the number of Rabi cycles, the AC Stark shift, the transition matrix element between bound and continuum states, and the competition between resonant and non-resonant ionization channels. The separation of the standard AT splitting is typically given by the product of the Rabi frequency and Planck's constant. However, the separation of dynamical AT splitting in a pump-probe scheme can be much larger than that of the standard AT splitting. Dynamical AT splitting can be treated as a consequence of temporal double-slit interference related to the phase jump of the initial ground state amplitude as it approaches zero. Due to the time dependence of the relative AC Stark shift between the initial state and the final continuum state, the photoelectrons ejected at different times have different energies. At two symmetrical time points—the rising edge and the falling edge of the laser pulse—the photoelectrons carrying the same energy can interfere, resulting in a multi-peak structure in the photoelectron energy spectra. This temporal double-slit interference is referred to as dynamic interference. Observing dynamic interference in ground-state atoms requires specific conditions. On one hand, the phase difference between the two photoelectron wave packets ejected at the rising and falling edges must be large enough to induce destructive interference at certain energies. On the other hand, the depletion of the initial state should not be too large to ensure that the second slit of the double-slit remains open. In particular, atomic stabilization is necessary to observe dynamic interference in ground-state hydrogen. Dynamic interference is also predicted in the double ionization of helium with a grid-like interference structure observed in the joint energy spectrum from the numerical solution of the full-dimensional TDSE. In the double ionization scenario, six ionization paths contribute to the grid-like interference structure. Initially, the multi-peak structure between the AT doublets is also attributed to dynamical interference. However, recent studies suggest that the multi-peak structure between the AT doublet may have other physical origins, as this structure can also appear in calculations using rectangular and half-Gaussian pulses, where the temporal double-slits cannot exist.Conclusions and ProspectsMost of the nonlinear effects of strong XUV pulses discussed in this review are still limited to theoretical predictions. While atomic stabilization in Rydberg atoms has been experimentally verified, experimental confirmation of atomic stabilization in ground-state atoms remains elusive. Measuring dynamic interference in atoms exposed to a single XUV pulse presents significant challenges since it requires not only extremely high pulse intensity but also proper pulse duration. Recently, the experimental measurement for the AT splitting in the XUV regime has been achieved. With the rapid advancement of free-electron laser sources globally, these intriguing physical effects predicted by the theory are expected to be verified experimentally, and more novel nonlinear physical effects are expected to be discovered.

SignificanceIn the past thirty years, measuring laser-induced fragments generated by atoms and molecules has become a key method for exploring and understanding atomic and molecular dynamics. Nanoparticles exhibit distinct size effects and surface near-field enhancement effects compared to individual atoms, giving them unique physical and chemical properties that have created immense technological and economic value in many application fields. The exploration of interactions between femtosecond laser pulses and nanostructures has led to many breakthrough scientific technologies, significantly advancing the fields of optoelectronics, nanoelectronic devices, nanomaterial processing, photocatalysis, biotechnology, and more. Understanding the ultrafast ionization dynamics of nanostructures is crucial for comprehending the fundamental physical processes at the atomic scale when nanostructures are excited by strong laser fields. This understanding is also valuable for regulating ultrafast ionization dynamics, promoting new nanotechnologies, and developing high-performance optoelectronic materials and chips. The scientific basis for these advanced technologies and cutting-edge applications lies in understanding the microscopic physical mechanisms of interactions between laser fields and micro-nano systems. With advancements in vacuum nanobeam source technology, the interactions between laser fields and individual nanoparticles using momentum detection spectrometers can be studied. Ultrafast femtosecond lasers, with their precise control time and frequency domains, offer high peak intensities and short pulse durations. The precise control over parameters such as wavelength, polarization, pulse width, intensity, and multi-pulse delay provides unprecedented methods for accurately measuring extreme ultrafast dynamic processes in nanomaterials, exploring strong field physical effects, and developing groundbreaking disruptive technologies. Utilizing nanoparticle beam targets in velocity imaging spectrometer systems has expanded laser ionization research from atomic to larger nanoscale systems, validating numerous fundamental physical results observed in atoms and driving forward the development of practical macroscopic applications.ProgressA monodisperse nano aerosol system has been proposed (Fig. 5). The development of a nanoparticle aerodynamic focusing system has been reported (Fig. 6). A nano velocity imaging spectrometer has been developed (Fig. 7). The physical mechanism of the M3C model has been reported (Fig. 13). The analysis of electron momentum in nanostructures influenced by carrier-envelope phase has been reported (Fig. 14). The femtosecond dynamics involved in the metallization of nanostructures have been explored (Fig. 15). The focusing effect of electrons emitted from nanoparticles has been reported (Fig. 21). Optical control of electron emission from nanostructures has been achieved (Fig. 25). A strong laser-induced minimal shock wave has been reported (Fig. 30). The synthesis of surface molecules on nanoparticles in a strong laser field has been reported (Fig. 33). Complete optical control of laser-induced dense plasma emission has been achieved (Fig. 36).Conclusions and ProspectsOverall, the study of strong field ionization in nanostructures remains an emerging field, with many aspects of ultrafast electron-ion dynamics still not fully understood. Future research promises to focus on the precise control of electrons and ion emission, as well as the ultrafast measurement and manipulation of surface molecular reactions. In addition, the formation of isolated nanoscale plasmas under intense laser fields provides an excellent platform for investigating the properties of small-scale plasmas. This includes exploring complex physical processes such as the expansion of dense plasmas, identifying and measuring various ion types within plasmas, generating dense ion sources, and examining the spatiotemporal phase transitions of plasmas.

SignificanceThe advent of high harmonic technology provides a new generation of high-quality light sources for modern high-resolution imaging applications. The high harmonic sources can provide highly coherent extreme ultraviolet/soft X-ray ultrashort pulses, leading to the rapid development of desktop high-resolution imaging technology in the past 20 years. Meanwhile, this technique can achieve nanometer-level spatial resolution and femtosecond-level temporal resolution by a variety of lensless diffraction imaging methods and has many applications in biology and materials. The isolated attosecond pulses, born in the 21st century, provide us with unprecedented temporal resolution, but the chromatic aberration introduced by their broad-spectrum characteristics has become a new problem in diffraction imaging methods.ProgressThere have been numerous reports on imaging applications in biological and material sciences by adopting quasi-monochromatic sources via various imaging techniques including traditional optical imaging, coherent diffractive imaging, Fourier transformation holography, and ptychography as shown in Figs. 1~9. Especially in recent years, there have been a lot of studies focusing on ptychography, including reflective modes and transmissive modes. Furthermore, the vortex structure has shown its unique advantages in ptychography and significantly improved the imaging qualities. Although compared with other methods, ptychography has the best imaging quality and algorithm stability, it is relatively difficult to combine with pump detection technology, and the ultrafast process should be highly repeatable. Thus, most of these ptychography experiments are subjected to static samples, which makes it difficult to show the time resolution advantages of high harmonics. Furthermore, long exposure time has always been a major shortcoming of ptychography. Although single-shot ptychography has been reported, it is only applicable to visible/IR regimes and still challenging under high harmonic illuminations. Imaging using attosecond pulse trains or isolated attosecond pulses is more challenging than quasi-monochromatic sources. The main difficulty is their broad spectrum characteristics, resulting in more unknown quantities to be solved from limited equations. Existing research mainly focuses on two solutions. One is to add a priori condition to reduce the number of unknowns, which can only conduct imaging on specific samples that meet the corresponding prior condition, limiting its application scope. The other is to match the number of unknowns by increasing the number of equations, which requires position scans or delayed scans. With the increasing spectral components, the required number of scan points will increase simultaneously to ensure a sufficient number of equations, thus resulting in the generally limited spectral resolution of such methods. Additionally, these methods are not compatible with single-shot imaging, limiting their applications in ultrafast sciences. Therefore, the development of new imaging methods and algorithms is still an important research direction for achieving combinations of high temporal and spatial resolution of attosecond pulses in imaging.Conclusions and prospectsAs high harmonic light sources are characterized by small size, selectable wavelength, good spatial coherence, and short pulse width, they become one of the most preferred choices for extreme ultraviolet/soft X-ray nanoimaging applications. As a quasi-monochromatic light source, the imaging algorithm of the filtered high harmonic light source has been relatively mature. The high harmonics generated by high-energy driving lasers can achieve single-pulse imaging, which provides the possibility to study ultrafast dynamics. Meanwhile, the high harmonics generated by the driving laser in the order of mJ can meet the needs of most static imaging applications and can be applied to many research fields such as biology and materials science, thus having the potential to become a new generation of high-end nanoscopy equipment. Nowadays, high harmonics generated from gases are constantly developing toward higher stability, higher energy, and shorter wavelengths to pave the way for ultrafast imaging with higher imaging quality and higher resolution. With the rapid development of high harmonic generation technology in the water window band, desktop water window live-cell imaging will become the next important research direction. Additionally, solid-state high harmonics are also a rising star developing rapidly in recent years. Although compared to gas-based high harmonics, solid-state high harmonics have low photon energy and damage to the sample, the peak intensity requirements of the driving laser are low, with higher stability and no need for vacuum. Therefore, the development of solid-state high harmonic lasers is also expected to further miniaturize the size of high harmonic microscopy equipment and reduce the cost. In addition to the common Gaussian light source, high harmonics of different spatial or polarization structures can also be obtained by light field synthesis or shaping techniques. With further development of high harmonics field control technology, new structures may also promote the further development of high harmonic imaging technology. As for imaging by adopting attosecond pulse trains or isolated attosecond pulses, there isn’t a satisfying solution. However, the rise of new technologies such as compressive sensing has injected new energy into imaging research, with many new methods developed. Compressed ultrafast imaging and compressed spectral imaging can introduce an additional temporal or spectral dimension to 2D imaging to achieve dynamical or hyperspectral imaging, which is similar to what we are eager to achieve in attosecond imaging. Meanwhile, deep learning is an attractive direction to explore. In the future, the combination of compressed sensing and deep learning with wide-spectrum CDI, wide-spectrum FTH, and wide-spectrum ptychography may provide more opportunities for us.

SignificanceObserving light information during atomic time processes to accurately reveal the physical, chemical, and biological phenomena and their evolution during atomic motion has always been a dream of scientists. Atomic time imaging with the sound intrinsic spatial resolution is applicable to studying ultrafast transient events in ultrafast physics, ultrafast chemistry, and ultrafast biology. The events include ultrafast optophysical processes in semiconductor and quantum well microstructures, excitation of excitons and carriers by light, high-order harmonic effects, ultrafast dynamic processes in condensed matter, ultrahigh-intensity laser wake field acceleration, formation and breaking of chemical bonds, transfer of protons and electrons, the influence of molecular vibrations and rotations on chemical reactions, energy transfer processes in photosynthesis, photoisomerization processes in visual systems, and charge and proton transfer in DNA. Currently, only all-optical imaging can be employed to achieve atomic time imaging, where light itself records the modulated light field of transient events or dynamic processes. This is based on the duality of light as both an information carrier and a research resource. From the perspective of the light field, the amplitude, phase, wavelength, wave vector, and polarization of the light field are included. From the perspective of photons, photon energy, momentum, spin angular momentum, orbital angular momentum, and nonlinear and quantum properties of photons are contained.ProgressUltrafast atomic time imaging has seen significant developments in recent years. In femtosecond holographic imaging, Dr. Martin Centurion was the first to achieve multi-frame encoded femtosecond time-domain holography (Fig. 5), while Dr. N. H. Matlisi first utilized chirped pulses and spectral interferometry to record the frequency-domain holography of laser-induced plasma wake fields (Fig. 6). In scanning streak tube compressed ultrafast photography (CUP), Gao et al. proposed the CUP technique to achieve two-dimensional imaging of non-repetitive ultrafast luminescent phenomena (Fig. 8). This caused a sensation in the ultrafast imaging field, which opened up a new avenue for atomic time imaging via adopting compressed sensing algorithms, thus leading to the emergence of T-CUP, CUSP, and other related derivative technologies. In spectrum-plane encoded atomic time imaging, Ehn et al. proposed the frequency recognition algorithm for multiple exposures (FRAME), which enabled ultrafast multi-frame imaging with high spatial and temporal resolution (Fig. 9). Zhu et al. proposed the frequency domain integration sequential imaging (FISI) technique, achieving the highest space-bandwidth product in ultrafast imaging to date (Fig. 10). In spectral encoding femtosecond imaging, Nakagawa et al. proposed the sequential timed all-optical mapping photography (STAMP), with a maximum framing frequency of 4.4×1012 frame/s, which was once considered the fastest photography in the world (Fig. 11), and this led to the development of technologies such as SF-STAMP (Fig. 12). The grating-sampling atomic time imaging technique (OPR) combines the grating sampling theory with spectral-time encoding technology by a grating plate, achieving all-optical high spatial and temporal resolution imaging with a grid principle of 2 trillion frames per second (Fig. 13). Multi-stage non-collinear optical parametric amplification (MOPA) idler imaging has parameters such as framing time, exposure time, spatial resolution, and frame size that are independent and unrelated, thus becoming an ideal imaging method. It yields sound effective framing frequency and high spatial resolution in single-shot atomic time-scale imaging (Fig. 14).Conclusions and ProspectsFurther development of the information theory of atomic time imaging is needed to evaluate and develop atomic imaging technology. We preliminarily improve Schardin’s space-time information theory, explore the optimal atomic time imaging system that is not limited by the Heisenberg uncertainty principle, and always pursue shorter exposure time, finer intrinsic spatial resolution, and greater spatial bandwidth products. Atomic time imaging faces new challenges, including advancing studies on high-speed imaging and computational femtosecond imaging information theory, promoting the combination of femtosecond imaging and picometer spatial resolution technology, and exploring new principles of femtosecond imaging, new optimal imaging systems, and reliable, reasonable enhancement of existing imaging technology performance. Additionally, it is necessary to promote the application of atomic time imaging technology in photon materials, plasma physics, live cells, and neural activity, and to push the timescale from femtoseconds to attoseconds. The development of attosecond imaging already shows its initial signs. Currently, the imaging of the electron wave packet motion in neon atoms and electron motion capturing in nitrogen molecules have been achieved, and the temporal resolution of transmission electron microscopy has been pushed to the attosecond scale. By directly measuring the relationship between the electromagnetic functions of natural and artificial materials with space and time, attosecond electron microscopy provides indispensable information for a deep understanding of the fundamental mechanisms of light-matter interactions, and is expected to promote development in fields such as near-field optics, passive and active deformable materials, photonic integrated circuits, photoperiodic photochemistry, and free-electron cavity optics.

