Understanding the dynamics of ferroelectric polarization switching under external stimuli is crucial for optimizing device performance, particularly in terms of operating speed and bandwidth. Here, we employ time-resolved second-harmonic generation (SHG) spectroscopy to probe ultrafast optically driven polarization modulation in bulk NbOCl2. Upon above-bandgap photoexcitation, a significant suppression of the SHG intensity occurs within subpicosecond time scales. The maximum modulation depth of the ferroelectric polarization reaches nearly 50% and scales linearly with pump fluences up to 10 mJ/cm2, reflecting the high stability of both electronic and lattice subsystems in NbOCl2. Supported by time-resolved reflectivity measurements, our findings reveal that polarization dynamics are primarily governed by photocarrier screening coupled with transient electron heating. These results highlight the promise of NbOCl2 for applications in high-speed optically controlled ferroelectric devices.

In situ ultrafast photocarrier dynamics of multilayer black phosphorus(BP)are investigated under pressure using op-tical pump-probe spectroscopy.Below 10 GPa,the transient reflectivity exhibits sub-picosecond saturable absorption(SA)followed by oscillations arising from longitudinal coherent acoustic phonons(CAPs).With increasing pressure,pronounced anomalies in carrier relaxation and CAP behavior are observed,including a strong enhancement of CAP amplitude around 2.0 GPa,associated with a pressure-induced Lifshitz transition.Above 10 GPa the ultrafast response switches from SA to absorption enhancement(AE),accompanied by the complete disappearance of CAPs,indicating the transition to a cubic metallic phase.The pressure-dependent behavior of the CAPs reflects an enhanced interlayer coupling along the cross-plane direction,while the transition from SA to AE dynamics signifies a fundamental shift in in-plane carrier transport.Meanwhile,first-principles calculations reveal a pressure-induced reconstruction of the electronic structure and an increase in the longitudinal acoustic phonon group velocity across Lifshitz transition,supporting the microscopic understanding for anomalous CAP dynamics based on enhanced deformation-potential coupling and electron temperature of Dirac carriers.The study provides critical insights into the pressure-tuned topological transitions and the role of Dirac fermions in the nonequilibrium dynamics of compressed BP.

Recently,a semiconducting material NbOCl2,a room-temperature ferroelectric with weak interlayer coupling and a strong nonlinear optical response,has attracted significant attention owing to its potential applications in ultra-compact on-chip photonic devices.In this study,we systematically investigate the photogenerated carrier dynamics,ferroelectric polarization behavior,and evolution of the electronic structure in NbOCl2 under high pressure using a diamond anvil cell(DAC)combined with in-situ measurement techniques,including femtosecond optical pump optical probe(fs-OPOP)spectroscopy,steady-state second harmonic generation(SHG),and time-resolved second harmonic generation(TR-SHG)spectroscopy.Our experimental results reveal that as pressure increases,the SHG intensity significantly decreases,with a turning point in its pressure dependence appearing around 2 GPa,suggesting a ferroelectric-to-antiferroelectric phase transition.Upon further compression to approximately 10 GPa,the system exhibits a coexistence of ferroelectric and antiferroelectric phases.The pressure-induced evolution of the optical bandgap,derived from high-pressure absorption spectroscopy,further indicates a close correlation between changes in the electronic structure and the SHG signal.Using high-pressure fs-OPOP spectroscopy,we observe exciton formation on a sub-picosecond timescale and rapid interband recombination involving surface defects on a picosecond timescale,with the relaxation times of both processes increasing with pressure.The bandgap narrowing under high pressure increases the energy difference between the pump photon energy and the bandgap,resulting in photogenerated carriers with higher initial kinetic energy and electron temperature,thereby prolonging hot carrier cooling and exciton formation,which is reflected in an increase in the fast time constant.The evolution of the slow relaxation time with pressure results from the competition among several factors,including bandgap narrowing,enhanced many-body effects,and changes in defect states.Furthermore,high-pressure TR-SHG experiments reveal that above-bandgap excitation induces modulation of the ferroelectric polarization.The relaxation behavior observed in these experiments is consistent with the photocarrier dynamics revealed by fs-OPOP spectroscopy under low-pressure conditions,supporting a physical picture in which photogenerated carriers regulate ferroelectric polarization via a charge screening mechanism.This work provides important experimental evidence for investigating the non-equilibrium dynamic behavior of ferroelectric materials and achieving ultrafast modulation of ferroelectric polarization under high-pressure conditions.

