Exploring fundamental limits to terahertz generation in electrooptic materials: From bulk to nanolayers

We review the progress made by us on the exploration of the fundamental limits to terahertz (THz) generation from several semiconductor electrooptic materials. Through the measurements of the THz output versus the pump beam in terms of incident angle, polarization, azithumal angle, and pump intensity, we have demonstrated that we can precisely determine the contributions made by the optical rectification and photocurrent surge. When a material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, these two mechanisms often compete with each other, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients are resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also illustrated that, for some materials, two-photon absorption can be one of the fundamental limits to the THz generation.     

Investigation of symmetries of second-order nonlinear susceptibility tensor of GaSe crystals in THz domain

Following our measurements and analysis made on several GaSe crystals, we demonstrated that terahertz (THz) generation from ultrafast laser pulses can be developed into a sensitive technique for investigating symmetries of second-order nonlinear susceptibility tensor of a nonlinear crystal. Indeed, for GaSe crystals, both Kleinman's symmetry condition and spatial symmetry were violated due to the contribution of ionic displacement to nonlinear polarization and deviation of GaSe lattice from hexagonal symmetry. When the pump photon energy was increased from that below the bandgap of GaSe to that above it, the mechanism for the THz generation was switched from optical rectification to photocurrent surge.     

Resonant terahertz generation from InN thin films

Highly efficient conversion from ultrafast optical pulses to their terahertz (THz) counterparts has been achieved with InN thin films. An average THz output power as high as 0.931 microW has been obtained for an average pump power of 1 W, corresponding to a normalized conversion efficiency of 190% mm(-2). Based on our measured dependences of the THz output power on pump polarization, incident angle, pump power, and InN film thickness, resonance-enhanced optical rectification is one of the most plausible mechanisms for the THz generation in the InN films.     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

Terahertz wave generation via pre-ionized air plasma

We report the terahertz(THz) wave generation from a single-color scheme modulated by pre-ionized air plasma via an orthogonal pumping geometry. It is found that the amplitude of the THz signal generated by the pump beam tends to decrease gradually with the increase of the modulation power. We believe that the ponderomotive force plays an important role in the process of the interaction between the pump beam and the pre-ionization beam. The hydrostatic state of the electrostatic separation field caused by the modulation beam will directly affect the generation efficiency of the THz wave. Our results contribute to further understanding of the theoretical mechanism and expanding of the practical applications of THz wave generation and modulation.     

GaSe晶体的双光子吸收对太赫兹输出的影响

根据光整流效应,利用超快激光脉冲泵浦GaSe晶体实现了0.2~2.5 THz的宽带太赫兹辐射输出。禁带中的电子在两个800 nm光子的作用下激发到导带中形成自由载流子,进而吸收所产生的太赫兹辐射,最终导致太赫兹的输出随泵浦功率的增加而趋于饱和。为了研究双光子吸收对太赫兹输出的影响,测量了800 nm处的GaSe晶体的双光子吸收系数,结果为 0.165 cm/GW。通过对太赫兹输出实验数据的拟合,得到GaSe晶体中自由载流子对太赫兹输出的吸收截面为1×10^-15 cm²。本文的研究结果可用于优化GaSe晶体在强激光泵浦下的太赫兹转换效率。     

激光在大气中驱动的强太赫兹辐射的理论和实验研究

两束双色激光脉冲能在大气中产生MV/cm的强太赫兹波.本文主要介绍了我们最近的三项理论和实验工作,澄清了双色激光方案的物理机制这个长期存在的问题,并对该方案进行了推广.为了在气体中有效地产生太赫兹波,在广泛研究的双色激光方案中两束激光的频率比ω_2/ω_1总是被取为1:2.首先从理论上预测采用其他频率比时,此方案仍能有效地工作,并通过实验进行证实.实验上观察到在新的频率比ω_2/ω_1=1:4,2:3下,也能有效地产生太赫兹波;观察到通过旋转较长波长的激光脉冲的偏振方向,能够有效地调节太赫兹波的偏振,但是旋转波长较短的激光脉冲的偏振方向,太赫兹波的偏振几乎没有变化,这违背了多波混频理论中极化率张量对称性的要求;采用不同的频率比时,太赫兹能量定标率并没有显示出明显的区别,这与多波混频理论预测的能量定标率不符.这些实验结果与等离子体电流模型及粒子模拟结果符合得很好.因此,该研究不仅对双色激光方案进行了推广,而且证实了其物理机制应该归结为等离子体电流模型.     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Manipulating Nonlinear Photocurrent from Interlayer Coupling in Bilayer Metamaterials for Polarized Terahertz Generation

