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Ferroelectric materials have been a key research topic owing to their wide variety of modern electronic and photonic applications. For the quick exploration of higher operating speed, smaller size, and superior efficiencies of novel ferroelectric devices, the ultrafast dynamics of ferroelectrics that directly reflect their respond time and lifetimes have drawn considerable attention. Driven by time‐resolved pump‐probe spectroscopy that allows for probing, controlling, and modulating dynamic processes of ferroelectrics in real‐time, much research efforts have been made to understand and exploit the ultrafast dynamics of ferroelectric. Herein, the current state of ultrafast dynamic features of ferroelectrics tracked by time‐resolved pump‐probe spectroscopy is reviewed, which includes ferroelectrics order parameters of polarization, lattice, spin, electronic excitation, and their coupling. Several potential perspectives and possible further applications combining ultrafast pump‐probe spectroscopy and ferroelectrics are also presented. This review offers a clear guidance of ultrafast dynamics of ferroelectric orders, which may promote the rapid development of next‐generation devices., In this review, the ultrafast dynamic features of ferroelectrics, including polarization, lattice, spin, electronic excitation, and their coupling, tracing by time‐resolved pump‐probe spectroscopy are systematically discussed and summarized. The various potential perspectives on ultrafast pump‐probe spectroscopy and the additional nondestructive functionality for ultrafast ferroelectric device design are also presented.

All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to control the high-field processes at the nanoscale. Here we report enhanced non-perturbative high-harmonic and terahertz emission from a LiNbO3 metasurface. The harmonic and terahertz intensity are enhanced by more than one order of magnitude compared to unpatterned samples. The enhanced high harmonics and terahertz waves are highly anisotropic with respect to the excitation polarization and are selective by the excitation wavelength due to its resonant features.

Terahertz (THz) generation in a gas medium is simulated by quantum calculations with multicolor intense laser pulses of various frequency ratios. By correlating THz radiation with the above-threshold-ionization photoelectron spectrum, we confirm that our previously proposed free-free transition model for the generation of THz radiation is also applicable to multicolor pulses. THz photon emissions can proceed through transitions between continuous states with similar energies that have opposite parities. This mechanism predicts that THz waves can be efficiently generated at special two-color laser frequency ratios when the multiphoton mixing condition is satisfied. Applying this model to multicolor fields (sawtooth wave shape), we provide a quantum-mechanical interpretation for the reason for THz radiation enhancement. A scheme combining multiple lasers to raise THz radiation satisfying multiphoton mixing conditions is also proposed.

During the OPA (optical parametric amplifier) processes pumped by a high-energy laser, the phase distortion of pump light will transmit to the signal light inevitably. As a result, the beam quality of amplified signal will become poor. In this article, a hybrid seeding scheme with a paired two-stage OPA system is used, where the first and second stages used the signal and the idler as seed sources, respectively. The numerical results show that the hybrid seeding OPA scheme can suppress the phase distortion transformation from pump to signal in both spatial and temporal domains. Consequently, the phase of output signal is well restored to its input state. Because of the restriction removal of beam quality for pump light under the conventional birefringent phase-matching method, this scheme had a good application prospect in high-energy OPA or optical parametric chirped pulse amplification systems.

Above-band-gap optical excitation of electron-hole pairs screens the doping-induced surface electric field and generates terahertz (THz) pulses via free-carrier transport. THz emission from a heavily doped silicon surface is much weaker than that of lightly doped samples. A polarity reversal of the THz electric field is observed in heavily doped p-type silicon, indicating that the doping related and carrier induced surface electric fields oppose each other. By comparing the penetration depth of the excitation laser with the thickness of the depletion layer for the doped silicon, it is shown that competition between diffusion and drift current causes the polarity reversal.

