Nonlinear cross-phase modulation with intense single-cycle terahertz pulses

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.     

All-optical ultrafast polarization switching of terahertz radiation by impulsive molecular alignment

We experimentally demonstrate ultrafast polarization switching of terahertz (THz) radiation generated by dual-color driving pulses composed of orthogonally polarized fundamental and second-harmonic waves, which can be controlled by field-free molecular alignment in air by modulating the relative phase between the two field components as a transient dynamic wave plate. By fine-tuning the time delay to properly match the molecular alignment revivals, a significant polarization modulation of the THz radiation is observed and both linearly and elliptically polarized THz radiations can be obtained.     

Tunable few-cycle and multicycle coherent terahertz radiation from relativistic electrons

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from ～0.26 to 2.6 THz with a bandwidth of ～0.16 THz.     

太赫兹液晶材料与器件研究进展

液晶是一种性能优异的可调控光电功能材料,基于液晶的太赫兹器件有着广泛的应用前景,但高性能太赫兹功能器件的研发仍处于初级阶段.本文综述了太赫兹领域液晶材料与器件的研究现状,探讨了液晶技术与太赫兹技术相结合的发展趋势.     

Femtosecond terahertz radiation from femtoslicing at BESSY

Femtosecond far-infrared radiation pulses in the THz spectral range were observed as a consequence of the energy modulation of 1.7 GeV electrons by femtosecond laser pulses in the BESSY storage ring in order to generate femtosecond x-ray pulses ("femtoslicing"). In addition to being crucial for diagnostics of the laser-electron interaction, the THz radiation itself is useful for experiments where intense ultrashort THz pulses of well-defined temporal and spectral characteristics are required.     

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.     

连续太赫兹波双物距叠层定量相衬成像

针对连续太赫兹波叠层成像重建算法收敛较为迟滞的问题,提出一种连续太赫兹波双物距叠层成像方法及相关重建算法,使用不同记录距离形成的差异化衍射图幅值作为重建算法记录平面的约束条件,增加了记录平面数据多样性和衍射信息冗余度.仿真结果表明,本方法可以加快算法收敛速率,有效减少迭代次数,提高连续太赫兹波定量相衬成像计算效率.随后构建了基于2.52 THz光泵连续太赫兹激光器的双物距叠层成像实验装置,应用双物距记录方法及改进算法重建获得了聚丙烯基字母图案样品的幅值和相位分布,结果表明改进方法可以减少算法迭代次数,提升计算效率,同时改善相位像重建结果保真度.     

Nonlinear optical terahertz wave sources

We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG, process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz, waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.     

Voltage-controlled liquid-crystal terahertz phase shifter and quarter-wave plate

Phase shift exceeding tau/2 at 1 THz is demonstrated by using electrically controlled birefringence in a homeotropically aligned nematic liquid crystal (E7) cell, 570 microm in thickness. The driving voltage required for a phase shift of 90 degrees is 125 V (rms). We demonstrate that the phase shifter works as an electrically switchable quarter-wave plate at 1 THz. The device can also be used as an electrically tuned phase compensator around the quarter-wave point near 1 THz.     

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。     

Modeling of terahertz radiation emission from a free‐electron laser

In this article, we report the generation of terahertz (THz) radiation using the interaction of a laser‐modulated relativistic electron beam (REB) with a surface plasma wave. Two laser beams propagating through the modulator interact with the REB, leading to velocity modulation of the beam. This results in pre‐bunching of the REB. The pre‐bunched beam travels through the drift space, where the velocity modulation translates into density modulation. The density‐modulated beam, on interacting with the surface plasma pump wave, acquires an oscillatory velocity that couples with the modulated beam density to give rise to a nonlinear current density which acts as an antenna to give THz radiation. By optimizing the parameters of the beam and the wiggler, we obtain power of the order of 10⁻⁴ using the current scheme.     

Study of terahertz interferometric imaging using optical techniques

<正>Principles of terahertz(THz) interferometric synthetic aperture imaging with heterodyne and optical techniques are presented.A THz interferometric experiment based on optical up-conversion is set up.The received THz signal is modulated into an optical carrier and transmitted in a fiber.To simulate phase differences between two THz receivers,the output of receiver is divided and a phase shifter is placed before electro-optical modulation(EOM).Interferometric spectra of these modulated optical signals are examined at different phase shifts.Otherwise,carrier suppression and phase error calibration are discussed for THz interferometric synthetic aperture imaging.     

