基于波前倾斜的太赫兹单次测量研究

太赫兹单次测量技术可以对一些超快不可逆过程进行快速测量,在蛋白质变性、核爆模拟分析等领域具有广阔的应用前景。从飞秒激光放大器发出的光脉冲分成两束并分别经过两块光栅产生波前倾斜,一束作为抽运光激发铌酸锂晶体,基于光整流效应产生高功率太赫兹波;另一束作为探测光,采用正交平衡探测法探测。利用该光路产生太赫兹脉冲并对其进行单次测量,系统的时间窗口为16.8 ps,频谱范围覆盖0.1～1.6 THz。     

太赫兹脉冲的正交平衡单次电光测量

提出了一种基于波长编码的超短太赫兹(THz)脉冲单次探测方法。该方法将新型正交平衡电光取样技术和波长编码技术结合起来,同时拥有这两种方法的优点:能对超快太赫兹脉冲实现高调制度和高信噪比的单次实时测量。为了实现单次测量,采用线性啁啾激光脉冲作为探测光,将太赫兹电场对探测脉冲的时域调制映射到频域,并用光谱仪对频域信息进行单次采集。在电光取样技术方面,采用正交平衡探测取代传统波长编码单次测量,通过设置两臂静态偏置相位,使它们大小相等、符号相反,实现对称推挽式调制,从而有效提高近0°偏置点附近的探测线性度和调制深度,并有效抑制动态噪声。     

用于波面倾斜产生高能量太赫兹波的接触光栅设计

基于非共线相位匹配的波面倾斜技术是目前采用飞秒激光产生超快太赫兹波最有效的手段,使用接触式光栅直接产生抽运飞秒脉冲的波面倾斜能够克服成像系统引入的畸变从而进一步提高该技术的效率。提出并利用简化的模态法设计了制备于铌酸锂晶体中用于波面倾斜技术产生高能量太赫兹波的内嵌式接触光栅。将Littrow角入射条件下的光栅衍射理论简化为光栅内双模式的干涉过程,极大地简化了光栅的参数设计。确定了满足波面倾斜条件的光栅常数,系统研究了衍射效率与光栅占空比和刻槽深度的关系,通过内嵌式光栅结构降低反射损耗,理论上能达到90%以上的-1级衍射效率,给出了合适的光栅加工参数。该系统设计方法对基于其他太赫兹波产生晶体的接触光栅设计具有参考意义。     

太赫兹辐射的时域光谱单发探测

随着大功率激光技术的发展,由其驱动产生的太赫兹辐射受到了广泛关注。然而,大功率激光装置的重复频率低,测量其产生的太赫兹辐射的难度较大。采用时域光谱单发探测技术可以克服这一探测难点,通过单次测量就能得到太赫兹辐射的电场波形信息。回顾了太赫兹辐射时域光谱单发探测的相关研究工作进展。根据频率-时间编码和空间-时间编码两种不同的原理,详细介绍了啁啾脉冲光谱编码探测和空间-时间编码探测等测量技术,并对各个测量技术的特点和参数进行了对比,最后对太赫兹辐射的单发探测作了总结和展望。     

基于频率梳的太赫兹辐射强度测量

为实现太赫兹辐射特性精准认知,开展太赫兹辐射绝对强度测量研究。通过光学频率梳产生太赫兹频率梳,利用太赫兹频率梳实现太赫兹辐射源空间强度测量。本文利用电光采样和光电导探测两种方式,实现了100 GHz辐射源空间辐射强度测量;将100 GHz辐射总功率溯源到标准太赫兹功率计,实现太赫兹辐射强度绝对测量。分析比较了利用800 nm空间光进行电光采样和利用1550 nm光纤激光进行光电导探测的测量结果。本文在不同距离下,对太赫兹辐射源的空间辐射绝对强度进行了测量,实验揭示了太赫兹辐射传输的空间演化特性。     