High-pressure synthesis of lutetium hydrides from molecular hydrogen (H2) and lutetium (Lu) is systematically investigated using synchrotron X-ray diffraction, Raman spectroscopy, and visual observations. We demonstrate that the reaction pathway between H2 and Lu invariably follows the sequence Lu ⟶ LuH2 ⟶ LuH3 and exhibits a notable time dependence. A comprehensive diagram representing the formation and synthesis of lutetium hydrides as a function of pressure and time is constructed. Our findings indicate that the synthesis can be accelerated by elevated temperature and decelerated by increased pressure. Notably, two critical pressure thresholds at ambient temperature are identified: the synthesis of LuH2 from Lu commences at a minimum pressure of ∼3 GPa, while ∼28 GPa is the minimum pressure at which LuH2 fails to transform into LuH3 within a time scale of months. This underscores the significant impact of temporal factors on synthesis, with the reaction completion time increasing sub-linearly with rising pressure. Furthermore, the cubic phase of LuH3 can be obtained exclusively through compressing the trigonal LuH3 phase at ∼11.5 GPa. We also demonstrate that the bandgap of LuH3 slowly closes under pressure and is noticeably lower than that of LuH2.

Two-dimensional noble transition metal chalcogenides (NTMCs) are of potential use for creating high-performance electronic and optoelectronic devices. However, the direct growth of high-quality multicomponent NTMC materials remains a significant challenge. Herein, we report the successful fabrication of quaternary AuPdNaS2 nanoribbons on a Au foil substrate by an ambient-pressure chemical vapor deposition approach. The vibrational properties and carrier dynamics of AuPdNaS2 nanoribbons are experimentally elucidated using temperature-dependent Raman spectroscopy and ultrafast optical pump-probe spectroscopy. In addition, the AuPdNaS2 photodetector exhibits a high photoresponsivity of 235.5 A/W and a good detectivity of 1.21 × 1012 Jones under 660 nm laser illumination. This work demonstrates the synthesis of quaternary AuPdNaS2 and paves the way for multicomponent NTMC materials in optoelectronic applications.

Coherent terahertz (THz) emission driven by ultrafast light-matter interactions is central to the development of advanced photonic and optoelectronic technologies. However, enhancing THz emission efficiency remains challenging due to the intrinsic dependence of generation mechanisms-optical rectification (OR) and shift current (SC)-on fixed material properties such as lattice symmetry and electronic structure. Here, we demonstrate hydrostatic pressure as an effective in situ control parameter for modulating THz emission in a two-dimensional GaTe crystal. Using ultrafast THz emission spectroscopy in a diamond anvil cell (DAC), we observe a more than 13-fold enhancement in THz output under compression. By tuning the excitation wavelength, we uncover a pressure-induced transition from bound-electron OR to free-carrier SC, featuring a systematic forward time shift in THz waveform. First-principles calculations reveal that the enhanced emission and time shift originate from pressure-driven changes in resonance frequency and charge density. These results highlight hydrostatic pressure as a powerful means to tailor nonlinear light-matter interactions and optimize coherent THz emission in low-dimensional systems.

Unveiling the nonlinear interactions between terahertz (THz) electromagnetic waves and free carriers in two-dimensional materials is crucial for the development of high-field and high-frequency electronic devices. Herein, we investigate THz nonlinear transport dynamics in a monolayer graphene/MoS2 heterostructure using time-resolved THz spectroscopy with intense THz pulses as the probe. Following ultrafast photoexcitation, the interfacial charge transfer establishes a nonequilibrium carrier redistribution, leaving free holes in the graphene and trapping electrons in the MoS2. When probed with intense THz pulses exceeding 34 kV/cm in a peak electric field, significant THz saturable absorption is observed over a period of 20 ps. Furthermore, the photoinduced change in the transmitted THz waveform, linked to the THz-driven nonlinear current, manifests as a substantial self-phase modulation. These nonlinear responses can be attributed to the competition between rapid carrier heating and slow carrier cooling via electron-electron and electron-phonon scattering in the charge-transfer-induced hole system of the graphene layer. This work demonstrates an integration of advantages arising from robust nonlinear absorption in graphene and enhanced photocarrier harvesting in transition metal dichalcogenides by exploiting heterostructure construction.

