Light-field-driven currents sensitive to pulse waveforms have established a promising foundation for the realization of petahertz (PHz) electronic devices. However, the influence of many-body interactions on photocurrent generation remains inadequately understood. Here, we theoretically investigate photocurrent generation in monolayer MoS2, highlighting the essential role of many-body effect on carrier dynamics and the optoelectronic response. We demonstrate that photocurrents depend critically on the electric-field waveform controlled by the carrier-envelope phase, and we reveal a distinct signature of multielectron interactions manifested as destructive interference of the maximum current with increasing field strength, due to specific intraband Coulomb scattering processes that can induce a primary band energy renormalization. Additionally, we propose an optical switch based on two orthogonally linearly polarized laser pulses with a controlled delay, capable of achieving current modulation at frequencies approaching 1 PHz. We present a strategy for constructing logic gates and emphasize the critical role of many-body interactions in the implementation of their functionality encoding. This optical switch also provides an all-electronic method to measure dephasing time on femtosecond timescales. Our findings deepen the understanding of many-body effects in petahertz electronics and facilitate the development of ultrafast optoelectronic devices based on two-dimensional materials.

Objective Recently, solid-state high-harmonic generation (HHG) has been observed in diverse materials, including dielectric crystals, semiconductors, two-dimensional layered materials, strongly correlated systems, and topological insulators. Topological insulators have attracted significant attention in HHG and strong field-driven dynamics due to their topologically protected surface states. These surface states exhibit spin-momentum locking, leading to unique optical responses in HHG such as even-order harmonic generation, non-integer harmonics, and anomalous dependence of harmonic intensity on driving laser ellipticity. However, the polarization characteristics of HHG in topological insulators remain systematically unexplored. We investigate the polarization properties of HHG in Bi2Se3 crystals driven by linearly polarized intense laser fields. By varying the crystal azimuth angle (8), we measure the evolution of the polarization state (polarization orientation angle alpha and ellipticity epsilon) of harmonics. The experiments reveal that HHG transitions from linear to elliptical polarization with rotating orientation angles as 8 changes, which is particularly pronounced in even harmonics. Theoretically, using the tight-binding approximation and semiconductor Bloch equations (SBEs), we demonstrate that the polarization states of even-order harmonics are primarily governed by the phase of complex inter-surface-state transition dipoles. These findings enhance our understanding of HHG mechanisms in topological materials and suggest new approaches for all-optical control of harmonic polarization. Methods We employ a linearly polarized mid-infrared femtosecond laser (central wavelength 3.8 mu m, pulse duration 60 fs, peak electric field 5.2 MV/cm corresponding to an intensity of 3.6x10(10) W/cm(2)) incident on a Bi2Se3 crystal at similar to 5 degrees angle to produce HHG (Fig. 1). High-harmonic emission is collected via a focusing lens and detected by a grating spectrometer. The polarization states of the HHG are measured using a Stokes parameter measurement device that includes a quarter waveplate (QWP) and a polarizer. Stokes parameter analysis determines harmonic orientation angle and ellipticity [Fig. 1(c)]. Theoretical simulations combine tight-binding models with semiconductor Bloch equations, which incorporates topological surface-state electronic structures to analyze how transition dipole phases and