Polarized terahertz (THz) wave generation is of great significance for chiral and anisotropic sensing applications. However, how to manipulate amplitude, polarization, and ellipticity of the THz generation is still a fundamental challenge. Herein, polarized THz wave generation is achieved from a bilayer metamaterial consisting of T-shaped structure (TSS) and split resonator rings (SRRs) by combining Maxwell and hydrodynamic equations. The elliptically polarized THz wave can be synthetized directly from horizontally and vertically polarized THz components due to the orthogonal nonlinear photocurrents along the arm-directions of TSS and SRRs, respectively. Besides, the ellipticity and the orientation angle of the THz polarization ellipse can be modulated by the twist angle between the SRRs and TSS layers. The maximum ellipticity can reach 0.34 while the orientation angle is tunable from −0.45 to 0.48π by tuning the twist angle. This work proposes an interlayer coupling method for the polarized THz sources based on metamaterials in potential circular dichroism and chiral sensing applications.     

多功能太赫兹超表面偏振控制器

本文提出了一种“金属栅-开口环/硅环-金属栅”结构的透射式超表面偏振控制器, 研究了入射角度和抽运光对该器件传输及偏振态控制性能的影响. 研究结果表明, 当线偏振太赫兹波垂直入射时, 可对0.39-1.11 THz频段的太赫兹波实现偏振方向90°旋转, 偏振旋转效率为99%, 损耗为1 dB. 对于斜入射的情况, 偏振转换性能在0-60°范围内基本保持不变, 且透过率达到90%以上. 同时, 通过调控抽运光强度的方式, 该器件能够实现对透射与反射太赫兹光束的强度调制, 调制深度均达到90%, 且可以实现太赫兹波偏振分束功能. 该器件可以作为未来太赫兹空间光通信和信息处理的宽带、角度不敏感、可调谐的偏振转换器和分束器.     

飞秒超强激光驱动太赫兹辐射特性的实验研究

强太赫兹源是太赫兹科学技术发展的关键,其中大能量强场太赫兹脉冲源在超快物态调控、新型电子加速器等领域具有重要的应用前景.超快超强激光与等离子体相互作用是近年来发展起来的一种新型的强场太赫兹辐射产生途径.本文报道了利用超强飞秒激光脉冲与金属薄膜靶作用产生太赫兹辐射的实验结果,研究了激光能量和离焦量对靶后太赫兹辐射能量的影响,并通过监测激光背向散射光谱,定性揭示了其变化规律与不同光强下的电子加热机制的相关性.实验表征了太赫兹辐射的频谱、偏振及聚焦光斑情况.测量结果表明,实验产生了脉冲能量达458μJ、聚焦场强高达GV/m量级的超宽带太赫兹辐射,为开展极端太赫兹脉冲与物质相互作用研究提供了一种新的强场太赫兹光源.     

非线性克尔效应对飞秒激光偏振的超快调制

研究了近红外飞秒激光的偏振在太赫兹频率的超快调制.利用抽运-探测光谱技术,通过改变两个脉冲之间的延迟时间可以控制光脉冲的旋转角.在Li：NaTb（WO₄）₂磁光晶体中观察到探测光的偏振随延迟时间变化的高速振荡,振荡信号的中心频率为0.19 THz.这种超快偏振调制现象可以解释为,抽运-探测实验构置中,前向传播的抽运光诱导的光学克尔非线性引起被晶体远端表面所反射的背向传播的探测光脉冲偏振面的额外旋转.通过改变抽运光的圆偏振旋性可以控制探测光调制信号的相位和振幅.实验结果表明,非线性光学克尔效应可以作为一种全新的手段,在磁光晶体中实现近红外飞秒激光以太赫兹频率的超快偏振调控.这将在超快磁光调制器等全光器件中得以应用.实验结果将有助于偏振依赖的超快动力学过程的研究.     

Circular photocurrent in Ag/Pd resistive films upon excitation by femtosecond laser pulses

This paper presents the results of the experimental investigation of the generation of nanosecond photocurrent pulses in silver–palladium (Ag/Pd) resistive films under excitation by laser pulses with a duration of 120 fs at a wavelength of 795 nm. The photocurrent was detected in the direction perpendicular to the plane of incidence of the laser beam on the film. The 20-μm-thick films under investigation were a porous polycrystalline material consisting predominantly of nanocrystallites of the palladium oxide PdO and the Ag–Pd solid solution. The direction of the photocurrent observed in the films depends on the sign of the circular polarization of the incident radiation. It was found that the observed photocurrent depends on the angle of incidence in accordance with the odd law and consists of the circular and linear contributions, which are dependent on and independent of the sign of the circular polarization, respectively. It was shown that the circular photocurrent is significantly higher than the linear photocurrent. It was established that, for both the circular and linear polarizations, the photocurrent is directly proportional to the power of the excitation radiation. For the linearly polarized laser radiation, the photocurrent depends on the polarization angle in accordance with the odd law. The regularities revealed are consistent with the mechanism of the generation of transverse photocurrent with the photon drag effect.     