高速太赫兹探测器

太赫兹(terahertz,THz)技术在高速空间通信、外差探测、生物医学、无损检测和国家安全等领域具有广阔的应用前景.能响应1 GHz调制速率以上THz光的高速THz探测器是快速成像、THz高速空间通信、超快光谱学应用技术和THz外差探测等领域的核心器件.传统的THz热探测器难以实现高速工作,而基于半导体的THz探测器在理论上可实现高速工作.光导天线具有超快的响应速度,可实现常温和宽谱探测;肖特基势垒二极管混频器、超导-绝缘体-超导混频器和超导热电子混频器具有转换效率高、噪声低等优点,可用于高速THz空间外差和直接探测;基于高迁移率二维电子气的天线耦合场效应晶体管灵敏度高、阻抗低,可实现常温高速THz探测;THz量子阱探测器是一种基于子带间跃迁原理的单极器件,非常适合高频和高速探测应用,亚波长金属微腔耦合机理可显著提高器件的工作温度及光子吸收效率.本文对上述几种高速THz探测器进行了综述并分析了各种探测器的优缺点.     

UP-conversion of terahertz radiation induced by photon drag effect

An all-optical approach to convert terahertz radiation (THz, wavelength λ₁) into infrared (IR, peak wavelength λ₂) is presented. We show that this up-conversion process is due to the photon drag effect induced by the THz radiation in intrinsic narrow-gap semiconductors followed by spatial redistribution of current carriers and band-to-band radiative recombination. The process results in non-selective high-speed (ns range rise/fall times) IR imaging of positive (conventional luminescence) and/or negative (negative luminescence) contrasts. Estimates made for an InSb pixelless converter at 300 K and moderate THz intensity (kW/cm²) show that this up-conversion process (with λ₁/λ₂>10²) can be observed with a conventional thermal imaging camera.     

High power terahertz pulses generated in intense laser-plasma interactions

Terahertz (THz) radiation has attracted much attention due to its wide potential applications. Though radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasma, with the advantage of no damage limit, is a promising medium to generate strong THz radiation. This review reports recent advances on strong THz radiation generation from low-density gases and high-density solid targets at different laser intensities.     

Control of peaks of terahertz radiation and tuning of its frequency and intensity

We propose to employ dark hollow laser beams (DHBs) for obtaining multifocal terahertz (THz) radiations which are quite useful in medical applications. The number of peaks in the THz field can be suppressed if we apply external magnetic field and use intense lasers so that the relativistic effects are prominent. It means we can achieve strong bifocal THz radiation with this scheme. However, multifocal THz radiation can be reconstituted by increasing the beating frequency of the lasers. The separation of the peaks can be controlled by wisely choosing the orders of the lasers, and also by varying the strength of the magnetic field. The magnetic field is found to enhance the THz field, and also it tunes the frequency of the THz field. The DHBs of higher and equal orders are found to be most significant for efficient THz radiation generation with respect to the role of the magnetic field and density ripples.     

全介质光栅在太赫兹波段的光调控特性

采用激光直写技术在100 μm厚的Si衬底上制备了全介质光栅结构, 利用近红外光抽运-太赫兹探测(near infrared pump-Terahertz probe)技术对该全介质光栅在THz波段的光谱响应及其光调控特性进行了测试, 最后结合电磁仿真结果, 对米谐振(Mie resonance)的形成机理和光调控机理进行了解释并对调控光作用下全介质光栅的电导率数值进行了估算. 研究结果表明: 在光栅与THz偏振垂直的情况下, 该全介质光栅在0-1.0 THz范围内有3个典型的米谐振峰且谐振模式各不相同; 随着调控光功率的增加, 3个谐振峰的谐振强度出现了不同程度的减弱, 其中第一个谐振峰的光调控幅度达到50%以上, 调控光作用下米谐振强度的减弱是由于光生载流子对入射THz波的吸收和散射导致了介质光栅内部感生电磁场减弱引起的. 上述工作对全介质超材料在THz波段的共振特性研究和相关光调控器件的研制具有重要参考价值.     

Creation of multi-frequency terahertz waves by optimized cascaded difference frequency generation

A new scheme which generates multi-frequency terahertz (THz) waves from planar waveguide by the optimized cascaded difference frequency generation (OCDFG) is proposed. A THz wave with frequency ω_T1 is generated by the OCDFG with two infrared pump waves, and simultaneously a series of cascaded optical waves with a frequency interval ω_T1 is generated. The THz wave with a frequency of M-times ω_T1 is generated by mixing the m-th-order and the (m+M)-th-order cascaded optical wave. The phase mismatch distributions of cascaded difference frequency generation (CDFG) are modulated by changing the thickness of planar waveguide step by step, thereby satisfying the phase-matching condition from first-order to high-order cascaded Stokes process step by step. As a result, the intensity of THz wave can be enhanced and modulated by controlling the cascading order of OCDFG.     

Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam

We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.     

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.