interband Berry phases modulate polarization. Results and Discussions By rotating the Bi2Se3 crystal (azimuth angle 8), we observe modulated polarization states in even-order harmonics (e. g., H6 and H8). Figure 3 shows the 8-dependent polarization modulation of the even-order harmonics (e.g., H6 and H8). As 8 increases from 0 degrees to 30 degrees, the orientation angle a of H6 decreases linearly from 90 degrees to near 0 degrees , which indicates polarization rotation from parallel to perpendicular relative to the driving field [Fig. 2(a)]. The ellipticity e exhibits periodic oscillations, peaking at 8=15 degrees (e=0.313), while maintaining linear polarization (e approximate to 0) at high-symmetry orientations (8 =0 degrees, 30 degrees, 60 degrees) [Fig. 2(b)]. This behavior stems from the threefold rotational symmetry of the Bi2Se3 crystal: along the high-symmetry orientations (Gamma-M and Gamma-K), parallel/perpendicular harmonic polarization alignment is enforced by mirror symmetry, whereas ellipticity is induced from the symmetry orientations through transition dipole phase differences (Fig. 6). Theoretical simulations, considering the strong-field-driven dynamics in topological surface states, reproduce the experimental observations [Figs. 7(a) and 7 (b)]. Interband Berry phase analysis reveals that the elliptical polarization originates from the spectral phase differences (Delta not equal 0 or pi) between orthogonal harmonic components, determined by the quantum geometric phase during electron-hole recombination. These phase differences stem from the complex transition dipole moments at the instant of electron-hole quasiparticle recombination, fundamentally linking polarization ellipticity to quantum geometric properties. Additionally, the crystal's threefold rotational symmetry (C3v) directly governs sinusoidal 8-dependent ellipticity oscillations in harmonics, which confirms symmetry-controlled polarization states (Fig. 4). Odd-order harmonics (e.g., H5, H7) exhibit polarization characteristics fundamentally distinct from their even-order counterparts. While H5 maintains near-parallel alignment relative to the driving field (maximum deviation is 16.3 degrees ), H7 undergoes polarization plane rotation from parallel (8=0 degrees ) to perpendicular (8=15 degrees ), followed by realignment to parallel (8=30 degrees ) with increasing azimuth angle [Fig. 3(a)]. These harmonics display significantly lower maximum ellipticity ( e max= 0.176) compared to even-order harmonics, with only moderate 8-dependent variations [Fig. 3(b)]. This contrast originates from their divergent generation mechanisms: odd-order harmonics predominantly stem from bulk-band optical responses, whereas even-order harmonics arise from topological surface state dynamics. This difference in generation mechanisms emphasizes the key role of band topology in controlling HHG processes. Notably, the anomalous polarization rotation observed in H7 likely originates from hybridization effects between topologically protected surface states and trivial bulk bands. Conclusions Through combined experimental and theoretical studies, we systematically characterize the polarization properties of HHG from Bi2Se3 and elucidate the physical mechanism governing ellipticity in even-order harmonics. Our findings reveal that the polarization states of even-order harmonics are predominantly determined by the orthogonal projection of the complex transition dipole moments at the instant of electron-hole recombination. By rotating the crystal azimuth angle, we achieve effective modulation of harmonic polarization states. This work not only advances the understanding of HHG mechanisms in topological materials but also proposes a strategy for HHG polarization control. These findings provide insights into nonlinear optical responses in topological states and establish a foundation for future all-optical harmonic polarization control.