Thanks to its unique frequency range, Terahertz (THz) wave is of fundamental importance in fusion plasmas diagnostics [1]. In this work, we demonstrate the experimental results of the laser induced plasma diagnostics at the THz frequency range [2]. Resonance absorption has been observed and identified through THz spectroscopy, which can be understood by a Lorentz-modified Drude model. The absorption is studied by varying the polarization and energy of the plasma-generated pulse. The results suggest no significant polarization dependence on the absorption features. As the laser intensity of the plasma-generated pulse grows, the central frequency of the resonance absorption increases, as well as the electron density. This indicates the intrinsic relationship between the plasma frequency and the resonance absorption. Therefore, the resonance absorption at the THz frequency range provides an alternative to characterize plasma properties, especially for the plasma with typical frequency at the THz range. Furthermore, our results are supplements to the existing theories on THz generation from the filament. When using the fil- ament as a source to generate the THz wave, the absorptions and the phase variations, as determined by the electron density, should be considered, both in calculations and experiments.

We report second harmonic generation with BBO crystal from intense few-cycle infrared femtosecond laser pulses, we managed to get broadband phase-matching with 14.2% conversion efficiency, and the result ultraviolet spectra supports 7.0 fs pulses.

We experimentally demonstrate that the optical-to-THz conversion efficiency in circularly-polarized two-color laser fields is higher than that in linearly-polarized two-color system.

We experimentally studied the dissociative double ionization of Ar2 in femtosecond laser fields of different light intensity. The experimental results show a significant regulatory effect of light intensity to dissociative double ionization of Ar2.

We establish a one-to-one mapping between the local phase slip and the spatial position near the focus by scanning a thin jet along the propagation direction of laser beams. The measurement shows that the optimal phase of THz can be utilized to characterize in situ the spatially dependent relative phase of the two-color field and the carrier-envelope-phase of the few-cycle pulse. The result is of critical importance for any phase-dependent application of strong laser-field including high-order harmonic and terahertz generation.

We study polarity reversal in terahertz pulses generated from p-doped Si(001) surfaces with different doping levels. The azimuthal dependence of both terahertz emission and second-harmonic generation caused by the bulk electric quadrupole and magnetic dipole process is measured to determine the contributions of optical rectification and the surface depletion field direction, respectively. By comparing the penetration depth of the excitation laser and the depletion field strength on Si surfaces, we show that competition between the photo-Dember effect and surface field accelerations causes the polarity reversal.

We present in situ measurements of the relative phase shift induced by Gouy phase shift near the focus in two-color field. The good agreement between the measurement and the theoretical prediction shows that the optimal phase of THz is robust enough as an useful tool to measure the relative phase change.

We experimentally demonstrated that the circularly polarized two-color femtosecond pulses with the same helicity can effectively enhance terahertz intensity by a factor of 5 in comparison with the linearly polarized counterparts. The quantum mechanical simulations confirm this enhancement based on the analysis of residual current calculated by time-dependent Schrodinger equation in three dimensions. The report not only provides a technique to increase the terahertz intensity but also arouses the interest to study the formation of the air plasma induced by circularly-polarized two-color laser-field.

We establish a one-to-one mapping between the local phase slip and the spatial position near the focus by scanning a thin jet along the propagation direction of laser beams. The measurement shows that the optimal phase of terahertz can be utilized to characterize in situ the spatially dependent relative phase of the two-color field. We also investigate the role of the Gouy phase shift on terahertz generation from two-color laser-induced plasma. The result is of critical importance for phase-dependent applications of two-color laser-field, including high-order harmonic and terahertz generation.

We study the generation of terahertz radiation from atoms and molecules driven by an ultrashort fundamental laser and its second harmonic field by solving time-dependent Schr\"odinger equation (TDSE). The comparisons between one-, two-, and three- dimensional TDSE numerical simulations show that initial ionized wave-packet and its subsequent acceleration in the laser field and rescattering with long-range Coulomb potential play key roles. We also present the dependence of the optimum phase delay and yield of terahertz radiation on the laser intensity, wavelength, duration, and the ratio of two-color laser components. Terahertz wave generation from model hydrogen molecules are further investigated by comparing with high harmonic emission. It is found that the terahertz yield is following the alignment dependence of ionization rate, while the optimal two-color phase delays varies by a small amount when the alignment angle changes from 0 to 90 degrees, which reflects alignment dependence of attosecond electron dynamics. Finally we show that terahertz emission might be used to clarify the origin of interference in high harmonic generation from aligned molecules by coincidently measuring the angle-resolved THz yields.