Nonlayered palladium sulfide (PdS) is of interest due to its rich physical properties and promising applications in optoelectronic devices. However, the growth of thin nonlayered PdS remains challenging because of its intrinsic 3D lattice structure. Here, the first demonstration of the direct synthesis of thin rectangular PdS ribbons/flakes on SiO2/Si substrates by a facile chemical vapor deposition (CVD) approach is presented. The atomic structure and high crystalline quality of CVD-derived PdS crystals are shown by scanning transmission electron microscopy. The nonlinear saturable absorption and absorption enhancement are revealed by using ultrafast optical pump-probe spectroscopy, and the photocarrier dynamics present the hot phonon bottleneck and Auger recombination effects. Additionally, the Raman vibration modes display the polarization-dependent properties verified by angle-resolved polarized Raman spectroscopy. Importantly, the photodetector based on PdS ribbon demonstrates a decent photoresponsivity of approximate to 7.7 x 103 A W-1. This results provide an effective way to form thin nonlayered PdS with potential applications in the field of photodetection. Thin nonlayered PdS crystals are synthesized on SiO2/Si substrates with a thickness down to 8.2 nm by a facile chemical vapor deposition strategy. The atomic structure and high crystalline quality of CVD-derived PdS crystals are shown by scanning transmission electron microscopy. Significantly, the PdS-based photodetector shows a high photoresponsivity of approximate to 7.7 x 103 A W-1 under 470 nm laser. image

Pentagonal palladium diselenide (PdSe2) stands out for its exceptional optoelectronic properties, including high carrier mobility, tunable bandgap, and anisotropic electronic and optical responses. Herein, we systematically investigate photocarrier dynamics in PdSe2 ribbons using polarization-resolved optical pump-probe spectroscopy. In thin PdSe2 ribbons with a semiconductor phase, the photocarrier dynamics are found to be dominated by intraband hot-carrier cooling, interband recombination, and the exciton effect, showing weak crystalline orientation dependences. Conversely, in thick semimetal-phase PdSe2 ribbons, the photocarrier relaxations governed by the electron-optical/acoustic phonon scattering strongly depend on the sample orientation, albeit with a degradation in in-plane anisotropy following hot-carrier cooling. Furthermore, we analyze the correlations between photocarrier dynamics and anisotropic energy dispersions of electronic structures across a wide range in k space, as well as the contributions from the anisotropic electron-phonon couplings. Our study provides crucial insights for developing polarization-sensitive photoelectronic devices based on PdSe2.

MnBi2Te4 can generate a variety of exotic topological quantum states, which are closely related to its special structure. We conduct comprehensive multiple-cycle high-pressure research on MnBi2Te4 by using a diamond anvil cell to study its phase transition behaviors under high pressure. As observed, when the pressure does not exceed 15 GPa, the material undergoes an irreversible metal–semiconductor–metal transition, whereas when the pressure exceeds 17 GPa, the layered structure is damaged and becomes irreversibly amorphous due to the lattice distortion caused by compression, but it is not completely amorphous, which presents some nano-sized grains after decompression. Our investigation vividly reveals the phase transition behaviors of MnBi2Te4 under high pressure cycling and paves the experimental way to find topological phases under high pressure.

VO2, which exhibits semiconductor-metal phase transition characteristics occurring on a picosecond time scale, holds great promise for ultrafast terahertz modulation in next-generation communication. However, as of now, there is no reported prototype for an ultrafast device. The temperature effect has been proposed as one of the major obstacles. Consequently, reducing the excitation threshold for the phase transition would be highly significant. The traditional strategy typically involves chemical doping, but this approach often leads to a decrease in phase transition amplitude and a slower transition speed. In this work, we proposed a design featuring a highly conductive MXene interfacial layer between the VO2 film and the substrate. We demonstrate a significant reduction in the phase transition threshold for both temperature and laser-induced phase transition by adjusting the conductivity of the MXene layers with varying thicknesses. Our observations show that the phase transition temperature can be decreased by 9 C-degrees, while the pump fluence for laser excitation can be reduced by as high as 36%. The ultrafast phase transition process on a picosecond scale, as revealed by the optical-pump terahertz-probe method, suggests that the MXene layers have minimal impact on the phase transition speed. Moreover, the reduced phase transition threshold can remarkably alleviate the photothermal effect and inhibit temperature rise and diffusion in VO2 triggered by laser. This study offers a blueprint for designing VO2/MXene hybrid films with reduced phase transition thresholds. It holds significant potential for the development of low-power, intelligent optical and electrical devices including, but not limited to, terahertz modulators based on phase transition phenomena.