Significance The appearance and rapid development of superintense and ultrafast laser have opened up many frontier areas in subjects such as atomic and molecular physics, strong field physics, and plasma physics, and superintense and ultrafast laser itself has become a powerful tool in numerous applications. The extreme nonlinear interaction between superintense and ultrafast lasers and atomic or molecular systems induces strong field ionization, attosecond radiation, femtosecond laser filamentation, and air lasing. Attosecond physics may develop new technological means for the study of electronic dynamics in complex systems that are relevant to physics, chemistry, biomedicine and other subjects. Laser filamentation and air lasing have brought new opportunities for applications based on laser plasma associated radiation, laser atmospheric remote sensing and material analysis, weather modification, and laser material processing. With the increase of laser intensity, physical processes enter the relativistic plasma regime, where many applications have been developed in laser - driven particle acceleration, laser - driven high - energy radiation sources, ultrafast electronic dynamics, laser nuclear physics, and laboratory astrophysics. Laser - driven ultra - high - gradient particle accelerators and high - energy ultrafast radiation are advantageous in producing compact particle and radiation sources with ultra - high peak brightness and ultrafast time duration, thus are unique in several key applications. Especially, laser - driven plasma wakefield acceleration is promising in generating high energy electrons to develop table - top X - ray free electron lasers. On the other hand, laser - driven ion acceleration is believed to bring new opportunities in tumor therapy, proton imaging, and fast ignition fusion. As the laser intensity further increases, the extreme strong field effect gradually becomes apparent and it enters the strong field quantum electrodynamics (QED) regime, resulting in a series of new phenomena such as quantum gamma radiation, generation of electron - positron pairs, vacuum polarization, QED cascades and so on. These studies would extend boundaries of laser - matter interaction. New properties such as spin and orbital angular momentum also emerge as new degree of freedom in laser - plasma interaction. The interaction between superintense laser and plasma is an important means of generating radiation at different wavelengths. The study of terahertz radiation sources driven by superintense lasers can further improve the laser energy conversion efficiency. The free electron and quasi - particle radiation driven by superintense laser is of great significance for the development of miniaturized and integrated new coherent light sources and terahertz - driven electron sources. Ultrafast lasers provide processing accuracy beyond the limits of optical diffraction, enabling high - quality micro - nano processing. In addition, micro - nano lasers are important for promoting the miniaturization and integration of optoelectronic devices. Progress This review first introduces nonlinear atomic and molecular physics driven by ultrafast lasers, followed by superintense lasers and relativistic plasma physics at higher intensities, and finally reviews the cross - disciplinary frontier applications of superintense ultrafast laser. This review will highlight the progresses and achievements made by the State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, in the long - term dedicated research on superintense ultrafast laser physics and its frontier applications. The laboratory has made significant progresses in the generation and control of high - order harmonics and single attosecond pulse, high brightness high - order harmonic coherent light sources, water window or keV high - order harmonic generation driven by mid - infrared wavelength lasers, and high - order harmonic generation in condensed matter. Important progresses in femtosecond laser filamentation measurement and control, femtosecond laser weather modification, and air lasing are also elaborated. In recent years, the laboratory has developed stable and usable high - quality laser - driven electron sources. The laser energy conversion efficiency and beam quality of high - energy and ultrafast radiation sources have also been improved. Compact free electron laser in the extreme ultraviolet band has been demonstrated based on laser - driven wakefield electrons for the first time in the world. Laser - driven ion acceleration has seen rapid development in the past decades, where new acceleration mechanisms have been proposed, and the laser energy conversion efficiency and the cut- off energies of protons are now sitting in the first class in the world. In the strong field QED regime, the radiation- reaction trapping of electrons and the upper limit of laser intensity caused by non- ideal vacuum are discovered. The exploits of vortex laser on particle acceleration and secondary radiations are developed in the laboratory. The laboratory has also successfully generated positrons using superintense laser and proposed new schemes for the detection of dark matter particles such as axion. Significant progress in the generation of intense terahertz radiation has been made. The quasi- particle radiation amplification mechanism and electron acceleration driving the terahertz radiation have been explored, which provides a unique platform for surface light sources and applications. In addition, a number of research achievements have been made in ultrafast laser micro- nano processing and micro- nano lasers. Conclusions and Prospects The development and application of superintense ultrafast laser technology have significant impacts on disciplines such as physics, chemistry, and biology. It is a highly competitive area of research worldwide. The progresses highlighted in this review will further guide the development of advanced laser technology and light sources, compact particle accelerators, and extend our knowledge on light- matter interaction. It is now an important stage to bring the research in laboratories to real world applications, where key aspects of the interaction should be controllable and the whole system must be stable or even cost efficient. On the other hand, the extreme field provided by 10-100 PW lasers shows unparalleled capacities in fundamental research, such as strong field QED, high energy density physics or even dark matter search. It relies on joint efforts among multiple disciplines such as plasma physics, theoretical physics, particle and nuclear physics and so on.