基于频率梳的太赫兹辐射功率密度测量

为实现太赫兹辐射特性精准认知,开展太赫兹辐射功率密度测量研究.通过光学频率梳产生太赫兹频率梳,利用太赫兹频率梳实现太赫兹辐射源空间辐射功率密度测量.本文利用电光采样和光电导探测两种方式,实现了100 GHz辐射源空间辐射功率密度测量;将100 GHz辐射总功率溯源到标准太赫兹功率计,实现太赫兹辐射功率密度绝对测量.分析比较了利用800 nm空间光进行电光采样和利用1 550 nm光纤激光进行光电导探测的测量结果.在不同距离下,对太赫兹辐射源的空间辐射功率密度进行了测量和量值溯源,实验揭示了太赫兹辐射传输的空间演化特性.     

太赫兹脉冲能量测量

利用太赫兹波的单啁啾脉冲电光探测技术,通过调节实验装置中λ/4波片晶轴与探测激光线偏振方向成45°,可以巧妙地得到太赫兹波的绝对电场强度.利用CCD阵列,通过抽运探测的方法可以得到太赫兹脉冲的光斑尺寸,进而在两步实验的基础上计算得到太赫兹脉冲的能量.提供了一种行之有效的测量太赫兹脉冲能量的实验方案.     

温稠密金的太赫兹时域光谱实验

太赫兹波为高能量密度物质提供了独一无二的诊断手段，但在大型高能量密度装置上实现极端条件下物质状态的太赫兹时域光谱诊断技术仍面临巨大挑战。本文报道了在低重复频率、高能量激光装置上开展的光脉冲泵浦-太赫兹探测实验。利用钛宝石飞秒激光器输出焦耳量级的单发脉冲，单发脉冲经磷酸二氢钾倍频后加热30 nm厚自支撑金膜，产生均匀的温稠密金等离子体；同时，将大孔径铌酸锂晶片通过光整流产生的单脉冲能量为7μJ的太赫兹脉冲作为探测光，利用金属阶梯镜实现了单发太赫兹波形探测，获得了温稠密金在太赫兹波段的时间分辨的电导率数据，为检验双温模型中电子-离子耦合系数等关键参数的准确性提供了新基准。     

太赫兹空气相干探测技术引起的太赫兹脉冲畸变

太赫兹空气相干探测技术是一种宽带探测技术,目前已被广泛应用于宽带太赫兹技术中。该技术的频率响应由探测激光脉冲宽度决定。因此,不同的探测激光脉冲在探测过程可能引起太赫兹脉冲的畸变。本文基于数值计算研究了空气相干探测技术引起的太赫兹脉冲的脉冲畸变和能量损失。结果显示,这种脉冲畸变和能量损失主要依赖探测激光脉冲宽度和待探测的太赫兹脉冲的中心频率。本项工作对于估测在宽带太赫兹技术中使用空气相干探测技术的性能和表现有很好的帮助。     

基于倾斜波前技术的高能强场太赫兹辐射脉冲源

回顾了基于倾斜波前技术并利用飞秒激光抽运铌酸锂晶体产生高能强场太赫兹辐射脉冲源的研究背景和发展历程,系统地介绍了倾斜波前装置中各组成部分对太赫兹辐射脉冲的能量转化效率以及发射特性的影响,提出了利用这种技术产生mJ量级的超强太赫兹源的可能途径,展示了强太赫兹源的潜在应用价值。     

Measurement of spatio-temporal terahertz field distribution byusing chirped pulse technology

The authors report the use of an optoelectronic system for the measurement of terahertz (THz) pulses by using chirped pulse technology. This system measures the spatio-temporal distribution of free-space pulsed radiation with an unprecedented data acquisition-rate. Using a linearly chirped optical probe pulse with an electro-optic crystal, a temporal waveform of a copropagating THz field is linearly encoded onto the frequency spectrum of the optical probe pulse and then decoded by dispersing the probe beam from a grating to a detector array. Acquisition of picosecond THz field pulses without using mechanical time-delay devices have been achieved. A single-shot electro-optic measurement of the temporal waveform of a THz pulse has been demonstrated. Unparalleled by other THz sampling techniques, this single-shot method provides what is believed to be the highest possible data-acquisition rate. Temporal resolution, sensitivity, optimal optical bias point of electro-optic modulation, potential applications, and possible improvements are also discussed. In principle, this technique can also be used in magneto-optic measurements     