SignificanceWith the development of fundamental physics, chemistry, materials science, and energy engineering, many processes that occur on the nanosecond (ns, 10-9 s) to femtosecond (fs, 10-15 s) time scales, known as ultrafast processes, have become crucial topics in research in related fields. Examples include ultrafast plasma dynamics and X-ray emission dynamics in nuclear fusion investigations. However, observing these ultrafast processes, which aim to record transient dynamics including information from at least two spatial dimensions, remains a significant technical challenge. Current methods for observing ultrafast processes, namely, ultrafast imaging techniques, can be divided into two categories: passive imaging techniques and active imaging techniques. Passive imaging techniques suffer from electronic bottlenecks such as the confinement of carrier movement speed, parasitic capacitance and inductance, and spatial dispersion of electrons, severely restricting their spatial and temporal resolution. Active ultrafast imaging techniques use illumination pulses to observe ultrafast processes, capturing light signals from the observation pulses themselves. This approach overcomes the limitations of electronic bottlenecks on imaging speed and quality in ultrafast imaging technology. With the rapid development of ultrashort pulse generation techniques, ultrafast imaging based on pump-probe, the most commonly applied active imaging technique, can achieve temporal resolutions as fine as a few femtoseconds and spatial resolutions down to sub-micrometer scales. However, pump-probe techniques can only capture a single snapshot of the observed ultrafast process in each observation. To capture the entire ultrafast process, it is necessary to continuously repeat the experiment. The delay between the pump pulse and the probe pulse (observation pulse) must be adjusted accordingly to capture different transient slices. Consequently, pump-probe techniques are not suitable for recording non-repeatable ultrafast processes with significant randomness. In recent years, many single-shot active ultrafast imaging techniques, capable of capturing multiple frames of ultrafast processes in a single observation, have garnered widespread interest. Techniques like FINCOPA, FRAME, and SNAP have emerged as powerful tools for observing critical ultrafast phenomena such as laser-plasma interactions, femtosecond laser ablations, and rapid phase changes. Among these, ultrafast imaging techniques based on spectral-temporal transform utilize different spectral components within a single pulse to record various temporal slices of the observed ultrafast process. Due to their unique ultrafast detection mechanisms, spectral-temporal transform-based techniques can continuously record ultrafast processes at extremely high frame rates exceeding 100 THz, acquiring hundreds of frames in a single shot. These capabilities are unmatched by other active ultrafast imaging methods, which allow for the revelation of transient details within ultrafast processes. In addition, the entire exposure duration of spectral-temporal transform-based ultrafast imaging techniques ranges from femtoseconds to nanoseconds, a wide observation time window unprecedented in other active ultrafast imaging methods. Considering the significance of ultrafast imaging techniques and the impressive capabilities of spectral-temporal transform-based methods in observing ultrafast processes, there is a clear need for a comprehensive review of state-of-the-art techniques. Such a review could inspire further development in ultrafast imaging technology and expand the application domains of single-shot ultrafast imaging techniques.ProgressThe first work on ultrafast imaging techniques based on spectral-temporal transform was completed by Goda’s group at Tokyo University in 2014. They captured 6 pictures with a frame rate exceeding 4.36 THz in a single shot. Since then, considerable advancements have been achieved in two main components: generating observation pulses with spectral-temporal transform characteristics and reconstructing the time-spectral image sequence. For generating observation pulses with spectral-temporal transform characteristics, both glass rods and grating pairs have been widely utilized to generate chirped pulses as observation pulses, with observation time windows ranging from femtoseconds to picoseconds. In 2020, Kannari’s group at Keio University broadened the observation time window to a few nanoseconds using a system called free-space angular-chirp enhanced delay (FACED) (Fig. 3). In 2023, Nakagawa’s group at Tokyo University further extended the observation time window to over 10 ns through techniques known as “spectral shuttle” (Fig. 4) and “spectrum circuit bridging” (Fig. 5), respectively. Both direct imaging methods, which rely on spatial-spectral mapping, and computational imaging methods, which utilize compression-reconstruction techniques, have been developed for the recovery of spectral-temporal image sequences. The direct imaging methods relying on spatial-spectral mapping were initially pioneered by Goda’s group in 2014 (Fig. 6) using a spectral mapping device (SMD), which was further advanced in terms of the number of frames acquired in a single shot by Nakagawa’s group in 2020 (Fig. 7). These spatial-spectral mapping methods also encompass spectral filtering introduced by Goda’s group in 2015 (Fig. 8), and spatial-point sampling methods initiated by Kannari’s group in 2020 (Fig. 9). Computational imaging methods for recovering the spectral-temporal image sequence were first developed by Downer’s group from the University of Texas at Austin in 2014 using a single-shot Fourier domain tomography method (Fig. 11). In 2018, Chen’s group pioneered computational ultrafast imaging techniques based on spectral-temporal transform using compressed sensing (Fig. 12), which significantly increases the number of pictures acquired in a single shot. Zhang’s group developed a single-shot polarization-resolved ultrafast mapping photography (PUMP) by combining spectral-temporal transform with multi-frame computational polarization imaging methods (Fig. 14).Conclusions and ProspectsFurther development of ultrafast imaging techniques based on spectral-temporal transform should first focus on evaluating the criteria of key parameters, such as the temporal and spatial resolution limits of the ultrafast imaging system, which are still debated. Enhancing temporal resolution further relies on using observation pulses with wider spectra while improving spatial resolution depends on advancements in both the imaging compression system and reconstruction algorithms.

SignificanceUltrafast laser pulses, with pulse widths as short as femtoseconds and even attoseconds, enable precise measurements with extremely high temporal resolutions. These capabilities have fostered numerous ultrafast spectroscopic detection and imaging techniques, establishing themselves as crucial tools in frontier photophysical research and applications. Several Nobel Prizes over the past 40 years underscore the promising future of ultrafast optics. However, the wave nature of ultrafast laser pulses imposes limitations on spatial resolution and penetration depth in certain scenarios. The optical diffraction limit of ultrafast laser pulses, for instance, sets an intrinsic physical constraint on spatial resolution. Additionally, strong optical absorption and scattering in opaque media restrict penetration depths to the nanometer scale. Therefore, it is imperative for the ultrafast optics community to develop complementary spatiotemporal imaging with ultrahigh resolution. Ultrafast acoustic pulses emitted during the interaction between ultrafast laser pulses and thin-film photoacoustic transducers hold promise for overcoming current limitations, which will open a new pathway for achieving spatiotemporal imaging with ultrahigh resolution. Firstly, the ultrahigh frequencies of these acoustic pulses, approaching THz, correspond to wavelengths below 10 nm, allowing for spatial resolutions in the order of a few nanometers. The mechanical wave nature of ultrafast acoustic pulses significantly strengthens their ability to penetrate opaque media. Secondly, these pulses can have extremely short durations, measured in picoseconds or even sub-picoseconds, enabling the functional imaging of buried interfaces and defects with unprecedented spatiotemporal precision. Thirdly, ultrafast acoustic pulses can be readily detected by ultrafast laser pulses, which provides a versatile approach for developing spatiotemporal imaging applicable across various materials and systems. Emerging sources of ultrafast acoustic pulses, such as 2D semiconductor heterostructures, show great potential for generating tunable ultrafast acoustic emissions. These sources offer a novel platform for studying interfacial photophysics and advancing the functional imaging capabilities of critical micro-devices and chips. Recent advancements in harnessing ultrafast acoustic pulses from 2D semiconductor heterostructures underscore their pivotal role in future studies of spatiotemporal imaging with ultrahigh resolutions. In this review, we will explore the history, recent progress, and future research opportunities in ultrafast acoustic pulses.ProgressWe review ultrfast acoustic pulses and discuss the challenge of controllable emission for ultrafast acoustic pulses. Firstly, we introduce optical emission schemes of ultrafast acoustic pulses (Fig. 1), where photoacoustic transducers of metal and semiconductor thin-film heterostructures are simultaneously excited by ultrafast laser pulses. We discuss the optical detection schemes of ultrafast acoustic pulses (Fig. 2), including the surface optical detection scheme of acoustic echoes in nontransparent media and the optical tracking detection scheme of acoustic traveling waves in transparent media. Secondly, we comprehensively present the history of ultrafast acoustic pulses, tracing it back to Bell’s pioneering exploration of the photoacoustic effect in 1880. Milestones of modern ultrafast acoustic pulse research are provided in detail (Fig. 3). Then, we point out the limitations of conventional photoacoustic energy materials, which pose a challenge for further advancements in this research field. Subsequently, we discuss the extraordinary photoacoustic energy conversion potential of 2D semiconducting materials. Recent progress and examples of coherent acoustic phonon oscillations in 2D semiconducting materials and heterostructures are presented (Figs. 4-6). These studies demonstrate that 2D semiconducting materials and heterostructures can serve as new ultrafast photoacoustic transducers capable of efficiently emitting ultrafast acoustic pulses with controllable wave parameters such as tunable wavelength, frequency, and pulse width. Next, recent experimental progress of ultrafast acoustic pulses employing 2D semiconducting heterostructures is further introduced (Figs. 7-8). The controllable emission properties of ultrafast acoustic pulses in 2D semiconducting heterostructures are of broad significance for realizing new ultrafast acoustic pulse emission sources, enabling functional imaging with ultrahigh spatiotemporal resolutions. Finally, we look ahead to the future development and applications of ultrafast acoustic pulses.Conclusions and ProspectsAs a unique carrier of ultrafast energy and information, ultrafast acoustic pulses offer physical advantages such as ultrashort pulse width, nanometer wavelength, and high penetration depth compared to ultrafast laser pulses. When combined with ultrafast optical spectroscopic techniques, fundamental studies and applications of ultrafast acoustic pulses can boost the interdisciplinary development of ultrafast optics. Although 2D layered semiconducting materials have presented great potential in constructing efficient and controllable sources of ultrafast acoustic pulses, the physical mechanism of photoacoustic conversion in van der Waals heterointerfaces and ultrafast measurement techniques needs comprehensive demonstration. Achieving ultrafast acoustic pulses with wavelengths in the nanometer range and pulse widths in the sub-picosecond domain presents challenges in developing corresponding spatiotemporal imaging techniques and instruments. We believe that research into controllable emission of ultrafast acoustic pulses will be applied to the noninvasive evaluation of micro-devices and chips, which will eventually pave the way for functional imaging with ultrahigh spatiotemporal resolutions.