We have observed the Berry phase effect associated with interband coherence in topological surface states (TSSs) using two-color high-harmonic spectroscopy. This Berry phase accumulates along the evolution path of strong field-driven electron-hole quasiparticles in electronic bands with strong spin-orbit coupling. By introducing a secondary weak field, we perturb the evolution of Dirac fermions in TSSs and thus provide access to the Berry phase. We observe a significant shift in the oscillation phase of the even-order harmonics from the spectral interferogram. We reveal that such a modulation feature is linked to the geometric phase acquired in the nonperturbative dynamics of TSSs. Furthermore, we show that the overwhelming Berry phase effect can significantly deform the quantum paths of electron-hole pairs, thus enhancing the ability to harness electron spin using lightwaves in quantum materials with strong spin-orbit interactions.

The absorption and emission of light by matter plays a crucial role in the development of science and technology. High Harmonic Generation (HHG) presents an extremely nonlinear optical radiation induced by the interaction of intense laser fields with matter. Over the past two decades, HHG in gaseous materials has been extensively studied and regarded as a vital tool for advancements in ultrafast science. The fundamentals of gaseous HHG can be explained by the semi-classical three-step model, the understanding of microscopic processes in HHG has laid the foundation for atto-second physics and metrology, including the ability to probe atomic structure and dynamics and molecular systems. Recently, there have been reports of HHG occurring in various solid-phase materials, the targets of solid-state high harmonic studies have been extended from bulk metals, semiconductors, and insulators to low-dimensional nanostructures. Two-dimensional materials can neglect the propagation effects in the direction of laser propagation, and thus become ideal materials for the study of high-harmonic carrier dynamics. An important phenomenon of HHG in solids is the anisotropy. Due to the modulation of the lattice symmetry, the harmonic signals generated by the driving light polarized along the different directions of the crystal are quite different, it has shown potential applications such as reconstructing crystal band structure, measuring Berry curvature, and investigating topological phase transitions. In this paper, we have investigated the process of HHG from monolayer h-BN by using the tight binding energy band and solving the two-band SBEs. We show that the yield of harmonics displays a periodicity of 60 degrees as the azimuthal angle between the h-BN and driving field are varied, consistent with the symmetry of the laser and the crystal. Notably, an intriguing pattern in the orientation-dependent HHG is observed. Specifically, we decompose the high harmonics into components parallel and perpendicular to the driving light, the parallel component of the odd-order harmonics in the cut-off region exhibits an angular shift of 30 degrees compared to the other orders, and this angular shift is independent of the change of driving light intensity. Comparison of the harmonic spectra of the driving light polarization along the zigzag direction and the armchair direction reveals that the harmonic spectrum has a sharp decrease (cut-off region) for harmonics above H17 when the driving field is along the zigzag direction. The harmonic intensities below the H17 are stronger in the zigzag direction than in the armchair direction, and the sharp decrease in the zigzag direction harmonics at the H17 results in the harmonic intensities being less than those in the armchair direction harmonics. Therefore, we believe that the angular shift of the intensity modulation of odd-order harmonic parallel components are related to the cut-off of the zigzag directional harmonic spectrum. Furthermore, we found the energy at which this angle shift occurs is strongly correlated with the bandgap energy of h-BN, especially when close to the M-momentum channel bandgap. Through detailed analyses, we determine that the phase shift in the intensity modulation of H17 arises due to the interference of different momentum channels and the interference of different polarity half-periods. We believe that this phenomenon is not coincidental, that the effect of energy band structure on harmonics is significant. The angular shift of odd-order harmonics holds important potential for developing techniques to probe the energy band structure of solids through HHG.

We demonstrated a scheme to differentiate the high-harmonic generation (HHG) originating from the surface states and bulk states of the topological insulator Bi2Se3. By adopting two-color mid-infrared laser fields on Bi2Se3, we found that the nonlinear response sensitively depends on the relative phase of the driving fields. The even harmonics arise from the sur-face states with a clear signature, whose modulation period equals the cycle of the second-harmonic generation (SHG) field. We reveal that the weak SHG perturbs the nontrivial dipole phase of the electron-hole pair in surface states, and thus leads to the modulation of HHG. It provides a means to manipulate the ultrafast dynamics in surface states through adopting a weak perturbing laser field.

Optical vortices with longitudinal orbital angular momentum (OAM) as a powerful tool for particle control, imaging and communication have been greatly developed. We introduce a novel property of broadband terahertz (THz) pulse, represented by frequency-dependent OAM orientation in spatiotemporal domain with transverse and longitudinal OAM projection. We illustrate a frequency-dependent broadband THz spatiotemporal optical vortex (STOV) in plasma-based THz emission driven by cylindrical symmetry broken two-color vortex field. We detect the evolution of OAM by time-delayed 2D electro-optic sampling combined with Fourier transform. This THz optical vortices tunability in spatiotemporal domain expands a new way for studying STOV and plasma-based THz radiation.