Employing a novel polarization-resolved detection, we precisely measured the polarization of terahertz emission from ionized gases in two-color field composed of circular polarized fundamental beam and linear polarized second harmonic. It shows that the polarization of the THz emission rotates with the phase delay and forms an attoclock. The polarization angular deviations of THz emissions in different laser intensities and different atoms reveal the distinct tunneling time delays. The polarization-resolved THz detection can be used to study tunneling ionization.

We report the joint measurements of terahertz wave and high harmonic spectra generated from aligned nitrogen molecules. Our results show the yields of terahertz wave and high harmonics are sensitive to the alignment angle. This observation clarifies that electronic structure play an important role in the generation of terahertz and high harmonics from molecules.

In this work, we characterize the transmission properties of the focusing-laser-induced filament by using terahertz (THz) time domain spectroscopy. Significant resonance absorption of the THz wave is identified, which is attributed to the dynamic screening of the ionized electrons. The polarization and intensity of the filament-generated pulse have been varied to study their influences on the resonance absorption. Our results suggest that the electron density of the filament can be determined by fitting the phase shifts and absorption properties around the absorption. This method provides not only an alternative to the plasma diagnostics with plasma frequency at the THz range, but also a supplement to the existing generation theory of THz waves from plasma.

We verify the microscopic mechanism of terahertz emission in two-color laser-produced plasmas by simultaneously measuring laser pulse phases, plasma currents, and terahertz radiation.

Terahertz radiation from tunneling ionization of gaseous atoms and molecules in the two-color laser fields with various polarizations has been investigated. We experimentally demonstrate that the efficiency of terahertz emission in the circularly polarized laser fields with the same helicity is 5 times higher than that with linearly polarized two-color femtosecond pulses in high laser intensity. By solving time-dependent Schrodinger equation, this enhancement is well explained based on the analysis of electron tunneling ionization and subsequent dynamics.

We perform a joint measurement of terahertz waves and high-order harmonics generated from Argon atoms driven by a fundamental laser pulse and its second harmonic. By correlating their dependence on the phase-delay of the two pulses, we determine the generation of THz waves in tens of attoseconds precision. Compared with simulations and models, we find that the laser-assisted soft-collision of the electron wave packet with the atomic core plays a key role. It is demonstrated that the rescattering process, being indispensable in HHG processes, dominant THz wave generation as well but in a more elaborate way. The new finding might be helpful for the full characterization of the rescattering dynamics.

We perform a joint measurement of terahertz waves and high-harmonics generated from argon atoms driven by a fundamental laser pulse and its second harmonic. By correlating their dependence on the phase delay between the two pulses, we determine the generation of THz waves in tens of attoseconds precision. Compared with simulations and models, we find that the laser-assisted soft collision of the electron wave packet with the atomic core plays a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way. The new finding might be helpful for the full characterization of the rescattering dynamics.

We reported a technology of gray-scale imaging using broadband terahertz pulse. Utilizing the spatial distribution of different frequency content, image information can be acquired from the terahertz frequency domain analysis. Unlike CCDs(charge-coupled devices) or spot scanning technology are used in conversional method, a single-pixels detector with single measurement can meet the demand of our scheme. And high SNR terahertz imaging with fast velocity is believed.

We have observed terahertz generation via injection current induced by harmonically related two-color beams in an unbiased ZnSe bulk at room temperature using a femtosecond Ti:sapphire oscillator. The terahertz intensity is just several times smaller than that obtained via optical rectification and further enhancements are believed possible. Experimental results demonstrate that the terahertz radiation is mainly attributed to the transition from the split-off band. This conclusion provides a novel approach to effectively generate a broadband and coherently controlled terahertz radiation, which leads to practical applications of terahertz radiation via this mechanism.