SignificanceCurrently, the rapid development of artificial intelligence (AI), cloud computing, mobile communications, the Internet of Things, and other fields has created a significant demand for advanced chips. Lithography is a core step in the manufacture of these chips, as its level directly determines the process and performance of the chip. The most advanced extreme ultraviolet (EUV) lithography machines currently use 13.5 nm light and are employed in high-volume manufacturing (HVM) of chips at 5 nm node and below. Throughout the production process, each step must be quantitatively measured to ensure that key parameters meet the process targets. While optical technology is predominant in semiconductor inspection and metrology, the sensitivity of traditional optical methods is gradually diminishing. Due to the substantial wavelength shift from 193 nm to 13.5 nm, EUV lithography necessitates new components and materials. Consequently, the detection of these components and research into material interactions need to be conducted anew under EUV light sources. Presently, EUV optical equipment mainly uses discharged produced plasma (DPP) and laser produced plasma (LPP) light sources. However, these plasma light sources have several drawbacks. They produce significant contamination, as plasma fragments can affect devices from the collection mirror to the sample, impacting their lifespan and operating environment. In addition, although plasma EUV light sources offer high power, its radiation of 4π solid angle leads to its low brightness, which will affect its application in high-resolution detection. Therefore, exploring new EUV light sources for quantitative detection is crucial. Since the first discovery of high-order harmonics (HHG) in 1987, extreme ultraviolet high-order harmonics (HHG-EUV) have been widely used in electron dynamics research and various spectroscopy and imaging studies due to its high coherence, short pulses, and high photon energy. High-order harmonics exhibit unique properties, such as good directionality, excellent temporal and spatial coherence, and a broadband spectrum ranging from extreme ultraviolet (XUV) to soft X-ray bands. This makes it feasible to use Table-top-terawatt (T3) lasers to obtain tunable coherent XUV and soft X-ray sources, which have become an important research tool in EUV lithography technology. Concurrently, research on the application of these ultrafast EUV light sources in lithography and semiconductor metrology is advancing rapidly.ProgressOur study reviews the development of high-repetition-rate and high-brightness ultrafast EUV light sources in recent years and their applications in semiconductor metrology. The high-order harmonic method can generate a single harmonic with a power of up to 12.9 mW in the EUV region, significantly expanding its application range. In the lithography process, the exposure step transfers patterns from the mask to the photoresist. During exposure, tiny mask defects can cause substantial changes in the critical dimension (CD) on the wafer. Defects above a certain size on the mask must be detected and repaired. Due to the high brightness, broad spectrum, and broad coherence of high-order harmonics, they currently have prospects in coherent diffraction imaging and coherent scattering imaging. Samsung has developed an EMDRS (extreme ultraviolet lithography mask defect review system) device to meet the review needs of EUV mask defects. Hyogo and RIKEN have jointly developed the HHG-CSM (HHG-coherent scatterometry microscope) device based on coherent scattering microscopy, which can observe line defects as small as 2 nm in mask inspection. Huazhong University of Science and Technology has proposed a new high-resolution mask defect detection method that can detect defects with high sensitivity and accurately inspect defects with higher resolution. In wafer inspection and metrology, inspection refers to detecting heterogeneities on the wafer surface or within the circuit structure, while metrology involves the quantitative description of structural dimensions and material properties on the observed wafer. KMlabs can perform interface detection within a certain depth range beneath the surface using coherent diffraction imaging. Researchers from ASML and Intel use 10-20 nm wavelength EUV scattering measurements to offer a promising next-generation metrology technology, expected to enable 3D nanometer-size measurements of transistors.Conclusions and ProspectsThe application of high-repetition-rate and high-brightness high-harmonic EUV light sources in semiconductor nanostructure detection has gradually emerged, with significant research results obtained in the actinic detection of mask defects and wafer metrology. The EUV light source generated by the high-harmonic process features a wide spectrum and high brightness, providing unique advantages in inspection and metrology. High-brightness EUV light sources enhance the detection resolution of EUV masks and wafer patterns, while multi-wavelength broadband spectra improve the accuracy of critical dimension measurement, overlay measurements, and complex three-dimensional transistor structures. This advancement is crucial for the development of future semiconductor processes.

SignificanceThe significance of ultrafast biophotonics lies in its ability to provide novel tools for exploring complex dynamic processes in living systems. Its extremely short timescales and high-resolution imaging capabilities enhance our understanding of the nature of life and drive advances in biomedicine and interdisciplinary integration. Ultrafast time-resolved technology captures transient changes in biomolecules and cells on femtosecond to picosecond timescales, which are challenging to observe under conventional conditions. Through ultrafast spectroscopy, time-resolved imaging, and related techniques, we can monitor critical events such as photophysical processes, energy transfer, and charge separation in real time, revealing the microscopic mechanisms of life activities. This understanding is crucial for comprehending the fundamental principles of living systems and exploring the molecular basis of disease development. Ultrafast biophotonics holds great promise in biomedicine. For example, in cancer diagnosis and treatment, ultrafast spectroscopy technology accurately distinguishes between the optical characteristics of normal cells and cancer cells, providing a reliable basis for early cancer detection. This technology helps to precisely identify tumor boundaries, improving surgical accuracy and success rates. It also enables real-time monitoring of drug distribution and metabolism in the body, supporting tailored treatment approaches. In drug development, ultrafast biophotonics detects structural changes in biomolecules and intermolecular interactions, which is critical for designing safer and more effective drugs. This technology also accelerates drug screening, improving the efficiency of research and development. In addition, ultrafast biophotonics offers a new perspective for biological research by enabling real-time observation of molecular-level changes during biological processes. This deeper understanding of chemical reactions and metabolic processes within organisms helps unravel mysteries in the life sciences and promises breakthroughs in biology. Research in ultrafast biophotonics has progressed through robust interdisciplinary integration, utilizing insights and methods from physics, chemistry, and biology. This integration requires collaborative efforts across disciplines to achieve significant breakthroughs. By combining principles from these diverse fields, ultrafast biophotonics not only drives innovation within its domain, but also catalyzes advances in related disciplines. Physics contributes fundamental theories and experimental techniques critical to understanding light-matter interactions at ultrafast timescales. Concepts such as nonlinear optics, femtosecond laser spectroscopy, and time-resolved imaging play a key role in elucidating biological processes with unprecedented temporal resolution. The application of these principles enables researchers to study molecular dynamics, protein folding kinetics, and cellular signaling pathways in real time. This capability is essential for unraveling the complexity of biological systems from the molecular to the cellular level. Chemistry provides essential tools for the synthesis of novel biomolecular probes and functional materials tailored for ultrafast biophotonic applications. Advances in synthetic chemistry facilitate the development of fluorescent dyes, quantum dots, and photonic crystals optimized for specific biological imaging modalities. Biology provides invaluable insight into the biological relevance and applications of ultrafast biophotonic techniques. By collaborating with biologists and biomedical scientists, photonics researchers gain access to biological samples, experimental models, and clinical data essential for validating their methods. This collaboration fosters a translational approach in which discoveries made at the bench are rapidly translated into clinical diagnostics and therapeutic strategies. In summary, the interdisciplinary nature of ultrafast biophotonics not only enriches the fundamental understanding across physics, chemistry, and biology, but also drives transformative advances in biomedical research and technology. By harnessing the collective expertise of these disciplines, researchers can address complex biological questions and accelerate innovations that benefit both scientific discovery and clinical practice. As the field continues to evolve, interdisciplinary collaboration will remain critical to shaping its trajectory and opening new frontiers in biophotonics and beyond. Ultrafast biophotonics plays a critical role in the study of living systems, the advancement of biomedical science, and the integration of diverse disciplines. Ongoing technological advances and in-depth research will continue to reveal the immense potential and value of ultrafast biophotonics in various fields. Therefore, summarizing existing research is essential to effectively guide future developments in the field.ProgressWe review the application and development of ultrafast biophotonics in biological systems. We summarize how ultrafast optical techniques detect processes occurring on picosecond to femtosecond timescales. By combining advanced microscopy imaging techniques with ultrafast methods, high temporal and spatial resolution can be achieved. We begin with the basic principles of pump-probe technology and two-dimensional infrared spectroscopy, highlighting their application to the detection of ultrafast biological processes. We then cover near-field ultrafast pump-probe technology, 4D optical microscopy, and 4D electron microscopy, emphasizing their high temporal and spatial resolution capabilities (Fig. 3). 4D optical microscopy allows long-term, high-resolution observation of mitosis in living cells (Fig. 6). Finally, we present various spectroscopic mechanisms resulting from nonlinear optical phenomena induced by ultrafast pulses. These mechanisms underlie multiphoton imaging techniques such as second harmonic generation (SHG) imaging, third harmonic generation (THG) imaging, two-photon fluorescence (2PF) imaging, three-photon fluorescence (3PF) imaging, and coherent anti-Stokes Raman scattering (CARS) spectroscopy. Typically, these imaging techniques can coexist in a multimodal microscope, providing multi-angle observation results for biological tissues (Figs. 11 and 12).Conclusions and ProspectsAs ultrafast techniques are increasingly integrated into biophotonics, the field will expand its research scope from single molecules to cells, tissues, and organs. As an emerging interdisciplinary field at the intersection of physics, chemistry, and biology, ultrafast biophotonics requires ongoing research and collaboration to advance its development and expand its applications in biomedical research and technology.

SignificanceAsynchronous optical sampling (ASOPS) based on pump-probe spectroscopy is a powerful technique for ultrafast optical measurements. It leverages linear scanning between different pulse sequences of dual-comb lasers to enable equivalent sampling of pump pulse signals by delayed probe pulses. This allows for the observation of transient responses in samples on a femtosecond or even sub-femtosecond scale. Traditional electronic systems are limited by the bandwidth of detector circuits, rendering them unable to directly detect instantaneous processes on picosecond or femtosecond time scale. Optical sampling technology overcomes this limitation by converting temporal resolution to spatial resolution, significantly expanding the effective bandwidth for sampling ultrafast processes. By utilizing equivalent sampling on ultrafast time scales, it reconstructs signals bypassing the limitations of electronic detection bandwidth, providing a simple and effective method for studying photoinduced transient properties of materials, such as carrier dynamics, surface acoustics, and ultrafast magnetodynamics. In addition, with scanning rates up to the kHz range, ASOPS pump-probe spectroscopy translates ultrafast behavior into a series of frames observable by humans, facilitating detailed analysis. Compared to other microscale spectroscopy techniques, such as electron spectroscopy, high-energy X-ray technology, and particle scattering spectroscopy, pump-probe spectroscopy requires less stringent experimental environments and sample preparation, making it a widely used in fields like femtosecond chemistry, physics, and biology.ProgressConventional ASOPS requires sampling across the entire pump pulse cycle, typically resulting in window lengths of nanoseconds. Although different transient processes require different time windows, most only need one or two hundred picoseconds to meet observation requirements, leading to a low duty cycle and a significant portion of irrelevant acquired data. Furthermore, the timing jitter caused by the laser drift of repetition rate on the femtosecond time scale is significant and complex active frequency stabilization devices are required, therefore increasing system cost and complexity. To improve sampling efficiency and address these limitations, researchers have developed various techniques based on traditional ASOPS in recent years, including electronically controlled optical sampling (ECOPS), optical sampling by cavity tuning (OSCAT), arbitrary detuning asynchronous optical sampling (AD-ASOPS), heterodyne interferometry via rep-rate exchange (PHIRE), variable repetition frequency ASOPS (VRF-ASOPS), and hybrid ASOPS. ECOPS enhances the effective duty cycle by actively manipulating the repetition rate. It uses two synchronized femtosecond lasers with identical repetition rates. A tunable phase signal modulates the probe laser while the pump laser maintains a constant repetition rate. This allows detection pulses to effectively “walk” across the pump pulse in the time domain through alternating modulation, enabling rapid and adaptable optical sampling. Since ECOPS involves undersampling at nonlinear intervals, interpolation for unequal time steps is necessary when frequency domain data is needed. Initial calibration is also required to adjust the scanning zero behavior. OSCAT offers a more cost-effective and compact solution compared to ASOPS and ECOPS. It uses a single femtosecond laser, splitting the same pulse into pump and probe beams with one beam experiencing a fixed delay introduced by a long fiber. The scanning range is determined by the repetition frequency tuning range and this fixed delay. While OSCAT has advantages in cost and compactness, its flexibility for different applications is limited by predefined scanning windows, and the asymmetry introduced by long fibers often requires additional dispersion compensation. AD-ASOPS employs two independent, free-running lasers for pump and probe, operating at different and potentially unstable repetition rates. It continuously monitors and evaluates the instantaneous frequency of the two lasers, identifying coincidence events where the pulses overlap. The result is reconstructed from these events through post-calibration, avoiding the complexity of traditional asynchronous sampling devices but introducing statistical data and longer sampling times. PHIRE is similar to OSCAT in that both input the laser output of a single comb into a Michelson interferometer with asymmetric fiber and use delay accumulation from long fibers to generate scanning pulse pairs. PHIRE inserts a modulator in the other arm to quickly switch repetition rates. VRF-ASOPS detects the instantaneous hot-electron excitation generated from a trigger sample, such as 100 nm platinum film, converting its corresponding repetition-frequency difference to response pulse pairs. This enables ASOPS measurement without stabilizers or feedback loops. The temporal resolution changes by adjusting the intracavity PZT. Hybrid ASOPS utilizes the high repetition frequency of the soliton microcomb to achieve parallel multi-point sampling of the frequency waveform generated by the measured radiation signal. This improves the acquisition rate without sacrificing resolution. The time-resolved spectra of hybrid ASOPS combine high resolution, high speed, and broad bandwidth, making it a powerful tool for exploring the complexities of materials at ultrafast timescales.Conclusions and ProspectsRecent years have seen increased attention to combining ultrafast spectroscopy and ultrafast microscopy. Ultrafast microscopy technology is based on the development of spectroscopy technology to study temporal and spatial dynamics simultaneously. Ultrafast microscopy, using ultra-short pulse sequences, provides high time sensitivity and high spatial resolution, key for studying zero-dimensional and one-dimensional materials, such as single molecules, metal nanoparticles, semiconductor quantum dots, or carbon nanotube, as well as local regions of two-dimensional heterogeneous chemical systems, such as organic semiconductor films, polycrystalline perovskite films, and biological media like cells or chromophores. With the mature development of ultrafast pump-probe spectroscopy, new ultrafast spectroscopy or microscopy technologies have emerged. These include the new time-domain stretch spectrometer combining dual-frequency-comb pump-probe technology and time-domain stretch spectrometer, attosecond-pump attosecond-probe based on full X-ray with sub-femtosecond resolution for understanding electron dynamics in quantum systems, and attosecond transient absorption caused by infrared pump and X-ray detection, paving the way for new imaging techniques in attosecond condensed matter system. The development of electrical phase-locking technology and optical frequency comb technology is pushing ASOPS pump-probe system towards miniaturization and lightweight, making them more reliable and cost-effective, and laying a solid foundation for further system improvements and developments.