Objective High harmonic is a crucial technology for generating bright and coherent light sources in extreme ultraviolet (EUV) and X-rays for a wide range of applications, including material science, chemistry, and biology. High harmonic is also used in attosecond science, which studies the ultrafast dynamics of electrons in atoms and molecules on their natural timescale of attoseconds (10(-18) s). High harmonic has traditionally been studied in gases and solids, but recent research has shown that it can also be observed in liquids. High harmonic in liquids offers several advantages over traditional gas-phase and solid-phase high harmonic. Firstly, liquids have higher electron densities than gases. Second, liquids can withstand higher laser intensities and can repair damage automatically compare with solids. Thus, high harmonic in liquids is a promising candidate for a compact and brighter EUV source. Therefore, it is crucial to reveal the underlying mechanism of liquid-phase high harmonic. However, there are many fundamental questions in liquid-phase high harmonic. In theory, Zeng et al. conducted a study in 2020 to investigate liquid-phase high harmonic by using a disordered linear chain and proposed a formula that could quantify the cutoff energy. Subsequently, Xia et al. proposed a statistical two-level model and revealed the role of localized charge-resonance states in high harmonic from disordered liquids. In experiments, Luu et al. reported the observation and detailed characterization of high harmonic in the EUV region from liquid water. Here, we study the high harmonic from liquid water by solving the semiconductor Bloch equations (SBE) in length gauge and investigate the modulation of the spectral shift in harmonic spectra driven by two-color laser fields. Our findings indicate that manipulating the relative phase of two-color laser fields can control the frequency and yield of odd harmonics and allow for the fine tuning of the high harmonic spectrum. We believe that our primary findings will be helpful for future studies on strong-field and attosecond electron dynamics in liquids. Methods First, by solving the SBE in length gauge, the high harmonic from water driven by two-color laser fields consisting of a fundamental field and its second harmonic is studied. Then, time-frequency analysis of the calculated high-order harmonic current is carried out by wavelet transform to gain more information about the high harmonic process. After that, the contributions from positive and negative half-cycles in the time domain are artificially separated and respectively transformed into the frequency domain to see how the inter-half-cycle interference affects the frequency shift of different harmonic orders. Next, the frequency shift of H9 and H10 from positive and negative half-cycles in different phase differences is calculated. Furthermore, the time of re-encounter in the positive and negative half-cycles dictated by the motion of electrons and holes in real space is calculated. Results and Discussions A typical high harmonic spectrum (Fig. 2) of water shows that the harmonic spectrum contains both odd and even harmonics. The generation of even harmonics is the consequence of the asymmetric two-color field. A clear sign of a plateau is shown, followed by a cutoff at the 23rd harmonic. Another important feature of the spectrum is that the harmonics are all blue-shifted. It can be attributed to the nonadiabatic effect. The time-frequency analysis (Fig. 3) shows that the high-order harmonics are mainly generated in the rising edge of the fundamental pulse. In the rising edge of the laser field, the high harmonic possesses a positive chirp and thus leads to a blue-shifted spectrum. Furthermore, it is shown that by changing the relative phase of two-color laser fields, the even-and odd-harmonics are periodically modulated (Fig. 4). As the relative phase is tuned from 0 to p, the redshift of odd-harmonics increases, and the odd-harmonics yield increases first and then decreases. As the relative phase is tuned from 0 to 0. 5 pi(0. 6 pi-0. 9 pi), the contribution of the positive half-cycle to odd-order high harmonic is greater (less) than that of the negative half-cycle (Fig. 5). As the relative phase is tuned from 0 to pi, high harmonic produced in the negative half-cycle is blue-shifted first and then red-shifted, while high harmonic produced in the positive half-cycle is always red-shifted (Fig. 6). According to the different contributions from different half-cycles (Fig. 5) and the phase difference results from the time interval between two adjacent half-cycles (Table 1), the interference between the positive and negative half-cycles (Eq. 8) is analyzed. In conclusion, these phenomena can be attributed to inter-half-cycle interference between positive and negative half-cycles. Conclusions In this study, high harmonic in liquid water driven by two-color laser fields consisting of a fundamental field and its second harmonic is investigated. Our analysis focuses on the spectral blueshift, and time-frequency analysis reveals that the high harmonics are mainly generated during the rising edge of the fundamental pulse, resulting in a blue-shifted harmonic spectrum. Besides, by tuning the relative phase of two-color laser fields, both the amplitude and the center frequency of high harmonic can be modulated. These phenomena can be attributed to the interference of high harmonic emitted from positive and negative half-cycles. This study might shed new light on the attosecond electron dynamics in liquids and the tuning of liquid-phase high harmonic.