不同抽运光强激发窄带隙半导体产生太赫兹辐射的研究

研究了窄带隙材料InAs和三种不同掺杂浓度的InN在不同抽运光强激发下产生太赫兹(THz)波的辐射特性.实验结果表明:在相同的抽运光强下,InN和InAs辐射的THz信号强度在同一量级,InAs较InN辐射效率要高一些.随着抽运光强的增大,这几种材料的发射光谱变得更宽,当抽运光增大到一定强度时,它们的发射光谱半极大值全宽(HMFW)趋于恒定.InN比InAs更容易在较低功率的抽运光作用下获得宽带太赫兹光谱.研究也表明,不同掺杂浓度对辐射THz波的强度及辐射效率有很大影响.这项研究对于探索半导体表面辐射太赫 关键词： InN InAs 太赫兹 抽运光强     

Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications

We review the generation of broadband THz radiation from femtosecond photo‐induced gas plasmas, with an emphasis on the highly efficient “AC‐bias” case where the plasma is generated and driven by a superposition of fundamental and second‐harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi‐quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four‐wave mixing model. We also employ these models to the case of few‐cycle pulses, where the observed THz emission is related directly to the carrier‐envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.}     

THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves

We propose a novel scheme for THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves (HSWs). The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity (RC) where low-order Stokes waves (LSWs) are converted to high-order Stokes waves again and again. The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation (OCDFG) where the poling periods of the optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally. Combined with the repeated and continuous frequency conversions, the optical-to-THz energy conversion efficiency (OTECE) exceeds 26% at 300 K and 43% at 100 K with pump intensities of 300 MW/cm².     

THz wave polarization-controlled spectroscopic imaging for anisotropic materials

We present a polarization-controlled terahertz (THz) wave spectroscopic imaging modality to investigate the anisotropy of the detected materials. The polarization of the emitted THz wave is controlled by changing the relative phase between the fundamental and second-harmonic waves in the two-color laser-induced air plasma THz generation configuration. The THz wave polarization direction is extracted by measuring the two electric field amplitudes when the polarization of the incident wave is controlled to be horizontal and vertical. The anisotropy of the industrial Sprayed-On-Foam-Insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrates that THz wave polarization-controlled imaging technique can be used for highly sensitive industrial nondestructive inspection and biological related characterization.     

Experimental study on terahertz radiation

In this letter, we describe a coherent subpicosecond terahertz (THz) spectroscopy system based on nonresonant optical rectification for the generation of THz radiation. We studied the two-photon absorption (TPA) of ZnTe induced by femtosecond laser pulses via THz generation, and its influence on the generation of THz radiation. Experimental results demonstrated that the intensity of pump beam against TPA must be traded off to get an optimum generation of THz radiation. As an example, we measured absorption spectrum of water vapor by time-domain spectroscopy (TDS) in the frequency range from 0.5 to 2.5 THzwith a high overall accuracy.     

光电导天线太赫兹辐射峰值调控研究

针对微结构光电导天线与飞秒激光之间相互作用效应以及辐射太赫兹波调控问题进行了研究。采用德鲁德-洛伦兹理论模型获得微结构光电导天线辐射光电流密度,通过时域有限差分把光电流密度迭代在激励网格上,结合麦克斯韦方程求解时变电磁场,并通过传输线格林函数获得多层介质近场到远场的辐射太赫兹波,建立了辐射光电流与辐射阻抗、电磁共振模式之间的关系模型,模拟仿真分析了微结构S型光电导天线太赫兹波辐射调控机理。研究结果表明:微结构改变了天线等效模型的辐射阻抗;同时得知耦合系数不为零时存在耦合作用,且随着耦合系数增大共振频率峰值发生辐射增强和位移;并通过设计S型光电导天线获得辐射峰值频率调整范围为0.50~0.80 THz之间,对比工形天线辐射峰值频率由原来的0.40 T移动到0.76 T,频率调整度75%,峰值辐射效率约提高70%。该研究工作为后续高功率光导天线太赫兹波辐射的共振中心频点以及结构设计奠定重要基础。     

The dependence on optical energy of terahertz emission from air plasma induced by two-color femtosecond laser-pulses

The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.