SignificanceUltrafast laser processing exhibits several key characteristics, including high processing accuracy, broad applicability to various materials, and three-dimensional non-contact processing. It holds significant potential for applications in electronics, medical devices, and scientific research. Traditional laser processing with a single focus achieves sub-wavelength resolution but may not always meet processing requirements due to limitations imposed by the optical system and Gaussian mode field. Light field modulation technology enables the alteration of the light field distribution at the focal plane by modifying the amplitude, phase, polarization, and other aspects of the laser wavefront, thus achieving specialized processing. In this review, we provide an overview of light field modulation principles, research progress, and application status. We cover non-diffractive beam processing, multifocal parallel processing, patterned light field processing, vector light field processing, and spot morphology correction while discussing the associated problems, challenges, and prospects.ProcessIn section 2, we introduce the principles of light field modulation, including modulation processes, propagation methods, and calculation techniques. The modulation processes consist of phase modulation, amplitude modulation, and complex modulation, each with its advantages and disadvantages discussed in subsection 2.1. Subsection 2.2 presents three common propagation paths: objective focus path, spatial diffraction path, and direct projection path, as illustrated in Fig. 1. The main diffraction processes differ among these paths, affecting calculation complexity and capacity usage. In subsection 2.3, we discuss several calculation methods for holographic phase and amplitude, including iterative calculation, forward modeling, and deep learning algorithms. We also introduce a closed-loop feedback method proposed to optimize phase, addressing practical issues like optical system aberrations and optical axis alignment errors. Section 3, the core chapter, focuses on research progress in non-diffractive beam processing, multifocal parallel processing, patterned light field processing, vector light field processing, and spot morphology correction. We examine how light field modulation technology optimizes focal spot morphology, machining accuracy, and processing efficiency. Subsection 3.1 introduces non-diffractive beams, including Bessel beams, needle-shaped beams, and Airy beams. Bessel beams, known for their light intensity invariance and self-reconstruction, are widely used in laser perforation, cutting, and two-photon polymerization. By adjusting the aspect ratio, suppressing side lobes, and selecting appropriate laser parameters, we enhance Bessel beam processing quality (Fig. 2). Needle-shaped beams (Fig. 3) and Airy beams (Fig. 4) offer unique performance due to their specific light field distribution. Subsection 3.2 discusses the multifocal arrays generated by light field modulation technology, which significantly improves processing efficiency. Besides, we compare focus uniformity, spacing, quantity, design flexibility, and application across two multifocal holographic phase calculation methods: iterative calculation (Fig. 5) and forward modeling (Fig. 6). The processing efficiency can be further improved by directly generating a patterned light field for processing through light field modulation technology. Therefore, subsection 3.3 focuses on the design and optimization methods for flat-top beams (Fig. 7) and other two-dimensional or even three-dimensional light fields (Figs. 8 and 9). We analyze the advantages and limitations of patterned light fields and address the practical issues that need attention during processing. Section 3.4 introduces several typical vector light field modulation methods and their applications in laser processing, emphasizing improvements in processing efficiency and resolution, as demonstrated in Figs. 10 and 11. In subsection 3.5, we introduce the application of light field modulation technology for spot topography correction, which compensates for defocusing caused by the refractive index mismatch of the laser passing through the dielectric surface (Fig. 12). Additionally, we cover the use of time-spatial modulation technology in spot topography correction (Fig. 13).Conclusions and ProspectsLight field modulation technology enhances processing accuracy and efficiency compared to traditional single-point laser processing. With the continued exploration of laser processing principles and innovations in modulation technology, new opportunities and challenges arise. Further research is needed to fully leverage the advantages of light field modulation technology and overcome current limitations in laser processing.

SignificanceThe widespread application of micro/nano processing technology has sparked interest in femtosecond laser processing, particularly for the fabrication of small-sized, high-precision, and three-dimensional structures. One of its key advantages lies in its ability to achieve ultra-high resolution beyond traditional diffraction limits, owing to nonlinear absorption effects such as two-photon absorption and threshold effects of focal excitation. Moreover, the integration of technologies like light field modulation greatly improves processing efficiency. Unlike traditional serial processing, which involves point-by-point scanning to create predetermined structures, light field modulation can transform a single focus into a multi-focus array, surface light field, or volume light field. This enables the rapid exposure of specific structures and provides flexible solutions for processing complex and time-consuming structures. Additionally, spatial modulation and spatiotemporal focusing technology can further improve axial processing resolution. Due to the significant advantages of femtosecond laser processing, it has found widespread applications in various fields such as micromachines, micro-optical devices, chiral research, and biomedicine. 1) Various micro-robots with different driving mechanisms can be fabricated using femtosecond laser processing, including ultrasonically driven robots, chemically driven robots, optically driven robots, electrically driven robots, and magnetically driven robots. These diverse driving mechanisms ensure that micro-robots can move precisely in different situations, enabling functions such as drug delivery and environmental monitoring. 2) Micro-optical devices have demonstrated great potential in fields such as imaging, sensing, communication, and signal processing due to their lightweight nature and high design flexibility. Utilizing techniques like two-photon polymerization, scientists can fabricate customized micro-optical devices with specific functions, such as achromatic lenses and gradient refractive index lenses. These micro-optical devices can also be integrated with fibers, photonic chips, CMOS cameras, and other components to form complete micro-optical systems. 3) Chiral structures created through femtosecond laser direct writing exhibit higher chiral optical responses compared to natural chiral materials, significantly advancing the field of chiral photonics. To expand the scope of chiral research, planar chiral structures, stacked planar structures, and three-dimensional spiral structures have been fabricated using femtosecond laser processing, providing a solid foundation for studying light responses in the orbital angular momentum dimension. 4) In the biomedical field, femtosecond lasers can be employed to fabricate cell culture scaffolds or construct three-dimensional capillary networks, thus promoting advancements in cytology and tissue engineering.ProgressResearchers have conducted a series of studies to enhance the processing speed of femtosecond laser two-photon polymerization. The traditional point-by-point scanning method is a slow and inefficient serial processing approach. To achieve rapid exposure of specific structures, researchers have explored techniques such as adding diffractive optical elements or utilizing methods like light field modulation to convert a single focus into a multi-focal array, surface light field, or volumetric light field. Figure 2 shows schematic diagrams of different femtosecond laser processing systems, including systems with diffractive optical elements or spatial light modulators. However, the spatial light modulation method also has a significant drawback, which is the lack of axial energy distribution control. This results in the laser energy density at the defocus surface typically reaching the threshold of two-photon polymerization, making it challenging to control the thickness of the structure. To address this issue, a new temporal modulation dimension has been introduced, enabling the femtosecond laser to focus simultaneously in both temporal and spatial domains. This allows the laser energy density on the focal plane to meet the threshold for two-photon polymerization, while the energy remains insufficient on the defocus plane, thereby improving the axial processing resolution. The femtosecond laser’s outstanding 3D processing capability has led to its widespread use in fields such as micromachinery, micro-optical devices, chirality research, and biomedical applications. Figure 4 showcases the magnetic-driven micro spiral structure and photothermal-driven micro robot created through femtosecond laser processing. By adjusting processing parameters, it becomes easy to control structural parameters such as height, radius, pitch, and cone angle. Following this, nickel and titanium metals are deposited onto the structure’s surface using magnetron sputtering, providing excellent magnetic responsiveness and biocompatibility. For the photothermal-driven micro robot, a gradient distribution of cross-linking density within the hydrogel is initially formed to create a joint deformation unit, followed by achieving photothermal control through the deposition of silver nanoparticles. Figure 6 displays the applications of femtosecond laser technology in integrated optical devices, integrating micro/nano optical structures with fibers, photonic chips, and CMOS cameras. Figure 8 illustrates the helical dichroism of various chiral structures, encompassing planar chiral structures, rotational stacking of planar structures, and stereo structures. Additionally, it highlights the utilization of chiral spiral arrays in information display and encryption.Conclusions and ProspectsLooking back over the past few decades, femtosecond laser processing has made significant strides in academic research. However, due to its high cost, two-photon polymerization technology has not yet achieved widespread applications in the industry. In the future, femtosecond laser processing technology will continue to advance towards higher processing resolution, shorter processing time, lower costs, and a wider variety of applicable materials.

SignificanceTopological photonics represents a forefront research field that integrates topological concepts from condensed matter physics, particularly topological insulators, into optical systems. These insulators, originally derived from the quantum Hall effect, denote materials where electron conduction is confined to their surfaces, exhibiting immunity to dissipation and backscattering even amidst defects and disorder. Leveraging the ease of control in photonic systems, topological photonics structures provide a convenient experimental platform for investigating related topological phenomena from condensed matter physics, such as the integer and spin quantum Hall effects. Furthermore, topological photonics fosters exploration into pioneering topological concepts, including higher-order topological insulators, anomalous Floquet topological insulators, Anderson topological insulators, and fractal topologies. Photonics has introduced innovative physical phenomena to topology, such as non-Hermitian and nonlinear topological photonics. Integrating topological theory into optics has introduced a new dimension of freedom to optical systems, offering novel techniques for stable transmission and manipulation of light fields and quantum states in integrated photonic devices. This encompasses robust state transformations under topological protection, topologically safeguarded quantum entanglement states, and advancements in topological quantum computation. Femtosecond laser direct writing (FLDW) technology provides exceptional 3D direct writing capabilities, enabling the fabrication of three-dimensional optical waveguides with arbitrary cross-sections and the construction of complex waveguide arrays. Waveguide devices made with FLDW have become crucial research platforms for optical communication, optical quantum computing, quantum algorithms, optical quantum storage, optical quantum simulation, topological photonics, and non-Hermitian systems. Since the paraxial equation of electromagnetic wave propagation in waveguides mirrors the single-particle Schr?dinger equation, waveguide systems are highly suitable for studying various two-dimensional photon evolution and distribution problems. FLDW allows flexible regulation of the waveguide evolution path and effective refractive index, enabling precise control over light coupling between waveguide arrays to simulate particle propagation in periodic potential fields. Consequently, FLDW has demonstrated various topological photonic structures in recent years, including Floquet photonic topological insulators, anomalous Floquet photonic topological insulators, nonlinear topology, non-Abelian topology, higher-order topological insulators, and fractal topology. Moreover, FLDW can introduce scattering points and machine segmented or curved waveguides to manage waveguide losses, thereby facilitating precise control over non-Hermitian effects. This capability advances the study of topological properties in non-Hermitian systems and opens new avenues for applications.ProgressIn this review, we comprehensively summarize studies on three-dimensional waveguide topological photonic structures in glass fabricated by FLDW. It covers periodic lattices that preserve or break time-reversal symmetry, as well as non-Hermitian topological waveguide structures. Firstly, the mechanisms and types of waveguides fabricated by FLDW are introduced, along with methods to improve their performance (Fig. 1). Subsequently, we systematically discuss research progress in optical waveguide topological photonics that break time-reversal symmetry. This includes the design principles of Hamiltonians to introduce artificial gauge fields, achieve equivalent magnetic flux, and thereby break time-reversal symmetry. Various FLDW constructions are highlighted in the review, such as Floquet photonic topological insulators, anomalous Floquet photonic topological insulators, Anderson photonic topological insulators, “chain-driven” honeycomb lattices, fractal photonic topological insulators (Fig. 2), and Aharonov-Bohm cages (Fig. 3) in glass. Furthermore, we explore optical waveguide topological insulators based on chiral symmetry, including the one-dimensional Su-Schrieffer-Heeger (SSH) model, the Aubry-André-Harper (AAH) model (Fig. 4), and two-dimensional higher-order topological insulators (Fig. 5). We also demonstrate their applications in quantum information processing (Fig. 6). The introduction of gain-loss or nonreciprocal coupling into topological photonic structures through suitable design is proved crucial for both fundamental physics and applications. For instance, the review reveals how non-Hermitian lattice engineering can tune the topological properties of an open system (Fig. 7). Additionally, the interaction between non-Hermitian modulation and topological phases can create novel non-Hermitian topological materials. The review also unveils the non-Hermitian skin effect, which leads to the breakdown of conventional bulk-boundary correspondence and the introduction of a generalized Brillouin zone (Fig. 8). Furthermore, dynamically encircling exceptional points can achieve robust asymmetric mode transformation (Fig. 9), offering a new method for robust state transformation and manipulation in integrated photonic devices (Fig. 10).Conclusions and ProspectsFLDW excels in 3D direct writing, swiftly and precisely fabricating intricate topological photonic structures. It enables the exploration of topological phenomena in both Hermitian and non-Hermitian systems. Glass, commonly used for topological optical waveguide arrays, presents challenges due to its material properties. Achieving electro-optic, acousto-optic, and magneto-optic modulation is rather difficult, hindering the creation of a tunable topological photonic platform. Additionally, the absence of path-dependent gain in glass results in considerable transmission loss in studies of non-Hermitian topology, thereby limiting its application in quantum information processing. To overcome these hurdles, FLDW is expected to craft topological photonic structures in materials like phase-change materials or laser glass. Current topological insulators based on optical waveguide arrays support only single-mode waveguides. Developing multi-mode waveguides or those supporting orbital angular momentum modes instead of single-mode would be pivotal for advancing topological insulators in optical information processing. In summary, FLDW has become indispensable for preparing topological photonic structures. As FLDW and material research continue to advance, they will drive further progress in topological photonics.