The impurities sandwiched between two plastic plates and the inside layout of the integrate circuit chip is mapped by terahertz time domain spectroscopy. They are both clearly showed in the amplitude and phase mapping of the transmitted terahertz waves.

We perform a joint measurement of terahertz waves and high-order harmonics generated from noble atoms driven by a fundamental laser pulse and its second harmonic. By correlating their dependence on the phase-delay of the two pulses, we determine the generation of THz waves in attosecond resolution.

Terahertz (THz) emissions of an isolated atom in an ultrashort (100 fs) laser field are simulated by solving the time-dependent Schr\"odinger equation. From numerical calculations with one- and three-dimensional hydrogen atom models and a short-range shallow potential model, it can be concluded that continuum THz emissions occur more readily following transitions involving intermediate states above rather than those well below the ionization threshold of the system. Line-shaped THz emissions from transitions between high-lying Rydberg states are also found. The models are also used to describe the observed enhanced terahertz emissions with a superposed second-order harmonic laser field or a spatially constant electric field. The dependence of the THz field strength on the intensities of the fundamental laser field and the superposed field is also examined. Strong field approximation is extended to analyze the general features of THz emissions resulting from continuum free-free transitions of an electron in strong laser fields. These calculations contribute to an understanding of the THz emission processes when strong laser fields interact with atomic and molecular systems that have larger ionization potentials and where multiphoton processes are involved in order to generate THz emissions effectively.

Fundamental and high-order phonon-phonon interactions dominate the far infrared absorption and dispersion spectrum of most semiconductors and dielectrics. We present the detailed investigation of the temperature dependence of the dielectric function of the semiconductor GaSe in the frequency range below 4 THz, between 100 K and 310 K, using terahertz time-domain spectroscopy (THz-TDS) technique. Combined with the Fourier translation infrared (FTIR) spectroscopy, the complete infrared absorption spectra, including both sum and difference two-phonon absorption, are obtained. From the dielectric function we determine the blue-shift of the fundamental phonon frequency Epsila at 0.59 THz with the decrease of the temperature. Furthermore, our experimental data enable assignment of the two-phonon terahertz absorption to difference combinations of fundamental phonon mode at critical points in the Brillouin zone center.

This paper presents the refractive index, absorption coefficient of layered GaSe crystal by means of terahertz time-domain spectroscopy. A unique resonance peak was observed at 0.59 THz (19.6 cm-1 ), which attributes to the E' phonon mode of epsiv-GaSe.

Argon atom ionizing in the ultrafast two-colour field has been used to investigate the physical mechanism of terahertz (THz) generation. We measured simultaneously pulse energies of THz and high-harmonics as the phase delay between the fundamental and its second harmonic was varied. The optimal phase delay of THz generation is determined according to the inherent attochirp of the emitted high-harmonics. The dynamic analysis of the tunnelling electron wave packet driven by the Coulomb–laser coupling shows that laser-assisted soft collisions of the electron wave packet with the atomic core play a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way.

Employing an orientation-modulated bias field, a polarization-sensitive scheme for terahertz air-biased-coherent-detection (THz-ABCD) is presented to directly measure the amplitude and polarization angle of THz field in the time domain. It can provide all characteristics of arbitrarily polarized THz wave with one single-scan measurement. Measuring convenience, broad bandwidth, and high angular resolution have been achieved. Polarization-sensitive THz time-domain spectroscopy can surely be developed based on this technology. Many other applications in the THz spectral region are also believed.

Terahertz radiation from two-color laser-produced plasma is studied with simultaneous measurements of absolute two-color phases, near-field plasma currents, and far-field THz radiation. This verifies the microscopic mechanism of terahertz radiation generation at various laser intensities.