Characteristics of the Beam-Steerable Difference-Frequency Generation of Terahertz Radiation

We have investigated the characteristics of a terahertz (THz) beam steering method based on a combination of difference-frequency generation (DFG) with the principle of the phased array antenna. In the DFG of THz radiation from a nonlinear optical crystal pumped by optical beams, the phase front of the THz radiation is indirectly tilted by adjusting the relative incidence angle between the pump beams to the crystal. A magnification of the steering angle with a factor of 193 is demonstrated as the most important effect provided by the method. The effect allows the use of a high-speed optical deflector for adjusting the incidence angle, accelerating the steering more than a hundred times compared with mechanical methods. The phase mismatching between the THz radiation and the pump beams as well as the refraction at the crystal surface limit the steering angle of the THz radiation to 56°, full width at half maximum.     

太赫兹波在介质中传播的FDTD分析

时域有限差分法是电磁场领域中应用最为频繁的数值方法之一,它可以有效地处理复杂媒质中颗粒对电磁脉冲信号的散射问题,以及超宽带电磁脉冲信号在色散媒质中的传播问题。几乎所有介质,无论其在太赫兹（THz）波段是否有吸收峰,对太赫兹波的传播都有影响。研究太赫兹波在介质中的传播情况,对优化太赫兹系统,分析与设计太赫兹应用技术有重要意义。本文模拟了太赫兹波在散射介质中的传播,说明了太赫兹波在内弹道干涉测速应用中的可能性;模拟了太赫兹脉冲在色散介质中的传播情况,为太赫兹时域光谱系统中晶体选择提供依据。     

High Energy THz Pulse Generation by Tilted Pulse Front Excitation and Its Nonlinear Optical Applications

Generation of high energy THz pulses by tilted pulse front excitation is reviewed. The basic idea and the realized setups are described. Properties of THz pulses generated by using different pump lasers are summarized. Nonlinear optical effects induced by the high energy THz pulses were observed, such as nonlinear refraction, self-phase modulation, saturation of THz absorption, and free carrier generation. The main results on these phenomena are described. The possibility of using THz pulses as a quasi-dc field for increasing the cut-off frequency in high harmonic generation is analyzed. The THz generation setup used in recent experiments is analyzed and arrangements for achieving better THz beam quality and higher THz pulse energy are considered. Finally, some results of calculations indicating significantly increased generation efficiency of 0.2-THz pulses compared to generation efficiency of 1-THz pulses are presented.     

ZnTe晶体对太赫兹波的电光响应及极化匹配

给出了ZnTe电光晶体折射率和吸收系数随太赫兹波频率而变化的计算曲线,比较了太赫兹波在ZnTe中传播时的相速度和群速度。通过与太赫兹频率和晶体厚度相关的电光效率响应函数,理论计算了ZnTe电光晶体对太赫兹脉冲的探测电光响应,得到了晶体厚度与探测到的太赫兹频谱宽度的定性关系,从计算结果中找到了ZnTe电光晶体在5.3 THz和6.2 THz等多个频点的探测盲点,这些探测盲点来自于ZnTe电光晶体与相应频点太赫兹波的栅格共振吸收。结合自制的大口径太赫兹光导天线和1 kHz脉冲重复频率的太赫兹时域光谱实验系统,通过差分探测技术,从实验上得到了太赫兹波极化方向与〈110〉型ZnTe晶体晶轴方向的六个最佳匹配角度,给出了太赫兹电场最大值随晶轴与太赫兹波极化方向之间夹角变化的曲线及经验公式,这将有利于在实践中对该现象的深入理解和对探测灵敏度的有效提高。     