SignificanceThe spectral band of 2-20 μm is defined as the mid-infrared (MIR) band, which is within the atmospheric transmission window and eye-safe range and plays an important role in environmental monitoring, optoelectronic countermeasures, lidar, and surgical operations. For example, MIR band lasers cover the absorption bands of many gas molecules, such as H2O, CO2, NH3, and O3, which can be employed in environmental monitoring and differential absorption lidars. Additionally, due to the large absorption coefficient and shallow penetration depth of 2-20 μm lasers in biological tissues, they are widely adopted in laser medical surgery. Currently, there are two major methods for generating MIR ultrafast lasers. The first is to directly generate MIR lasers based on the energy level structure of the gain medium itself, such as solid lasers, gas lasers, fiber lasers, and quantum cascade lasers (QCLs). The other is to adopt nonlinear frequency conversion technology to generate MIR ultrafast lasers, such as supercontinuum light generation, optical parametric processes, difference frequency, and four-wave mixing. QCLs are a new type of semiconductor laser, which enables semiconductor lasers to operate in the MIR band. The working principles of traditional semiconductor lasers and QCLs are shown in Fig. 1. QCLs are characterized by compact structure, easy wavelength tuning, and long lifetime. However, their manufacturing process is complex and expensive, and features high environmental requirements and poor beam quality, thus limiting its application range. Its application is limited to long-distance remote sensing detection and optoelectronic countermeasures. The core technology of mid-infrared solid-state lasers is to employ laser crystals as gain media, to dope impurities in the crystals to change their energy level structure, and finally to achieve light amplification via the cavity to produce MIR laser output. Among them, the main doped impurities are rare earth ions (Tm3+, Ho3+, Er3+) and transition metals (Cr2+, Fe2+). As shown in Fig. 2, it shows the energy level transition of rare earth doped ions. The band generated by MIR ultrafast solid-state lasers is in the range of 2-5 μm. MIR solid-state lasers have the characteristics of high conversion efficiency and high stability, but due to the immature material processing and preparation process of laser gain media, the laser will produce thermal effects and quantum losses during the oscillation process, with limited output wavelength. MIR solid-state lasers realized by doping transition metals into crystals have advantages in outputting high-power pulses while operation at high temperatures will reduce the luminescence life of the doped crystals. MIR lasers generated based on nonlinear frequency conversion technology mainly utilize nonlinear effects in nonlinear crystals. These include difference frequency generation (DFG), optical parametric amplification (OPA), optical parametric generation (OPG), and optical parametric oscillation (OPO) (Fig. 3). Among them, DFG combined with a high-stability near-infrared ultrafast fiber laser as a pump and signal source is more conducive to the miniaturization and high stability of MIR ultrafast lasers. Additionally, the intra-pulse difference frequency generation (IP-DFG) technology based on a short-wavelength ultrafast fiber laser source is a simple nonlinear frequency conversion method. We describe the generation of MIR lasers, the basic principles and research progress of IP-DFG technology, and the application of ultrafast MIR lasers. Finally, the future development and application prospects of the IP-DFG system are presented.ProgressThe IP-DFG process (Fig. 9) mainly employs an ultra-wide light source that can cover the range of both signal and pump light sources to directly complete the difference frequency process in the nonlinear crystal to generate MIR laser pulses. If the spectral width of the incident light source is not wide enough, a nonlinear compression scheme must be added before the difference frequency stage. The wide spectrum light source is mainly obtained by spectral broadening in positive dispersion fiber, soliton self-compression in negative dispersion fiber, Kerr lens mode-locked direct output, and other methods. Subsequently, we summarize the nonlinear coefficient, transparency range, thermal conductivity, damage threshold, and other physical and nonlinear optical properties of commonly adopted nonlinear crystals, such as PPLN, GaSe, OP-GaP, CSP, and LGS. Then, we analyze both the detailed properties of different crystals and research the progress of IP-DFG based on these crystals to generate corresponding ultrafast MIR lasers in detail. Finally, the advantages and applications of IP-DFG technology based on ultrafast fiber lasers as the light source are summarized. Meanwhile, we expect to adopt ultrafast fiber lasers as the light source based on the cascaded IP-DFG technology to control the new MIR few-cycle pulse waveform by adjusting the carrier-envelope phase (CEP) shift frequency of the driving pulse.Conclusions and ProspectsIP-DFG system based on fiber lasers as the driving light source has a compact structure, high stability, and low cost. As high-energy ultrashort pulse fiber laser technology becomes increasingly mature, it is very helpful to enhance the nonlinear effect. Therefore, utilizing a wide-spectrum femtosecond laser as a driving source is an important direction for the future development of this field. Additionally, the MIR light source generated by IP-DFG in an ultrafast fiber laser features a wide spectrum, high signal-to-noise ratio, strong coherence, and large pulse energy, with irreplaceable special applications in environmental detection, medical treatment, and industrial processing. With further exploration and research by scientific researchers, the spectrum of MIR lasers will be wider, with narrower pulse, higher peak power, and broader application prospects.

SignificanceNonlinear optical crystals exhibit secondary or higher-order nonlinear optical effects under strong laser fields and are commonly used for wavelength/frequency conversion of lasers. They have played a vital role in various fields, such as nonlinear optics, laser communication, laser technology, and military medicine. Common nonlinear optical crystals include lithium niobate (LiNbO3, abbreviated as LN), lithium tantalate (LiTaO3, abbreviated as LT), and potassium titanium phosphate (KTiOPO4, abbreviated as KTP). Key indicators for evaluating the performance of nonlinear crystal materials include large nonlinear optical coefficients, wide transparency range, and high damage threshold. Modulation of nonlinear coefficients is crucial for enhancing the performance of these materials. For instance, by periodically adjusting the nonlinear coefficients (i.e., spontaneous polarization or ferroelectric domains) of the multifunctional ferroelectric LiNbO3 crystal, nonlinear photonic crystals (NPCs) could be fabricated, significantly improving the conversion efficiency of processes like frequency doubling and spontaneous parametric down conversion. Currently, the primary techniques for fabricating NPCs in LiNbO3 crystals are electrical poling and optical poling. While electrical poling yields excellent performance and is commercialized, its technological process is complex and requires custom masks for each NPC design, increasing manufacturing costs. Traditional electrical poling techniques have demonstrated unique strengths in creating one-dimensional (1D) and two-dimensional (2D) NPCs. However, it encounters significant hurdles when attempting to fabricate three-dimensional (3D) NPCs, presenting substantial experimental challenges within the field of nonlinear optics over the past two decades. There are two primary types of optical poling techniques: light-assisted poling and all-optical poling. Light-assisted poling is characterized by the use of ultraviolet laser irradiation to facilitate the poling process. In contrast, all-optical poling employs either continuous or pulsed lasers to accomplish the poling procedure. By focusing a femtosecond laser into LiNbO3 crystals using a lens or microscope objective, the material's nonlinearity at the focal point can be effectively modulated. If material nonlinearity is modulated throughout a three-dimensional space, the fabrication of 3D NPCs can be achieved. This all-optical poling technique has become extensively adopted in precision material processing, referred to as femtosecond laser direct writing. The utilization of femtosecond laser direct writing overcomes the obstacles associated with manufacturing 3D NPCs, significantly contributing to achieving high-efficient nonlinear frequency conversion and nonlinear beam shaping.ProgressFemtosecond laser direct writing technique offers rapid processing speed, high processing accuracy, and flexibility, enabling fast and efficient fabrication of 3D micro-/nanoscale photonic structures. The modulation of nonlinear optical coefficients depends not only on the selected femtosecond laser parameters but also on the intrinsic properties of nonlinear optical crystals, such as bandgap and dispersion. In 2018, with femtosecond laser direct writing technique, Xu et al. fabricated 3D NPCs in ferroelectric barium calcium titanate (Ba0.77Ca0.23TiO3, abbreviated as BCT) crystals by inducing ferroelectric domain inversion (Fig. 1) using femtosecond laser direct writing. Similarly, Wei et al. induced ferroelectric domain erasure in LiNbO3 crystals to fabricate 3D NPCs (Fig. 7). These studies mark the earliest reports on 3D NPC fabrication using femtosecond laser direct writing, turning the exploration of 3D NPC capabilities into a research focus in nonlinear optics. In the past five years, the notable achievement of femtosecond-laser-induced nanodomains has significantly advanced the field of femtosecond laser modulation of ferroelectric crystal domains (Fig. 3). Additionally, in the domain of femtosecond laser modulation of quartz-crystal nonlinear coefficients, the introduction of the additional periodic phase (APP) theory (Fig. 11) has enabled the generation of high-efficiency deep-ultraviolet lasers. Furthermore, the experimental demonstration of second-to-fifth harmonic generations (Fig. 12) marks substantial progress. The utilization of femtosecond laser direct writing technique in fabricating NPCs has overcome a significant challenge in producing 3D NPCs in nonlinear optics over the previous two decades. This development has not only extended the application range of femtosecond laser direct writing but also reinforced its crucial role in nonlinear optics.Conclusions and ProspectsWith femtosecond laser direct writing, we can periodically modulate the optical nonlinearity of nonlinear crystals leading to the formation of 1D, 2D, and 3D NPCs. This capability is of great significance for achieving high-efficient nonlinear frequency conversion and nonlinear beam shaping applications. Our study succinctly summarizes recent advancements in modulating the nonlinear properties of ferroelectric and quartz crystals using femtosecond laser direct writing. For ferroelectric crystals, the main focus lies on domain inversion, domain erasure, and domain modification. We elucidate the connections and differences among these processes and highlight their potential applications in nonlinear frequency conversion, nonlinear holography, and nonlinear beam shaping. In the case of quartz crystals, we primarily utilize APP phase matching as a key technique to introduce the applications of laser-written APP quartz crystals. These applications include the generation of ultraviolet lasers and the production of high-order harmonics. As our understanding of the interactions between femtosecond lasers and nonlinear crystals deepens, along with the emergence of new nonlinear optical materials and beam-shaping-based femtosecond laser direct writing, further groundbreaking results are anticipated in the field of femtosecond laser modulation of crystal optical nonlinearity.

SignificanceSuperintense ultrafast laser pulses, characterized by their extremely short duration and high instantaneous power, generate extreme physical conditions, including ultrafast temporal scales, ultra-high energy densities, and ultra-strong electromagnetic fields. These pulses represent a cutting-edge scientific frontier with profound implications for fundamental science, life sciences, national defense, etc. Since the advent of lasers, researchers have strived to produce superintense ultrafast pulses with narrower pulse widths and higher peak powers. The development of mode-locking techniques, Ti-sapphire gain media, prism pairs, gratings, and other dispersion compensation techniques has made sub-100 fs pulses achievable. To further reduce femtosecond pulse widths, more precise dispersion compensation is required. Chirped mirrors have become essential for generating sub-10 fs pulses. In 2001, leveraging advanced chirped mirror technology, the Vienna University of Technology in Austria first generated and measured isolated attosecond pulses from femtosecond pulse lasers interacting with noble gas targets, marking the beginning of the attosecond era. In addition to efforts to shorten pulse widths, increasing peak pulse power has been a major goal. The advent of chirped pulse amplification (CPA) and optical parametric chirped pulse amplification (OPCPA) has enabled the production of petawatt (PW)-femtosecond (fs) pulses. Peak power has rapidly exceeded 4.2 PW-20 fs and even 10 PW-25 fs, creating unprecedented extreme physical conditions. As laser technology has evolved, ultrafast laser coating technology has evolved with it, and vice versa. Ultrafast laser coatings are crucial for guiding laser beams and managing dispersion within laser systems. Their performance metrics including reflectivity, bandwidth, dispersion control, and laser damage resistance significantly affect the performance of ultrafast laser pulses. This area continues to be a dynamic research focus. By employing gradual changes in optical thickness in layer structures, ultrafast laser coatings achieve wide bandwidth with high reflectivity while allowing the light of different wavelengths to propagate through different optical paths, thus providing precise dispersion compensation. This capability is critical for superintense ultrafast pulse technology. As ultrafast lasers approach petawatt and exawatt peak powers and pulse widths approach single optical cycle, the performance requirements for bandwidth, dispersion control, reflectivity, and damage thresholds become more stringent. Enhancing the overall performance of ultrafast laser coatings in these areas is vital for the generation of superintense ultrafast and attosecond pulses and is therefore a focal point of research.ProgressWe provide a comprehensive review of advances in ultrafast laser coatings, focusing on systems such as superintense ultrafast lasers and attosecond lasers. Our review addresses performance requirements related to damage thresholds, operational bandwidth, reflectivity, and dispersion control. We detail the fundamental design principles of ultrafast laser coatings that effectively balance these parameters. To address the challenge of dispersion oscillation in ultrafast laser coatings, we present detailed principles and methods for suppression. In response to the demand for broad bandwidth and high threshold in ultrafast laser coatings, we discuss theoretical design methods for improving broadband thresholds. We outline high-threshold deposition techniques, high-precision film thickness monitoring, and group delay dispersion (GDD) testing techniques for ultrafast laser thin films, providing a basis for their precise preparation and accurate performance testing. Regarding the damage characteristics of ultrafast laser coatings, we explore nonlinear optical effects under ultrashort pulses, describe the degradation patterns of coating performance before catastrophic damage, and discuss damage morphology, evolution processes, and phase damage mechanisms. Finally, we summarize the applications of ultrafast laser coatings in superintense ultrafast lasers, attosecond lasers, compression systems, and mid-infrared ultrafast laser systems.Conclusions and ProspectsThe advancement of ultrafast laser technology is creating new demands and challenges for ultrafast laser coatings, while advances in coating technology are driving the development of laser technology. Future research will focus on a comprehensive approach that includes theoretical design, precision manufacturing, damage characteristics, and system applications of ultrafast laser coatings.