We investigate off-axis phase-matched terahertz(THz)radiation in laser plasma pumped by few-cycle laser pulses.We find that the THz amplitude and angular distributions in the far field are sensitively dependent on the pump pulse's focal carrier-envelope phase(CEP).Ring-like profiles of THz radiation are obtained at CEP values of 0.5 π and 1.5π,due to the inversely symmetric local THz waveforms emitted before and after laser focus.Off-axis phase-matched THz radiation offers a tool to accurately measure the CEP of few-cycle pulses at the center of a medium.

Sub-optical-cycle electron dynamics in materials driven by intense laser fields can be investigated by high harmonic generation. We observed frequency shift of high harmonic spectrum near the band gap of monolayer MoS2 experimentally. Through semi-classical quantum trajectory analysis, we demonstrated that the phase of transition dipole moment varies according to the recombination timing and momentum of tunneled electrons. It results in either blue- or red-shift of harmonic frequencies, determined by the modulated energy gap by transition dipole phases (TDPs) and Berry connections. Our finding reveals the effect of TDPs on high harmonic frequency in non-central symmetric materials.

Three-dimensional topological insulators feature unconventional two-dimensional surface states, the carriers in which are helical Dirac fermions and protected from backscattering. Thus, they exhibit novel electronic response upon illuminate ultrashort and intense laser light. We briefly reviewed recent studies on ultrafast phenomena from the surface of the topological insulators driven by laser pulse ranging from visible to THz frequency. Ultrafast dynamics of Dirac fermions can be excited by helical photons and driven by strong light field. Many unique nonlinear behaviors have been demonstrated, such as the excitation of helicity-dependent photocurrent, the formation of Floquet-Bloch bands, lightwave-driven Dirac currents and the generation of optical high-harmonic emission. This review aimed at understanding the microscopic mechanism of the ultrafast charge and spin dynamics in topological surface states and its prospects for coherent manipulation of Dirac fermions by laser light.

The enhancement of even-order harmonics near the cut-off of high-order harmonic spectra from monolayer MoS 2 has been experimentally observed recently by several groups. Here we demonstrate that this enhancement can be interpreted as a result of spectral interference between half-cycles with opposite polarity by adopting a fully quantum mechanical calculation. We found that, due to the energy modulation induced by Berry connections, only half-cycles with the same polarity can generate high-order harmonics near the cut-off frequency, thus the lack of destructive interference leads to the enhanced intensity of the corresponding even-order harmonics. The explanation is supported by the frequency shift of the measured harmonic peaks. Our finding revealed the role of inter-half-cycle interference in high-harmonic generation (HHG) from non-centrosymmetric materials.

We present a velocity-gauge model for the generation of even-order high harmonics, and reveal that the even-order harmonics originate from the multiple-step transitions among the energy bands in momentum space, while the odd-order harmonics are mainly from direct transitions. The lower valence band is found vital for the generation of even harmonics. Relative intensity of even-order harmonics versus the odd orders is calculated and shows a growing trend as the laser field amplitude increases.

We present a scheme to produce the selected left or right elliptically polarized high-order harmonic generation. By adopting bichromatic counter-rotating colinear elliptically polarized pulses, their interaction with oxygen molecule are calculated under the framework of time-dependent density functional theory. Here we show that the helicity-selective high harmonics with tunable ellipticity are generated by changing the ellipticity of one of the pump pulses. Because of the interference of the degenerate highest occupied molecular orbitals, tunable ellipticity in a wide spectral range is obtained. This scheme can be used to synthesize attosecond pulses with controlled ellipticity and helicity.

Three-dimensional topological insulators are a phase of matter that hosts unique spin-polarized gapless surface states that are protected by time-reversal symmetry. They exhibit unconventional charge and spin transport properties 1 , 2 . Intense laser fields can drive ballistic charge dynamics in Dirac bands 3 , 4 or they can coherently steer spin 5 and valley pseudospin 6 . Similarly, high-harmonic generation (HHG) in solids provides insights into the dynamics of the electrons in topological insulators 7 – 13 . Despite several theoretical attempts to identify a topological signature in the high-harmonic spectrum 14 – 16 , a unique fingerprint has yet to be found experimentally. Here, we observe HHG that arises from topological surface states in the intrinsic topological insulator BiSbTeSe 2 . The components of the even-order harmonics that are polarized along the pump polarization stem from the spin current in helical surface states, whereas the perpendicular components originate from the out-of-plane spin polarization related to the hexagonal wrapping effect 17 . The dependence of HHG on surface doping in ambient air also suggests the presence of a Rashba-split two-dimensional electron gas, whose strength can be enhanced by an increase in the intensity of the mid-infrared pump.