Photocarrier dynamics of the ZrTe 3 under pressure are investigated using optical pump-probe (OPOP) spectroscopy in combination with a diamond anvil cell. A prominent laser heating effect is manifested, characterized by significant changes in the profiles and an elongation of the echo period as the pump fluence is increased. Furthermore, this heating effect is found to be enhanced at pressures below 2 GPa, gradually diminishing until it completely disappears at 6 GPa. Additionally, the estimated sound velocity at high pressures indicates a rapid increase with pressure. This study not only assesses the potential application of ZrTe 3 in the field of ultrafast optoelectronic devices but also provides fundamental understanding on the electron structural transition under pressure.

Transient terahertz responses and terahertz emission performances for the GaTe under pressure are examined by time-resolved terahertz spectroscopy. The terahertz emission strength rockets with increasing pressure whereas rapidly declines above 10 GPa, implying the insulator-metal transition. Decay time of the pumped hot carriers also shows incontinuity at the corresponding pressure.

Graphene hosts intriguing photocarrier dynamics such as negative transient terahertz（THz） photoconductivity, high electron temperature, benefiting from the unique linear Dirac dispersion. In this work, the pressure effects of photocarrier dynamics of graphene have been investigated using in situ time-resolved THz spectroscopy in combination with diamond anvil cell exceeding 9 GPa. We find that the negative THz conductivity maintains in our studied pressure range both for monolayer and bilayer graphene. In particular, the amplitude of THz photoconductivity in monolayer graphene manifests an extraordinary dropping with pressure, compared with that from the counterparts such as bulk silicon and bilayer graphene.Concomitantly, the time constant is reduced with increasing pressure, highlighting the pressure-induced hot carrier cooling.The pressure dependence of photocarrier dynamics in monolayer graphene is likely related with the enhancement of the interfacial coupling between diamond surface and sample, allowing for the activity of new electron–phonon scattering. Our work is expected to provide an impetus for the studies of high-pressure THz spectroscopy of two-dimensional materials.

The lateral and temporal walk-offs between fundamental wave and second harmonic wave are compensated simultaneously, which enhances THz conversion efficiency by a factor of 3 in our homemade air plasma system. Second harmonic generation efficiency is also optimized for THz radiation. Together with a dual-wavelength half-waveplate, a high THz conversion efficiency of > 0.06% is achieved at 800 nm excitation in the dry air environment, corresponding to an overall enhancement factor of 6. In the detection part, electro-optic sampling and air-biased coherent detection are experimentally investigated. By using our air plasma system, two characteristic peaks (4.85 and 5.8 THz) of the rust sample are identified.

We investigate the nonequilibrium photocarrier dynamics of WTe2 under pressure using the optical pump-probe spectroscopy. The pressure dependences of the electronic relaxation manifest anomalous changes around 0.8, 3.5, and 6 GPa, indicating the abruptions in the electron-phonon interactions. In addition, the coherent phonon oscillations originating from shear mode suddenly disappears above 3.5 GPa, which marks the onset of Td-1T' structural phase transition. Supported by the theoretical calculation, we unveil the electronic topological transitions (ETTs), especially an emergence of a new type-II Weyl point for Td-WTe2 under pressure. Our work demonstrates a novel route to probe the ETTs under pressure.

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

二维PtSe2具备宽可调带隙、高稳定性等优点,在新型光电器件方面具有极大应用价值.利用时间分辨太赫兹光谱研究了不同厚度PtSe2中的光生载流子超快动力学,发现该材料瞬态太赫兹光电导的幅度及其激发光强度依赖性随材料厚度的增加呈现出显著的非线性增加趋势.通过太赫兹光电导频谱分析,获得了光生载流子浓度、散射时间、背散射因子等动力学参数,并结合激发波长依赖的太赫兹弛豫动力学,推测束缚激子和自由载流子的竞争是引起这种厚度非线性关系的主要原因.此外,基于光泵浦-光探测光谱证明了 PtSe2中的激子效应及半导体-半金属转变.该工作演示了层数对PtSe2中非平衡态动力学的有效调控,对贵金属基二维材料在光电器件方面的应用具有指导意义.