ObjectiveWith the development of ultrafast laser technology, especially the generation of high power few-cycle femtosecond laser pulses with their carrier-envelope phase (CEP) locking, attosecond laser pulses based on high-order harmonic generation (HHG) via interaction between high power femtosecond lasers and noble gas have become a leading direction in ultrafast sciences. In 2001, the attosecond pulse train and isolated attosecond pulse (IAP) were realized successively by Pierre Agostini’s team and Ferenc Krausz’s team respectively, and the generation and measurement of attosecond pulses made scientists master a powerful tool to look into ultrafast processes inside materials at unprecedented precision, thus giving birth to a new field and research direction: attosecond science. Characterized by unique specifications such as very short pulse duration with high photon energy and sound coherence, the attosecond light beam provides unprecedented new tools that can go deep into the interior of atoms to measure their electronic dynamic behavior, thus triggering a revolution in basic research. Therefore, the 2023 Nobel Prize in Physics was awarded to three physicists, Pierre Agostini, Ferenc Krausz, and Anne L’Huillier, for their pioneering contributions to the experimental method of generating attosecond light pulses to study the electron dynamics in matter. Many countries have invested a lot of funding to carry out relevant studies. In particular, Europe soon established the attosecond alliance. Meanwhile, Ferenc Krausz led the establishment of the Max Plank center for attosecond science including scientists from China, Japan, the Republic of Korea, Australia, and other countries. Meanwhile, it is worth mentioning that under the proposal of G. Mourou and other scientists, the European Union started to conduct a research program on the extreme light infrastructure (ELI) early in 2006. This program included the ELI-attosecond light physical science (ELI-ALPS) facility located in Hungary with attosecond light sources as the main beams, in which attosecond beams with different specifications, high harmonics, terahertz, and ion electron beams were contained. In 2019, partial beams were completed and open to users. In China, theoretical and experimental studies on attosecond pulse generation and application are also significant. The first IAPs were demonstrated by the Institute of Physics, Chinese Academy of Sciences (CAS) in 2013. Subsequently, Xi’an Institute of Optics and Precision Mechanics of the CAS, the National University of Defense Technology, and Huazhong University of Science and Technology also obtained IAP generation. To better realize the application of attosecond light pulses in multiple fields and make them serve more users, in September 2017, the Institute of Physics, CAS built a large ultrafast dynamic and image system as one of the four extreme conditions of the “Synergetic Extreme Condition User Facility (SECUF)”, with the support of National Major Basic Science Infrastructure Projects. The attosecond laser station is responsible for the generation and application of IAPs based on high harmonic generation with a pulse duration less than 100 as in extreme ultraviolet (XUV) and is equipped with time-resolved angle-resolved photoelectron spectrometer (ARPES), photoelectron microscope (PEEM), cold target recoil-ion momentum spectrometer (COLTRIMS) and other terminal devices. Thus, this provides users time resolution from attosecond to femtosecond and momentum, and energy resolution measurements are conducted for studying ultrafast dynamics of physical, chemical, and biological materials on an atomic scale.MethodsAccording to the construction content and goal of attosecond laser stations, we have constructed four attosecond and femtosecond light sources in the XUV range. Each beam contains an XUV light source designed specifically for application experiments with its end station. The first beam employs the high-energy few-cycle fs laser as the driving laser to produce broadband XUV isolated attosecond light for attosecond photoelectron spectroscopy and attosecond transient absorption spectroscopy. This will be adopted to study the electron dynamics in atoms, molecular and condensed matter by utilizing attosecond streaking and transient absorption. The second XUV beam leverages the high repetition rate and high power fs laser as the driving laser to produce narrowband femtosecond XUV pulses for time-resolved ARPES to study the electronic dynamics on the timescale of fundamental correlations and interactions in condensed matter. The third beam adopts high-energy few-cycle fs lasers at 10 kHz to produce broadband XUV pulses for attosecond coincidence spectroscopy in a COLTRIMS to research the ultrafast dynamics and reactions in atomic and molecular systems. The fourth beam utilizes high repetition rate few-cycle lasers to produce broadband attosecond XUV pulses for time-resolved PEEM to study the ultrafast dynamics of plasmons in nanostructures and surfaces of solid materials with high temporal and spatial resolutions simultaneously.Results and DiscussionsAfter five years of development of four XUV beams and their end station, the attosecond laser station can output XUV coherent radiation with photon energy of nearly 100 eV and IAPs with pulse width of 86 as. The time-resolved ARPES beam employs a femtosecond laser with an average power of 280 W in 57 fs at repetition rate of 500 kHz to produce high-order harmonics of 20-50 eV. A narrowband high-order harmonic light source is selected by a monochromator and combined with a femtosecond IR laser to form a pump-probe pulse, which is jointly focused on the ARPES sample. The generated photoelectrons are detected by an energy analyzer. By scanning the precise delay between the pump light and detection light, the time resolution is demonstrated to be 125.75 fs, with the energy resolution of 43.9 meV. The minimum temperature of the sample is 3.8 K. As for PEEM, the spatial resolution of PEEM with 21.6 eV high harmonic light source generated by few-cycle lasers at 100 kHz is demonstrated to be 20 nm. As for COLTRIMS, the momentum resolution of electrons is 0.03 a.u., and the momentum resolution of ions is 0.04 a.u. All four beams in the attosecond laser station work normally and are provided for users. For further development, we will develop the sub-50-as XUV light source using mid-IR fs lasers at 2.2 μm. We will also develop time-resolved ARPES based on TOF (time-of-flight spectrometer)-ARPES and momentum microscopes to realize fs resolved, high energy resolved, and spatial resolved measurements.ConclusionsThe attosecond laser station, which has four XUV beamlines with its own end station, was built in Huairou District, Beijing as a condition of SECUF. Based on few-cycle femtosecond pulses and HHG technology, the experimental station can provide IAPs with a pulse duration of less than 100 as in the XUV range. Meanwhile, time-resolved ARPES, PEEM, COLTRIMS, and other end stations are included to help users study fundamental ultrafast processes in physics, chemistry, biology, and material sciences with temporal resolution from femtosecond to attosecond on atomic scale.

ObjectiveHigh-average-power and high-repetition-rate femtosecond fiber lasers are widely used in industrial and scientific domains. However, the presence of excessive nonlinearity and transverse mode instability poses a constraint on further scaling of energy and power within the fiber lasers. Coherent beam combination (CBC) has risen as a viable solution to the constraint, enabling the extension of average power and pulse energy limits while preserving beam quality. Under ideal conditions, the laser power and energy of the combined beam from N channels can reach N times that of a single amplifier. However, in real-world applications, variations in beam quality and discrepancies in the spatiotemporal properties of the beams result in power losses during the combining process. These losses are quantified by the combining efficiency. Recently, an average power of 1 kW with a pulse energy of 1 mJ and an average power of 1 kW with a single pulse energy of 10 mJ are achieved through 8-channel and 16-channel fiber laser CBC, respectively. Yet, the proliferation of combination paths not only increases system complexity but also affects stability. Moreover, due to gain narrowing and incomplete dispersion compensation, compressed pulses typically exceed 300 fs, presenting hurdles in achieving Fourier limit pulse duration. Although methods such as spectral shaping and post-compression can further shorten the pulse duration, these methods undoubtedly further increase the complexity and operational difficulty of the system. Hence, we focus on reducing the number of combination paths and improving dispersion compensation to achieve clean ultrashort femtosecond pulses while maintaining the existing power level.MethodsTo streamline the number of combination paths, enhancing the power of a single amplifier is crucial. By boosting the power of a single amplifier beyond 200 W, the number of required combination channels can be reduced by a factor of 2 to 3. An ultrafast femtosecond fiber system comprising four coherently combined large mode-area rod-type photonic crystal fibers as the main amplifier is constructed. To ensure high beam combining efficiency and subsequent high-quality pulse compression, it is necessary to strictly control the power of each amplifier to ensure the same B-integral. Meanwhile, due to the effect of nonlinear polarization rotation, a large amount of laser power cannot participate in beam combination. Circularly polarized amplification is an effective method to minimize nonlinear phase accumulation and ensure high beam combining power. The phase stabilization is achieved using H?nsch-Couillaud (HC) detectors after beam combination. While spectral pre-shaping of seed light is effective for optimizing the duration of compressed pulses, its practical operation is cumbersome. Hence, we use the tunable pulse stretcher (TPSR) to pre-compensate the dispersion of the seed light. This matches the pre-compensate dispersion with the accumulated dispersion during subsequent amplification and compression processes, further optimizing the pulse duration.Results and DiscussionsThe average output power of each channel ranges from 215 to 222 W, with a fitted slope efficiency exceeding 65%. The maximum discrepancy between the channels is no greater than 3%. This precision enables our four-channel CBC system to achieve an output power of 776 W with a pulse energy of 0.97 mJ, and a combination efficiency of 89%. The active phase stabilization system ensures excellent power stability for the four-channel CBC fiber lasers, with a root mean square of 0.59%. Despite the differences in the output beam profiles of different channels, the combined beam still exhibits a circular Gaussian profile. The beam quality is analyzed by M2 measurement using the 4σ-method showing an almost diffraction limit beam quality of M2<1.25 on both axes. Remarkably, we accomplish these results using only four amplifiers, whereas previous researchers required eight or more, effectively reducing system complexity. After compression by gratings, the combined beam exhibits a pulse duration of 445 fs and significant high-order dispersion residue, as shown in Fig. 6. By optimizing dispersion, particularly high-order dispersion, using TPSR, we reduce the pulse duration to 227 fs and significantly increase the proportion of main pulse energy, as illustrated in Fig. 7. Spectral phase analysis shows a significant reduction in second to fourth order dispersion. The compression efficiency reaches 93.3%, with compressed power at 724 W and pulse energy at 0.9 mJ.ConclusionsWe present an ultrafast femtosecond laser system based on CBC. The system achieves an average power of 724 W and a pulse energy of 0.9 mJ through CBC of four channels. This approach effectively overcomes the power limitation of a single rod-type photonic crystal fiber. By employing an active phase stabilization system, the combination efficiency of the system reaches 89%, while the combined beam maintains good beam quality with M2<1.25. Furthermore, the use of TPSR for pre-management of pulse dispersion enables precise compensation of the residual dispersion after pulse compression, successfully optimizing the pulse duration full width at half maximum (FWHM) from 445 to 227 fs. The system demonstrates the effective potential of coherent beam combining in achieving high average power and large pulse energy femtosecond lasers. In the future, it is expected that by increasing the chirp broadening of pulses, reducing the repetition rate, and incorporating spatial-temporal coherent beam combining, the peak power and energy of laser pulses can be further enhanced.

ObjectiveExtreme ultraviolet (XUV) optical frequency combs have significant applications in precision measurement physics and ultrafast science. They are essential tools for high-resolution XUV spectroscopy, such as precise spectroscopic measurements of few-electron atomic or ionic systems (e.g., He+ and Li+), which probe the limits of quantum electrodynamics (QED) theory. Combining XUV optical frequency comb spectroscopy with thorium-229 nuclear transition could advance the development of a novel optical clock—the nuclear optical clock. In ultrafast science, utilizing the high-order harmonic generation (HHG) process inherent in XUV optical frequency comb generation reveals ultrafast phenomena under conditions of extremely high repetition rates (>50 MHz). In addition, XUV optical frequency combs provide high-repetition-rate and low-flux XUV light sources, which facilitate time- and angle-resolved photoelectron spectroscopy studies. The spectral coverage and output power are critical parameters for assessing the performance of an XUV optical frequency comb and realizing its broad applications. Extending its spectral coverage and increasing the output power are core objectives in developing XUV optical frequency combs and in the present study.MethodsTo optimize the key parameters of an XUV optical frequency comb, we enhance the peak power within the femtosecond enhancement cavity (fsEC) by compressing the pulse duration of the driving laser source. Firstly, we utilize a solid-core photonic crystal fiber for spectral broadening. The driving laser, assisted by our self-constructed beam stabilization system, is coupled into the solid-core photonic crystal fiber. The fiber’s photonic bandgap structure enables high-efficiency transmission of specific wavelengths through the solid fiber core, resulting in spectral broadening via the self-phase modulation (SPM) effect. We then use multiple chirped mirrors for compressing pulse duration. The multilayer structure of the chirped mirrors ensures precise dispersion compensation by providing different group delays for different frequency components. Finally, in the high-order harmonic generation and output coupling stage, we employ a bow-tie-shaped fsEC with six mirrors and in-cavity focusing to increase the peak power of the driving laser. The amplified driving laser pulses interact with the gaseous medium at the focal point, generating XUV light, which is then coupled out by either an Al2O3 Brewster plate or a micro-nano grating mirror (GM).Results and DiscussionsOur driving laser has a repetition rate of approximately 80 MHz, a maximum output power of 100 W, a central wavelength of 1035 nm, and a pulse duration of 490 fs. Fig. 2(a) shows the spectral broadening achieved with the solid-core photonic crystal fiber, and Fig. 2(b) shows the pulse shapes before and after compression. We successfully compress the pulse duration from 490 fs to 56 fs using chirped mirrors. With the aid of the fsEC and an amplification factor of over 100, we achieve a peak intensity of 9.3×1013 W/cm² at the focal point. Subsequently, we use an Al2O3 Brewster plate to couple out the XUV radiation and image the harmonic profile on a sodium salicylate plate. Figs. 5(a)-(c) show the fluorescence images for Xe, Kr, and Ar as the gaseous medium, respectively. The discrete spots on the fluorescence screen correspond to different harmonic orders. From the fluorescence images, we determine that the highest harmonic order generated is the 35th, corresponding to a photonic energy of about 42 eV (approximately 30 nm in wavelength). By replacing the output coupling device with a GM, we measure the power of the 17th harmonic (61 nm) and obtain a maximum average power of 9.3 μW when using Xe as the working gas, as shown in Fig. 6.ConclusionsXUV optical frequency combs are high-quality, narrow linewidth, table-top coherent XUV light sources. In this study, we aim to expand the spectrum and enhance the power of the XUV optical frequency comb by compressing the pulse duration of the driving laser and increasing the peak power within the femtosecond enhancement cavity. Using a solid-core photonic crystal fiber for spectral broadening and chirped mirrors for precise dispersion compensation, we achieve a pulse duration of about 56 fs and a peak intensity of 9.3×1013 W/cm2 within the fsEC. Using high-order harmonic generation processes with noble gases such as Xe, Kr, and Ar, we achieve an XUV optical frequency comb with the shortest wavelength of 30 nm (35th harmonic) and an output power of 9.3 μW at 61 nm (17th harmonic). This study lays a solid foundation for the succeeding precision spectroscopic measurements of few-electron atoms and molecules using the XUV optical frequency comb.