Terahertz Focusing and Polarization Control in Large-Area Bias-Free Semiconductor Emitters

We show that, when large-area multiplex terahertz semiconductor emitters, that work on diffusion currents and Schottky potentials, are illuminated by ultrashort optical pulses they can radiate a directional electromagnetic terahertz pulse which is controlled by the angular spectrum of the incident optical beam. Using the lens that focuses the incident near-infrared pulse, we have demonstrated THz emission focusing in free space, at the same point where the optical radiation would focus. We investigated the beam waist and Gouy phase shift of the THz emission as a function of frequency. We also show that the polarization profile of the emitted THz can be tailored by the metallic patterning on the semiconductor, demonstrating radial polarization when a circular emitter design is used. Our techniques can be used for fast THz beam steering and mode control for efficiently coupling to waveguides without the need for THz lenses or parabolic mirrors.     

基于非周期极化铌酸锂晶体产生任意频率太赫兹辐射

理论上研究并论证了飞秒激光在非周期性极化畴反转的铌酸锂晶体(LN)中由光整流效应产生任意频率、窄带的太赫兹(THz)波辐射的可行性.为实现对THz波谱的整形,文中利用模拟退火(SA)算法设计出铌酸锂晶体的极化畴反转结构的最佳模型,由此可产生任意预置时间波形的THz辐射场.结果表明,SA算法是一种有效的通过设计晶体极化畴...     

不等光程法太赫兹波段材料光学参数的精确测量

近20年,太赫兹科学技术发展迅速,太赫兹波与物质之间的相互作用成为研究热点。理论推导并实验验证了一种提高太赫兹波段材料折射率、吸收系数等参数测量精度的方法。以两种厚度的高密度聚乙烯板（3.5 mm、5.0 mm）为样品,将不等光程测量法与常规太赫兹时域光谱测量方法进行比较,分别获得了0.5~2.5 THz频率范围的高密度聚乙烯的折射率和吸收系数,结果显示两种方法测量样品的折射率和吸收系数有较大的差异。常规法和不等光程法得到的折射率分别为1.505和1.483。常规法在1.465 THz附近出现一个峰值,而不等光程法得到的吸收系数在1.166 THz和2.431 THz出现特征吸收峰。证明不等光程法能更加精确提取材料光学参数,并可以揭示常规方法无法测得的材料特征吸收峰,为太赫兹波段精确测量材料光学参数提供了新的方法。     

Error analysis of polyamide plate thickness detection based on Terahertz Time Domain System

The analysis on the thickness of polyamide plate using Terahertz Time Domain System (THz-TDS) in reflection mode is carried out. The refractive index, one of the optical parameters in terahertz band, is solved through the mathematics model, and its value is 1.88. A kind of polyamide plate sample with four kinds of thickness is designed and the ability of THz-based method to detect defects or foreign bodies in fiber glass is verified by attaching metal plates to the back of fiberglass. By the comparison of traditional method and THz method, the terahertz method has a measurement error between 2.5% and 10%. As the thickness increases, the error tends to increase. The reason about the deviations is analyzed, as well as the systematic factors affecting the thickness measurement accuracy, in order to improve the accuracy of THz thickness measurement system and provide theoretical basis for designing terahertz thickness measurement system in the future.     

用飞秒激光触发GaAs光电导体产生THz电磁波的研究

报道了用半绝缘GaAs材料研制的光电导偶极天线在飞秒激光脉冲触发下辐射THz电磁波的实验结果.GaAs光电导偶极芯片的两个欧姆接触电极间隙为3mm,采用Si3N4薄膜绝缘保护,在540V直流偏置下被波长800nm,脉宽14fs,重复频率75MHz,平均功率130mW的飞秒激光脉冲触发时产生THz电磁波.用电光取样测量得到了THz电磁脉冲的时域波形和频谱分布.THz电磁波的辐射峰值位于0.5THz左右,频谱宽度大于2THz,脉冲宽度约为1ps.