Tan Yong, Zhao Hang, Wang Wei-Min, Zhang Rui, Zhao Yue-Jin, Zhang Cun-Lin, Zhang Xi-Cheng, Zhang Liang-Liang
Key Laboratory of Terahertz Optoelectronics (MoE), Department of Physics, Capital Normal University, Beijing 100048, China.
Beijing Key Laboratory for Precision Optoelectronic Measurement Instrument and Technology, School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China.
Phys Rev Lett. 2022 Mar 4;128(9):093902. doi: 10.1103/PhysRevLett.128.093902.
Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1-18 THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.
固体和气体都已被证明可作为太赫兹（THz）相干探测的材料。基于气体的相干探测方法需要高能量的探测激光束，而基于固体的方法探测带宽有限。液体是否可用于太赫兹探测并缓解这些问题尚未见报道，由于最近观察到液体中能高效产生太赫兹波，这成为一个适时且有趣的话题。在此，我们提出一种基于液态水的太赫兹相干探测方案。当太赫兹脉冲和基频激光束在自由流动的水膜上混合时，随着等离子体形成会产生二次谐波（SH）光束。将这种太赫兹诱导的二次谐波光束与控制二次谐波光束相结合，我们成功实现了频率范围为0.1 - 18太赫兹的太赫兹场的时间分辨波形。所需的探测激光能量低至几微焦耳。在可比的探测条件下，我们方案的灵敏度比基于空气的方法高1个数量级。该方案对太赫兹极化以及基频和控制二次谐波光束之间的相位差敏感，这为优化和极化敏感探测带来了直接途径。太赫兹诱导光束的能量缩放和极化特性表明其产生可归因于四波混频过程。这种产生机制使得探测激光、太赫兹诱导的二次谐波和太赫兹场之间具有简单的关系，有利于探测装置的稳健性和灵活性。