ObjectiveWith the rapid development of laser technology, the generation of isolated pulses with a timescale down to 43 as is achieved. This breakthrough enables the exploration of electron dynamics on an ultrashort timescale. One promising method for investigating the sub-femtosecond dynamics of electronic systems is the attosecond transient absorption spectrum (ATAS). In the previous study, the electron dynamics in atoms are investigated using ATAS, and the offset time in the Autler-Townes split is observed. Recently, the research on ATAS is extended to graphene, where we also observe the offset time in the fishbone structure of ATAS in graphene. In this work, we will reveal the underlying mechanism of the offset time in graphene.MethodsGraphene is a two-dimensional single layer of carbon atoms arranged in a honeycomb lattice structure. In this study, we consider four-energy bands of graphene to calculate the time-dependent density matrix equations. To uncover the underlying mechanism of the offset time, we simplify the two-dimensional four-band model into a single-electron model located at the Van Hove singularity on the one-dimensional two-band structure. Using this simplified model, an analytical model of offset time is established.Results and DiscussionsOur numerical simulation results reveal the fishbone resonance structure around the Van Hove singularity points, and the offset time is observed. Using the simplified model, our numerical results for ATAS are qualitatively consistent with those calculated by the four bands model. This consistency suggests that the simplified model is a viable tool for investigating the offset time. Based on the simplified model, we derive an analytical model of the offset time, and the offset time predicted by the analytical model is qualitatively consistent with the simulation results of four-band density matrix equations. We also extend our calculation to more pump laser wavelengths and laser pulse periods, and the simulation results indicate that the analytical model can predict the outcome of numerical simulations.ConclusionsWe numerically simulate the ATAS of graphene using the four-band density matrix equations of graphene. Our simulation results reveal the existence of the offset time in zero-order fringes of the fishbone structure similar to the atomic offset time. To reveal the underlying mechanism of the offset time, we simplify the four-band model to a single-electron model located at the van Hove singularity on the one-dimensional two-energy band, and establish an analytical model of the offset time based on this model. The offset time predicted by our analytical model can qualitatively predict numerical simulation results under different laser wavelengths and laser pulse periods. Our analytical theory can be verified experimentally, facilitating the study of ultrafast electron dynamics in two-dimensional crystals.

ObjectiveIn the context of previous research in strong-field physics, the laser field has frequently been regarded as a classical field, largely overlooking the quantum effects of the field. The advent of specific quantum optical technologies has resulted in the emergence of quantum light sources that meet the requisite standards for strong-field physics. Consequently, there is a necessity to consider the quantum effects of the laser field. Recent studies have demonstrated that strong-field processes driven by quantum optical sources exhibit new phenomena. For example, high harmonic driven by squeezed state light is squeezed light. However, there are numerous phenomena related to strong-field physics under quantum optical drive remain unexplored, and significant gaps in our understanding of the quantum effects involved persist. This study examines the momentum distribution and energy spectrum of hydrogen atoms under coherent state light (quasi-classical light) and bright squeezed vacuum state light (quantum light lacking a classical counterpart), providing a comprehensive investigation of the phenomenon of above-threshold ionization.MethodsIn the interaction between atoms and quantum light, the density matrix of the atom-quantum light system satisfies the complete quantum time-dependent Schrödinger equation. The linear nature of the density matrix, as dictated by the Schrödinger equation, allows any quantum light field to be decomposed into a linear combination of coherent states. This decomposition is achieved by transforming the Schrödinger equation into a summation of solutions corresponding to coherent state light. Consequently, the solution of the density matrix is a non-coherent superposition of the solutions of the Schrödinger equation for coherent state light. By separately solving the Schrödinger equation for each coherent state, the Schrödinger equation for the density matrix of the atom-quantum light system can be indirectly obtained. This approach effectively employs numerical methods to explore the three-dimensional time-dependent Schrödinger equation and investigate phenomena such as strong-field ionization of hydrogen atoms under the influence of a quantum light field.Results and DiscussionsWe demonstrate the broadening effect observed in the photoelectron spectrum under the influence of bright squeezed vacuum state light. It combines the quantization of the light field with solutions derived from the time-dependent three-dimensional Schrödinger equation for hydrogen atoms (Figs. 2 and 5). Additionally, this work discusses and elucidates this phenomenon (Figs. 4 and 6). Our investigation delves into how the quantum properties of the light field impact the interference structure observed in the photoelectron momentum distribution. Our findings highlight that when exposed to bright squeezed vacuum state light, the photoelectron energy spectrum exhibits higher cutoff energy compared to coherent state light. Moreover, the photon statistics of the quantum light field have a notable impact on the interference patterns of the photoelectrons (Figs. 3 and 7). Specifically, the intra-cycle interference fringes are diminished under the influence of bright squeezed vacuum state light, while the inter-cycle interference, known as the above-threshold ionization ring, persists. Remarkably, the holographic interference of photoelectrons, characterized by interference fringes between directly ionized electrons and forward re-scattered electrons, remains observable. We enhance our comprehension of the quantum effects induced by optical fields on strong-field ionization processes. Moreover our research holds the promise for providing additional insights into imaging atomic and molecular structures, as well as probing ultrafast dynamics through strong-field ionization.ConclusionsOur research aims to analyze disparities in the photoelectron energy spectra and momentum distributions of hydrogen atoms driven by light at 400 nm and 800 nm. Regardless of the wavelength, the photoelectron energy spectra driven by BSV light exhibit a broader distribution compared to those driven by coherent state light. This is primarily due to the broader quasi-probability distribution of BSV light compared to coherent state light. Furthermore, under 400 nm coherent state light, the momentum distribution of the photoelectrons reveals interferences caused by inter-cycle channel (ATI), which is preserved by BSV light. In the 400 nm case, the ATI in BSV light is attributed to the narrower final ionization amplitude distribution, which reduces the level of smearing of the periodic interferences. Conversely, under 800 nm coherent state light, the momentum distribution of the photoelectrons exhibits multiple structures. In contrast, BSV light preserves only the fork-like structure while other interference patterns are attenuated. This observation underscores the quantum nature of the light field and enhances comprehension of the strong-field ionization process within the domain of quantum optics.

ObjectiveStrong-field Rydberg state excitation (RSE) is a significant physical phenomenon observed when atoms or molecules are exposed to intense laser fields. Numerous theoretical and experimental studies have explored the RSE process to understand its underlying physical mechanisms. However, existing theoretical methods, such as solving the time-dependent Schr?dinger equation (TDSE), while accurately describing the physical process, do not provide a clear physical picture. Consequently, the mechanism of RSE, particularly in elliptically polarized laser fields, remains unclear. To better investigate the RSE in strong fields, we develop a quantum model for resonance excitation based on S-matrix theory. This model describes the electron transition from the ground state to the field-dressed Rydberg state in strong laser fields, allowing for a detailed investigation of the physical characteristics of strong-field RSE.MethodsA quantum model based on S-matrix theory is developed to describe the transition of electrons from the ground state to the Rydberg state in intense laser fields, using the length gauge. Furthermore, we consider the effect of the intense laser field on the Rydberg state, treating the field-dressed Rydberg state as the final state within an elliptically polarized laser field characterized by a sine-square envelope containing 30 optical cycles.Results and DiscussionsWe first use the quantum model to calculate the evolution of the excitation rates of the 4s, 4p, 4d, and 4f states at different laser intensities for laser pulses with an ellipticity of 0.5 (Fig. 1) and provide corresponding explanations. Next, we investigate the ellipticity-dependent excitation rates of different excited states at two laser intensities of 1.4×1012 W/cm2 and 6.82×1013 W/cm2. When the laser intensity is 1.4×1012 W/cm2 (two-photon transition), the excitation rate of the 4s state decreases with increasing ellipticity, while the excitation rate of the 4d state increases (Fig. 2). These findings are consistent with both TDSE results and the perturbation theory presented in a recent paper. When the laser intensity is 6.82×1013 W/cm2 (three-photon transition), the excitation rate of the 4p state initially increases, reaching a maximum at an ellipticity of 0.3, and then decreases as ellipticity increases. In contrast, the excitation rate of the 4f state increases with increasing ellipticity [Fig. 2(b)]. Our calculations reveal that at higher laser intensities, the influence of the laser field on the spatial phase distribution of Rydberg states becomes significant, while at lower intensities, this influence can be ignored [Figs. 2(c) and 2(d)]. In addition, we present the excitation rates of excited states with different magnetic quantum numbers at a laser intensity of 6.82×1013 W/cm2 (Figs. 3 and 4), which demonstrate the transition selection rules for orbital angular momentum quantum numbers and magnetic quantum numbers. We also provide a theoretical analysis using a circularly polarized laser pulse as an example.ConclusionsWe develop a quantum model describing electron transitions from the ground state to the Rydberg state in intense laser fields based on S-matrix theory. Using this model, we calculate the ellipticity-dependent excitation probabilities of a lithium atom to various Rydberg states in laser fields. For two-photon resonance transitions at relatively low laser intensities, the excitation process described by the quantum model aligns with perturbation theory, where the influence of the laser field on the excited state can be ignored. However, for three-photon resonance transitions at higher intensities, the laser field not only alters the energy levels of the Rydberg state but also affects its spatial phase distribution. Furthermore, theoretical analysis reveals the characteristics of the transition selection rule for angular momentum and magnetic quantum numbers in the multi-photon excitation process in elliptically polarized fields: if the electron absorbs an even (odd) number of photons during resonance excitation, the parity of the orbital angular momentum (magnetic quantum number) of the initial state will match (differ from) that of the Rydberg state. Our work lays the foundation for a deeper understanding of the resonance excitation process.

ObjectiveWith increasing concern for food safety, the standards for food-safe materials have become more stringent. Polyether ether ketone (PEEK) is a high-performance thermoplastic with advantages such as high mechanical strength, good stability, and low density, making it widely used in aerospace, medical, and food industries. However, traditional PEEK surfaces are inherently hydrophilic, making them prone to attracting liquids like oil or milk, which can lead to contamination risks during food processing. Modifying PEEK surfaces to be hydrophobic can help maintain their self-cleaning properties. However, the traditional preparation of these self-cleaning surfaces often involves chemical methods, such as chemical etching, electrochemical etching, or chemical vapor deposition, which introduce additional chemical reagents and may pose food safety concerns. In this study, we use femtosecond laser processing technology to create micro/nanostructures on the PEEK surface, achieving a chemically-free self-cleaning surface. This method provides high precision, non-contact processing, and high efficiency without the need for chemical reagents, making it highly suitable for applications requiring stringent food safety.MethodsIn this study, femtosecond laser technology is applied to fabricate micro/nanostructures on PEEK materials to explore their self-cleaning capabilities. Initially, the influence of laser power density on the hydrophobicity of the fabricated structures is examined. Based on these findings, the effects of laser scanning speed and scanning times on hydrophobicity are further investigated. It is observed that rapid surface scanning enhances the self-cleaning properties of the structures. In addition, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) is used to measure the carbon (C) and oxygen (O) content in the structures before and after processing to analyze its influence on self-cleaning ability. Finally, a self-cleaning capability test is conducted using water, juice, and milk to evaluate the performance of the structures.Results and DiscussionsBy optimizing the laser processing parameters, the fabricated micro/nanostructures demonstrate superhydrophobicity and self-cleaning properties with respect to milk and juice. Initially, the processing of the PEEK material results in micro/nanostructures with a water contact angle of 148.2° and a minimum sliding angle of 10.3°. In addition, these structures exhibit excellent self-cleaning effects for liquids such as milk and juice, with contact angles of 145.9° for milk and 143.2° for juice [Figs. 4(a) and (b)]. After further rapid surface scanning, the hydrophobicity is enhanced, reaching a contact angle of 152.5°. The liquid-repellent effects of milk and juice also improved [Figs. 4(c) and (d)]. Moreover, after 50 cycles of milk testing, the fabricated surface maintains good self-cleaning performance (Fig. 5).ConclusionsWe successfully fabricate a micro/nanostructure with excellent self-cleaning capabilities on the surface of PEEK material using femtosecond laser processing. By adjusting the femtosecond laser processing parameters, such as power, scanning speed, line spacing, scanning times, and power density, the adhesion properties of the PEEK surface are optimized. The optimized surface achieves the desired hydrophobicity and self-cleaning ability for milk and juice. Testing demonstrates that the surface exhibits excellent superhydrophobicity, with a maximum contact angle of 152.5° and a minimum sliding angle of 10.3°. The results of this study are expected to provide a theoretical foundation and technical support for developing new food contact materials to enhance food safety.

