太赫兹脉冲整形技术研究进展

太赫兹(THz)脉冲整形技术在量子理论、生物医学成像、安全检查、亚毫米波通信等领域都具有重要的学术价值和应用前景。概述了基于飞秒脉冲的整形技术和新型太赫兹辐射材料整形技术,分析了太赫兹脉冲整形器件整形技术目前的研究状况,并对各种整形方法的发展进行了展望。     

聚四氟乙烯材料的太赫兹特性

太赫兹(THz)辐射能够穿透很多对可见光不透明的非金属、非极性材料,而用X辐射对这些材料成像的对比度又相对低,因此,太赫兹成像在安全检测和生产质量控制等领域日益受到重视。对成像材料的太赫兹特性的实际测量是太赫兹成像技术的重要组成部分。利用CO_2激光抽运太赫兹激光对聚四氟乙烯材料的太赫兹吸收特性和透过光斑轮廓进行了实验研究,获得聚四氟乙烯在70.51μm,96.5μm,118.83μm,122.4μm,158.51μm,184.31μm和214.58μm波长的吸收系数。     

金属光栅发出太赫兹脉冲

英国斯特拉思克莱德（Strathclyde）大学的研究人员发现，发射超短脉冲到镀金纳米结构光栅是产生太赫兹脉冲简单易行的方法。利用该方法产生的太赫兹波功率和利用最好的无机晶体相当，而且可以进一步优化产生更高的功率。     

基于光栅倾斜脉冲前沿的太赫兹脉冲单次测量

太赫兹脉冲单次测量技术对于不可逆或者单次超快过程的太赫兹光谱研究具有重要意义。为了实现无畸变的高频率分辨率的太赫兹脉冲单次探测,利用光栅产生了一束具有倾斜前沿的飞秒探测脉冲,并利用该探测脉冲实现了对太赫兹脉冲时域电场波形的单次测量,测量时间范围达到20 ps,频谱覆盖0.1 THz~2.5 THz范围。且测量的结果与使用传统电光采样方法测量的结果相符合。对于基于光栅产生倾斜脉冲前沿的太赫兹脉冲单次测量方法进行了建模分析,获得了光栅和光学元件参数的选取,以及光路设计等指导性结论。     

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。     

太赫兹探测与通信技术新进展

回顾了太赫兹技术的最新进展,并总结了未来的几个重要方向,重点介绍几种关键的太赫兹技术,包括太赫兹源、太赫兹传输、太赫兹调制和太赫兹检测。简要介绍了当前太赫兹技术的发展现状和应用情况,如射电天文学、物体无损检测、医疗成像、安检等太赫兹探测应用和太赫兹通信概况。对太赫兹探测和通信最新技术进展进行了回顾,给出了该领域的前瞻研究方向。     

脉冲太赫兹波成像与连续波太赫兹成像特性的比较

对脉冲太赫兹(THz)波成像和连续波太赫兹成像进行了对比研究.在两个系统下分别进行实验,对两个成像系统的成像机理、系统分辨率、系统噪声、成像速度、信息量、价格、复杂性、便携性及其应用进行多方面的比较.研究结果表明,脉冲系统可以获取更多信息,连续波系统简单便捷,两者在应用上具有各自的特点,同时又有很好的互补性.另外,就脉冲太赫兹波成像的多波长特点,探讨了太赫兹波多光谱成像识别方法.     

太赫兹波探测器技术研究进展

对现有的几种常见太赫兹探测技术进行了总结,介绍了探测太赫兹脉冲信号的THz-TDS技术、外差探测的相干探测技术以及基于热吸收的直接探测技术。分析了等离子波探测器相对于传统电子和光子探测器的优势,并重点介绍了以石墨烯为材料的新型太赫兹探测器的研究进展。     

基于强场太赫兹脉冲的扫描隧道显微镜

纳米尺度上的超快动力学研究对现代纳米技术的应用具有重要的指导意义。太赫兹辐射在电磁波谱中独特的位置使其被广泛应用于各种物性的研究。然而衍射极限的存在限制了太赫兹辐射在纳米和亚纳米尺度上的应用。为了应对这一挑战,太赫兹扫描隧道显微镜（THz-STM）应运而生。通过原子级针尖来增强并束缚住太赫兹辐射,将太赫兹成像的空间分辨率提升了多达6个数量级。为了进一步推动国内THz-STM相关研究的发展,重点介绍了THz-STM的发展历程、基本原理、系统构成以及潜在应用,包括在半导体和单分子中的超快动力学研究,并对该领域的下一步发展进行了展望。     

