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首页> 外文期刊>Frontiers of optoelectronics in China >Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics
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Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics

机译：太赫兹波气体光子学研究超宽带太赫兹时域光谱

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摘要

Recently, air plasma, produced by focusing an intense laser beam to ionize atoms or molecules, has been demonstrated to be a promising source of broadband terahertz waves. However, simultaneous broadband and coherent detection of such broadband terahertz waves is still challenging. Electro-optical sampling and photocon-ductive antennas are the typical approaches for terahertz wave detection. The bandwidth of these detection methods is limited by the phonon resonance or carrier's lifetime. Unlike solid-state detectors, gaseous sensors have several unique features, such as no phonon resonance, less dispersion, no Fabry-Perot effect, and a continuous renewable nature. The aim of this article is to review the development of a broadband terahertz time-domain spectrometer, which has both a gaseous emitter and sensor mainly based on author's recent investigation. This spectrometer features high efficiency, perceptive sensitivity, broad bandwidth, adequate signal-to-noise ratio, sufficient dynamic range, and controllable polarization. The detection of terahertz waves with ambient air has been realized through a third order nonlinear optical process: detecting the second harmonic photon that is produced by mixing one terahertz photon with two fundamental photons. In this review, a systematic investigation of the mechanism of broadband terahertz wave detection was presented first. The dependence of the detection efficiency on probe pulse energy, bias field strength, gas pressure and third order nonlinear susceptibility of gases were experimentally demonstrated with selected gases. Detailed discussions of phase matching and Gouy phase shift were presented by considering the focused condition of Gaussian beams. Furthermore, the bandwidth dependence on probe pulse duration was also demonstrated. Over 240 times enhancement of dynamic range had been accomplished with n-hexane vapor compared to conventional air sensor. Moreover, with sub-20 fs laser pulses delivered from a hollow fiber pulse compressor, an ultra-broad spectrum covering from 0.3 to 70 THz was also showed. In addition, a balanced detection scheme using a polarization dependent geometry was developed by author to improve signal-to-noise ratio and dynamic range of conventional terahertz air-biased-coherent-detection (ABCD) systems. Utilizing the tensor property of third order nonlinear susceptibility, second harmonic pulses with two orthogonal polarizations was detected by two separated photomultiplier tubes (PMTs). The differential signal from these two PMTs offers a realistic method to reduce correlated laser fluctuation, which circumvents signal-to-noise ratio and dynamic range of conventional terahertz ABCD systems. A factor of two improvement of signal-to-noise ratio was experimentally demonstrated. This paper also introduces a unique approach to directly produce a broadband elliptically polarized terahertz wave from laser-induced plasma with a pair of double helix electrodes. The theoretical and experimental results demonstrated that velocity mismatch between excitation laser pulses and generated terahertz waves plays a key role in the properties of the elliptically polarized terahertz waves and confirmed that the far-field terahertz emission pattern is associated with a coherent process. The results give insight into the important influence of propagation effects on terahertz wave polarization control and complete the mechanism of terahertz wave generation from laser-induced plasma. This review provides a critical understanding of broadband terahertz time-domain spectroscopy (THz-TDS) and introduces further guidance for scientific applications of terahertz wave gas photonics.
机译：最近，通过聚焦强激光束以使原子或分子电离而产生的空气等离子体已被证明是宽带太赫兹波的有希望的来源。然而，同时宽带和这种宽带太赫兹波的相干检测仍然具有挑战性。电光采样和光电导天线是太赫兹波检测的典型方法。这些检测方法的带宽受到声子共振或载流子寿命的限制。与固态检测器不同，气体传感器具有几个独特的功能，例如无声子共振，色散少，无Fabry-Perot效应和连续可再生性质。本文的目的是回顾宽带太赫兹时域光谱仪的发展，该光谱仪同时具有气态发射器和传感器，主要基于作者的最新研究。该光谱仪具有高效率，感知灵敏度，宽带宽，足够的信噪比，足够的动态范围和可控制的极化特性。通过三阶非线性光学过程实现了利用环境空气检测太赫兹波的方法：检测通过将一个太赫兹光子与两个基本光子混合而产生的二次谐波光子。在这篇综述中，首先提出了宽带太赫兹波检测机制的系统研究。用选定的气体实验证明了检测效率对探针脉冲能量，偏置场强，气体压力和气体的三阶非线性磁化率的依赖性。通过考虑高斯光束的聚焦条件，对相位匹配和Gouy相移进行了详细的讨论。此外，还证明了带宽对探测脉冲持续时间的依赖性。与传统的空气传感器相比，使用正己烷蒸气可实现超过240倍的动态范围增强。此外，利用从中空纤维脉冲压缩机发出的20fs以下的激光脉冲，还显示了覆盖0.3至70 THz的超宽光谱。此外，作者开发了一种使用偏振相关几何结构的平衡检测方案，以改善常规太赫兹空气偏置相干检测（ABCD）系统的信噪比和动态范围。利用三阶非线性磁化率的张量特性，通过两个分开的光电倍增管（PMT）检测到具有两个正交极化的二次谐波脉冲。来自这两个PMT的差分信号提供了一种减少相关激光波动的现实方法，该方法可以规避常规太赫兹ABCD系统的信噪比和动态范围。实验证明了信噪比提高了两个因素。本文还介绍了一种独特的方法，该方法可通过带有一对双螺旋电极的激光诱导等离子体直接产生宽带椭圆偏振太赫兹波。理论和实验结果表明，激发激光脉冲与产生的太赫兹波之间的速度失配在椭圆偏振太赫兹波的特性中起关键作用，并证实了远场太赫兹发射模式与相干过程有关。研究结果深入了解了传播效应对太赫兹波偏振控制的重要影响，并完善了激光诱导等离子体产生太赫兹波的机理。这篇综述提供了对宽带太赫兹时域光谱（THz-TDS）的批判性理解，并为太赫兹波气体光子学的科学应用提供了进一步的指导。

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