ObjectiveIn the current rapid development of optoelectronics technology, gallium arsenide as a semiconductor material plays a crucial role in optoelectronic devices such as lasers and photodetectors in the field of communication. As a typical representative of compound semiconductor materials of Ⅲ-Ⅴ groups, gallium arsenide has a direct bandgap of dual-energy valleys and high mobility, and its excellent optoelectronic and thermal properties are highly favored. However, increasing the absorptivity and resistivity of the material should be considered to enhance its key properties such as responsivity and thus expand the application fields of GaAs more comprehensively and further optimize the GaAs performance, especially in the detection field. In the study of GaAs, laser ablation technology has great potential. We focus on the enhancement of photovoltaic properties by laser ablation technique. The high energy density and ultrafast time scale of femtosecond laser pulses can form plasma on the surface of semiconductor materials and produce micron-scale micro-nano-structures, thus enhancing light absorption. Our goal is to understand the effects of different environments on the surface properties of GaAs by systematic experiments and in-depth theoretical analysis, and finally provide more comprehensive references for the future practical applications of this material.MethodsWe employ intrinsic GaAs material and conduct laser ablation experiments by adjusting various processing parameters including laser power, scanning speed, spot size, and scanning spacing. After determining the optimal processing parameters, a confined gas cavity is introduced and laser ablation experiments are carried out under four different environments, including air, vacuum, nitrogen, and sulfur hexafluoride. Additionally, the surface morphology of the material is observed and analyzed in detail using scanning electron microscopy (SEM), with the mechanism of surface structure formation studied in depth. To analyze the changes in the properties of the material surface, we perform X-ray photoelectron spectrometer (XPS) tests. Meanwhile, infrared spectroscopy tests performed on each sample show that the absorbance of the samples processed under different environments is significantly increased. Additionally, by analyzing and discussing the Ⅰ-Ⅴ curves of the samples, the pressure resistance changes of the samples after laser ablation in different environments are observed to reveal a systematic improvement in the optoelectronic properties of the materials.Results and DiscussionsUnder different environments, femtosecond laser ablation leads to the formation of different structural morphologies on the GaAs surface. In air and nitrogen, the surface structure is relatively regular. In the vacuum, the splashing of GaAs can form raised impurities due to surface thermal adsorption. In addition, in sulfur hexafluoride, irregular ellipsoidal structures appear (Fig. 3). In the high-energy laser field, sulfur hexafluoride molecules may dissociate and react to produce reactive species such as fluorine, which may affect the material surface during laser processing. The presence of elemental fluorine in samples processed only in sulfur hexafluoride is confirmed by XPS tests, and the oxygen content of samples processed in air is significantly higher than that in other environments according to the test results. Fourier transform infrared spectroscopy (FTIR) analysis of the sample surfaces indicates that the transmittance of the samples is significantly reduced after laser ablation. The transmittance of the samples processed in the vacuum is decreased to about 1% (Fig. 6). When laser light is directed at the surface structure of a material, this may result in multiple reflections, scattering, or localization of the incident light in the material, increasing the propagation path of the light through the material. This enhances the interaction between the light and the material, thus increasing the probability of the material absorbing the light (Fig. 7). To better analyze the conductivity properties of the materials, we measure Ⅰ-Ⅴ curves for each sample. The samples processed in sulfur hexafluoride exhibit higher resistivity in both dark and light conditions (Fig. 9). According to the specific needs, processing in different gas environments can be chosen to achieve the most suitable properties.ConclusionsWe investigate the optoelectronic property changes of the GaAs surface by femtosecond laser ablation in different environments. SEM tests show that the changes in the processing parameters have an effect on the surface micro-nanostructures, and different environments also change the structural morphology. Meanwhile, the material is further tested and analyzed by XPS tests and we find the changes in the surface properties of the material. The processed material exhibits a significant increase in absorptivity compared to the original sheet, and a resistivity increase in light conditions. In particular, the samples processed in vacuum conditions show the highest light absorption, while the samples processed in sulfur hexafluoride gas present better pressure resistance in dark and light conditions. According to specific needs, processing in different gas environments can be selected to yield the most suitable performance. As a result, GaAs materials with surface micro-nano-structures exhibit higher responsiveness in detection applications and can be adopted for optimizing detector performance. This provides new ideas and solutions for developing more efficient optoelectronic devices. Finally, we expect to provide valuable technical methods and experimental data for the development and application of new optoelectronic materials and thus promote the progress and innovation of optoelectronic technology.

ObjectiveTerahertz (THz) waves hold significant application potential in fields such as spectroscopy, integrated optics, and imaging. With the advancement of high-power femtosecond laser technology, lithium niobate crystals and waveguides have emerged as efficient, high-beam quality, and stable THz sources for generating THz radiation. Notably, the tilted-pulse-front method has led to significant progress in generating strong THz radiation from lithium niobate crystals. However, the spectral center frequency of strong THz sources generated by lithium niobate prisms is typically challenging to modulate, limiting their use in tunable multi-wavelength applications. We present a tunable THz source based on a lithium niobate waveguide, wherein the waveguide’s thickness is controlled to modulate the central frequency of the emitted THz spectrum, achieving tunable single-frequency THz wave output. In addition, we utilize an interdigital photoconductive antenna to detect the THz signals generated by the lithium niobate waveguide, enhancing the system’s signal-to-noise ratio.MethodsThe experimental setup involves a laser with a wavelength of 800 nm, a pulse width of 35 fs, and a repetition rate of 500 Hz. A cylindrical lens with a focal length of 70 mm is used to modulate the laser beam into a line laser mode suitable for transverse excitation. Two off-axis mirrors with a focal length of 101.1 mm are used to collect the THz waves generated by the lithium niobate waveguide. An interdigital photoconductive antenna is used to collect the far-field THz signals, with a wide-angle silicon lens that efficiently collects elliptical THz spots, ensuring high detection efficiency without the need for additional optical components.Results and DiscussionsIn the transverse excitation mode, by comparing the time-domain and spectral characteristics of THz waves generated by lithium niobate waveguides of varying thicknesses and lengths, the following conclusions are drawn. 1) The center frequency of THz waves generated by the lithium niobate waveguide is affected by the waveguide’s thickness, but not by its length. 2) The physical principle underlying narrowband THz wave generation in lithium niobate waveguides is that only THz frequencies that confirm to the waveguide phase matching mode can be transmitted over long distances within the waveguide. 3) Measurement of THz waves generated by lithium niobate waveguides of identical thickness but different lengths shows that as the waveguide length increases, the center frequency of the THz waves gradually approaches the phase-matching frequency, consistent with the waveguide mode dispersion frequency. Only THz frequencies with a phase velocity matching the group velocity of the femtosecond laser can be effectively transmitted over long distances and out of the waveguide. 4) The high signal-to-noise ratio of the interdigital photoconductive antenna enables precise measurement of the THz waves reflected from the waveguide’s end face, further confirming that the THz wave transmission in the lithium niobate waveguide is consistent with the phase matching of the waveguide mode.ConclusionsDuring femtosecond laser transverse excitation of lithium niobate waveguides, the pump laser energy is continuously converted into THz waves. By adjusting the thickness and length of the waveguide, the center frequency and intensity of the generated THz waves can be modulated. Compared to traditional forward excitation modes, the transverse excitation mode allows for the generation of higher energy narrowband THz pulses. It is expected that by cascading lithium niobate waveguides of varying thicknesses and lengths, a continuously tunable narrowband pulsed THz radiation source can be realized under the same pump laser conditions.

ObjectiveThe spintronic terahertz (THz) emitters developed in recent years have shown numerous advantages such as ultrabroadband, low cost, easy integration, and tunable polarization. As a result, they are increasingly demonstrating significant practical value in THz technology applications. However, advancing the development of THz time-domain spectroscopy (THz-TDS) systems crucially depends on combining high-efficiency, broadband spintronic THz emitters with low-cost, miniaturized titanium sapphire (Ti∶sapphire) laser oscillators. In our study, we use a miniaturized direct diode-pumped Ti∶sapphire ultrafast oscillator to drive the THz spectroscopy system, achieving efficient THz emission from antiferromagnetic | ferromagnetic | heavy metal (IrMn3|Co20Fe60B20|W) heterostructures under no external magnetic field conditions. We not only verify the spintronic THz radiation mechanism but also find that the heterostructure has a stronger radiation signal before the focusing lens focus. These experiments demonstrate that miniaturized, low-cost direct diode pumped Ti∶sapphire ultrafast oscillators are the preferred choice for direct application in spintronic THz emission spectroscopy systems, which is a powerful tool for analyzing the interactions between femtosecond lasers and different materials. This research lays the foundation for further promoting the application of miniaturized ultrabroadband THz time-domain spectral imaging technology driven by femtosecond lasers.MethodsWe utilize a direct diode pumped Ti∶sapphire ultrafast oscillator to drive the THz time-domain spectroscopy system. This laser adopts a new direct diode pumping scheme, combining the advantages of traditional Ti∶sapphire lasers and fiber femtosecond lasers, and features high performances with small size and low cost. It can provide laser pulses with a center wavelength of 800 nm, pulse duration of 47 fs, repetition rate of 80 MHz, and output power of 625 mW. The laser output beam is divided into three paths: the first excites a photoconductive antenna to generate THz pulses; the second excites another photoconductive antenna to detect THz time-domain waveforms; the third passes through a focusing lens with a focal length of 20 mm and a chopper with a frequency of 1600 Hz. The laser power on the THz emitter is about 130 mW, which excites THz waves on the studied materials. We characterize the performances of the THz time-domain spectroscopy system by testing the absorption peaks of nano metasurface samples at different positions under TE and TM wave incidences.Results and DiscussionsIn this work, we use THz emission spectroscopy to investigate the THz radiation of IrMn3|Co20Fe60B20|W in the atmospheric environment. The results and discussions are summarized as follows: first, the sample is placed at the focal point of the focusing lens and pumped by a femtosecond laser without an external magnetic field. We observe an emitted THz signal up to 1.5 THz. The amplitude of THz signals changes periodically along with the rotation of the sample’s azimuth angle (Fig. 4). These experimental results are consistent with the spintronic THz emission mechanism. The exchange bias effect between antiferromagnetic and ferromagnetic materials causes Co20Fe60B20 to saturate magnetization, and the spin current generated inside it undergoes spin-to-charge conversion (SCC) at the interface due to the inverse spin Hall effect (ISHE), radiating a THz wave. Consequently, we can flexibly modulate the polarization of the THz waves by changing the sample azimuth angle. Second, we change external conditions to investigate the dependence of THz radiation on the laser pump power, in-plane magnetic field, and incident plane. We exclude other mechanisms by conducting left-right flipping and up-down flipping experiments (Fig. 5). It is confirmed that THz emission is induced by SCC caused by ISHE because both flipping methods result in THz pulses with opposite phases. In addition, the materials have not been magnetized due to the incomplete exchange bias effect of the sample. When the in-plane magnetic field is applied to IrMn3|Co20Fe60B20|W, the THz radiation increases by approximately 66.7%. An opposite polarity THz pulse is obtained by reversing the in-plane magnetic field (Fig. 6). Furthermore, THz emission intensity is enhanced by increasing the pump power, showing a linear dependence relationship (Fig. 6). Finally, we move the position of the sample before and after the focusing lens focus and measure the THz radiation output. It is found that THz emission amplitude is maximum when the sample is positioned 8 mm in front of the focus point instead of being located exactly at the focus point (Fig. 8). We have ruled out the possibility of the laser pump fluence reaching the sample saturation threshold. The reason might be that the coupling between the THz signal and the photoconductive antenna is better when the laser is converging, resulting in a larger detected THz amplitude.ConclusionsOur research takes the lead in using a compact direct diode-pumped Ti∶sapphire laser oscillator as the pumping source to drive the THz spectroscopy system and studies the THz radiation mechanism and performance of the IrMn3|Co20Fe60B20|W heterostructure at room temperature without external magnetic field conditions. We not only observe the phenomenon of THz emission enhanced by applying an in-plane magnetic field, increasing the pump power, and placing the sample before the focus but also realize the control of THz linear polarization. This research promotes the development of THz spectroscopy technology based on direct diode pumped Ti∶sapphire laser oscillators with ultrashort pulses (less than 15 fs) to achieve broader band, smaller volume, and more flexible THz manipulation.