皮焦耳光脉冲能量测试

研制了用于测量皮焦尔量级光脉冲的微能量计。系统采用PIN光电二极管完成光电转换,由电荷积分器对光脉冲产生的电荷进行积分,得到与能量成正比的电压,从而实现对光脉冲能量的测量。结果表明,该微能量计的测量精度达1pJ,为微弱能量信号的测量提供了一种理想的解决方案。     

Real-Time Detection of Terahertz Pulse Amplitude and Position

A novel and continuous detection scheme for the pulse amplitude and temporal position of a terahertz time domain system is presented. Currently, we have achieved a sampling time of 25 Hz and a resolution of less than 70 fs. The method is therefore very well suited for online measurements in production processes to monitor the thickness and inhomogeneities in the composition of non-conducting materials.     

Tunable Terahertz Signal Generation by Chirped Pulse Photomixing

Tunable signal generation in the gigahertz to terahertz range was demonstrated by photomixing of a chirped optical pulse and its time-delayed replica. The chirped pulses were created by chromatically dispersing femtosecond pulses from a mode-locked fiber ring laser, and frequency tuning was achieved by adjusting the time delay and/or the amount of group velocity dispersion. It is shown that the signal phase is locked to the femtosecond-laser pulses and that an extinction ratio close to 100% can be obtained. Analytical calculations on the signal chirp introduced by higher order dispersion are also presented and compared with the experimental data     

Fundamentals of Measurement in Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a main spectroscopic modality to fill the frequency range between a few hundred gigahertz to a few terahertz. This spectrum has been known as “terahertz gap” owing to limited accessibility by conventional electronic and optical techniques. Over the past two decades, THz-TDS has evolved substantially with enhanced compactness and stability. Since THz-TDS is becoming an industrial standard, the performance and precision of the system are of prime importance. This article provides an overview on terahertz metrology, including parameter estimation, signal processing, measurement characteristics, uncertainties, and calibrations. The overview serves as guidance for metrology and further developments of THz-TDS systems.     

太赫兹功率测试研究与实现

为了给太赫兹源、太赫兹探测器等太赫兹仪器的研制和生产提供测试保障,研制了一种基于锁相放大原理的太赫兹功率测试仪器。目前,许多商用锁相放大器价格高、体积大且结构和功能比较复杂,不适合一些便携式检测系统。该仪器主要采用基于AD630的锁相放大方案来实现对微弱信号的检测。这种锁相放大器成本低、设计结构简单、灵活性强且集成度高。当标准太赫兹功率计测试到的太赫兹功率为96.8 mW和2.98 μW时,采用本文方案研制的太赫兹功率测试仪器测试到的功率分别为97.9 mW和3 μW,误差范围在±5% 以内。基于CD552-R3弱信号锁相放大方案只能提取到毫伏级信号,而本文方案则可提取到微伏级信号。因此,采用本文方案探测太赫兹功率时,量程更宽且稳定性更高。     

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO₃ crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO₃ crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.     

太赫兹高分辨率雷达杂波测量与分析

太赫兹高分辨率雷达杂波特性是太赫兹雷达检测技术的基础,该文分析了传统杂波概率统计模型及其相应的参数估计方法,在此基础上,利用研制的太赫兹高分辨率雷达进行了杂波测量实验,并对实测数据进行了分析与检验。实验结果表明,G0 分布模型对太赫兹频段杂波的统计特性拟合度最优。      

Uncertainty in Terahertz Time-Domain Spectroscopy Measurement of Liquids

Terahertz time-domain spectroscopy (THz-TDS) is a significant technique for characterizing materials as it allows fast and broadband measurement of optical constants in the THz regime. The measurement precision of the constants is highly influenced by the complicated measurement procedure and data processing. Taking THz transmission measurement of liquids into account, the sources of error existing in THz-TDS process are identified. The contributions of each source to the uncertainty of optical constants in THz-TDS process are formulated, with particular emphasis on the effect of multilayer reflections and plane wave assumption. As a consequence, an analytical model is proposed for uncertainty evaluation in a THz-TDS measurement of liquids. An actual experiment with a Di 2-Ethyl Hexyl Phthalate (DEHP) sample is carried out to show that the proposed model could be a basis to evaluate the measurement precision of optical constants of liquids.