Broadband terahertz (THz) emission generated from laser induced gas plasma provides an effective tool for studying nonlinear spectrum, imaging and remote sensing. Recently, the contribution of plasma oscillation to the THz emission was revealed from the nitrogen molecules pumped by intense two-color laser pulses. Plasma oscillation contributes only to the THz emission at relatively low plasma density due to negligible plasma absorption. More generally, with the THz emission generated from the ionizing gaseous medium, the surrounding plasma is expected to play an important role in the generation process. For the THz radiation from laser filament, the plasma region is extended in the laser propagation direction, and the effect of surrounding plasma on the emitted THz spectrum needs studying. In this work, we investigate the relation between pump power and filament length from THz spectrum emitted by air filament driven by two-color laser pulse. The time domain spectrum of THz field is recorded by an electro-optic (EO) sampling technique. In our experiments, significant frequency shifts are observed as the pump power and the filament length increase, and we find that the center frequency of the THz radiation is shifted towards longer wavelength, which is the so called red-shift of the THz spectrum. This red-shift is independent of THz radiation angle. The observations are explained by the plasma absorption inside the air filament. Our theoretical model is based on three mechanisms: the ionization-induced photocurrent, the plasma current oscillation and the plasma absorption. We coherently add up all the local THz fields inside the air filament, and simultaneously consider the plasma absorption induced correction of the THz spectrum. The simulation well reproduces the experimental observation. The skin depth decreases as the plasma density increases, thus the plasma absorption dominates the red-shift process. If the skin depth is larger than the filament length, the plasma oscillation contributes to the THz spectrum dominantly, and thus leading to the blue-shift of THz spectrum. Our results indicate that for the extended filament length or higher plasma density, the combining effect of photocurrent, plasma oscillation and absorption, results in the observed low-frequency broadband THz spectrum. Our study offers a method of coherently controlling the broadband THz spectrum.

Powerful tcrahcrtz (THz) radiation is generated by the interaction between laser and plasma of a super short laser source; such THz radiation is usually single-shot. Herein, we propose a single-shot measurement strategy that involves splitting the THz beam, measuring the THz radiation in the horizontal and vertical directions using two independent spectral detection systems with chirped pulses, and finally obtaining the polarization state. We perform a proof experiment based on this strategy and measure an elliptically polarized THz waveform via the single-shot method. Experimental results agree well with the model analysis, verifying the feasibility of our strategy in addition to providing a new plan for single-shot detection of the polarization state of elliptically polarized THz radiation.

With the development of high-power lasers, terahertz (THz) radiation driven by high-power lasers has received extensive attention. However, it is difficult to measure the terahertz radiation produced by high-power laser devices because of their low repetition rate. This can be solved by using a single-shot time-domain spectrum detection technique. Using this technique, the electric field waveform information of terahertz radiation can be obtained in a single measurement. In this study, we review the research progress with respect to the single-shot time-domain spectrum detection for THz radiation. Based on frequency-time encoding and space-time encoding, we introduce spectral encoding detection using a chirped pulse, space-time encoding detection, and other techniques. In this review, the characteristics and parameters of each measurement technique arc compared. Finally, we summarize and discuss the future prospects of single-shot detection for THz radiation.

We experimentally observe the bond stretching time of one-photon and net-two-photon dissociation pathways of singly ionized H_{2} molecules driven by a polarization-skewed femtosecond laser pulse. By measuring the angular distributions of the ejected photoelectron and nuclear fragments in coincidence, the cycle-changing polarization of the laser field enables us to clock the photon-ionization starting time and photon-dissociation stopping time, analogous to a stopwatch. After the single ionization of H_{2}, our results show that the produced H_{2}^{+} takes almost the same time in the one-photon and net-two-photon dissociation pathways to stretch to the internuclear distance of the one-photon coupled dipole-transition between the ground and excited electronic states. The spatiotemporal mapping character of the polarization-skewed laser field provides us a straightforward route to clock the ultrafast dynamics of molecules with sub-optical-cycle time resolution.