铌酸锂强场太赫兹非线性时域光谱系统

Lithium Niobate Strong⁃Field Terahertz Nonlinear Time⁃Domain Spectroscopy System

强场太赫兹(THz)时域光谱技术在强场THz科学技术与应用中具有重要作用,材料、物理、化学、生物等领域诸多涉及强场THz与物质强非线性相互作用的研究都离不开强场THz时域光谱技术。然而,受限于高效率、高光束质量、高稳定性、高重复频率强场THz辐射源的性能,强场THz时域非线性光谱技术发展缓慢。针对强场THz非线性光谱技术及其潜在应用中存在的难题与挑战,在商用kHz钛宝石激光器的驱动下,笔者设计并实现了一套基于铌酸锂倾斜波前技术产生强场THz的高度集成化时域光谱系统。在3 mJ激光能量泵浦下,利用该系统在室温下实现了单脉冲能量为6.5μJ、峰值场强约为350 kV/cm的THz强场产生,该系统具备强场THz非线性光谱测试、THz泵浦‑THz探测、光泵浦‑THz探测、THz发射谱测量等多种超快时间分辨测量功能,是研究强场THz非线性效应的有效实验手段。

Objective The strong-field terahertz(THz)time-domain spectroscopy is fundamental in strong-field THz science,technology,and applications.Strong-field THz time-domain spectroscopy is also indispensable in many fields including materials,physics,chemistry,and biology,which involve strong nonlinear interactions between strong-field THz and matter.However,the unavailability of a highfield free-space THz source with high repetition rate,excellent beam quality,and high stability hinders its development.In this study,we designed and independently developed a highly integrated strong-field THz nonlinear time-domain spectroscopy system based on a lithium niobate(LN)strong-field THz source.The proposed system is driven by a kilohertz femtosecond laser amplifier and exhibits the functions of a strong-field THz nonlinear spectrum,THz-pump THz probe(TPTP),strong-field THz-pump optical probe,optical-pump THz probe(OPTP),and THz emission spectrum.This highly integrated strong-field THz nonlinear timedomain spectroscopy system is a powerful tool for analyzing the nonlinear effects of strong-field THz waves.Methods We developed a strong-field THz nonlinear time-domain spectroscopy system.We employed a Ti∶sapphire femtosecond laser amplifier that provided laser pulses with a center frequency of 800 nm,pulse duration of 35 fs,repetition rate of 1 kHz,and maximum pump power of 5 mJ.The laser input to the system was split using an 80∶20 beam splitter.The transmitted beam(80%)was employed as a pump beam to generate strong-field THz waves from the LN crystal through optical rectification based on the tilted pulse front technique.The strong-field THz waves generated by the LN crystal was used to induce and probe the nonlinear effects.The residual 20%femtosecond laser energy was divided into three beams:the first for the optical pump,the second for generating a weak-field THz probing beam in a ZnTe emission crystal(ZnTe 1),and the third for electro-optic sampling of both the pump and probing THz temporal waveforms.Three delay lines were employed to synchronize the strong-field THz,optical pump,and electrooptic sampling.THz temporal waveforms were detected by an electro-optic sampling system consisting of a ZnTe crystal,quarterwave plate,Wollaston prism,and two photodiodes for the coherent detection of THz pulses based on the principle of the electro-optic effect.Results and Discussions In this work,the study on LN strong-field THz nonlinear spectroscopy is summarized as follows.First,for the strong-field THz generation and detection system(Fig.2),the energy conversion efficiency of near-infrared light to THz waves was approximately 0.22%.At the focus of off-axis parabolic mirror 2(OAP2),the calculated peak electric field can reach 350 kV/cm.The focused THz beam profile can be detected using a THz camera and temperature-sensitive paper.Based on this strong-field THz static nonlinear spectroscopy system,we observed the nonlinear absorption caused by the intervalley scattering of doped silicon induced by strong-field THz using the Z-scan technique(Fig.3).In addition,the strong-field THz-induced nonlinear transmission self-frequency modulation of the THz nonlinear metasurface further demonstrates the excellent ability of strong-field THz nonlinear spectroscopy in the frequency domain(Fig.4).Second,the pump-probe technique is an essential research method for performing strong-field THz nonlinear spectroscopy.The TPTP technique can be achieved by introducing a THz probing beam generated by a ZnTe emission crystal,a coaxially aligned THz pump(generated by LN),and a THz probe,followed by focusing on the sample with an off-axis parabolic mirror(Fig.5).The THz probing beam was modulated by a chopper with a 500 Hz rotor frequency to obtain a pure probing signal.A THz polarizer was positioned behind the sample with its polarizing orientation perpendicular to the THz pump to further restrict the transmitted THz pump.Using the TPTP technology with a spectral resolution,we observed dynamic changes in the resonant frequency of the THz probe transmission spectra induced by a strong THz field on nonlinear THz metasurface samples.This phenomenon demonstrates that carrier production in this SRRs sample is caused by the impact ionization of high-resistance silicon.Third,we realized the OPTP technique by introducing an 800 nm pumping beam.The strong-field THz waves generated by the LN crystal can be used as the probing beam.By adjusting the incident THz field strength,the transmission signal self-modulation induced by the strong-field THz under an 800 nm pump is measured(Fig.6).For more common OPTP applications,the weak-field THz generated by the ZnTe emission crystal is used as the probing beam,which is primarily used to investigate the ultrafast dynamics of carriers in semiconductors(Fig.7).By adding a spectral resolution,the photoexcitation dynamics with different frequencies can be analyzed more comprehensively.Finally,the proposed strong-field THz nonlinear time-domain spectroscopy system exhibits THz emission spectral capability.When stimulated by a femtosecond laser,the induced THz pulse carries a significant amount of physical information in its waveform.Taking the spin THz emission as an example,we demonstrate the flexibility of the system in examining the emission characteristics of the W/CoFeB/Pt thin-film structure(Fig.8).Conclusions Strong-field THz nonlinear spectroscopy has become a critical method for studying the nonequilibrium behaviors resulting from strong THz-matter interactions.Based on the self-developed LN strong-field THz nonlinear-time-domain spectroscopy system,various experimental methods of strong-field THz nonlinear spectroscopy were studied and explained,demonstrating the unique ability and essential role of strong-field THz nonlinear spectroscopy in basic research.In addition,with a slight adjustment,the system can also be used for two-dimensional THz spectroscopy,THz electron acceleration,and the THz Kerr effect.This highly integrated and miniaturized THz time-domain spectrometer provides comprehensive research capabilities and potential for nonlinear THz spectroscopy in physics,materials,biology,and engineering applications.