单发太赫兹时域光谱技术

太赫兹时域光谱技术(THz-TDS)广泛应用于材料、生物医学、化学、药学、安检等诸多领域。传统扫描式THz-TDS技术需要通过改变探测光延时逐点扫描并重构时域信号，仅适合于具有较高重复频率且稳定的太赫兹辐射源情形下的样品探测。在低重复频率或涨落较大的太赫兹辐射源情形下和不可逆过程中样品的探测，扫描式THz-TDS不再适用，需要使用单发THz-TDS技术，单发THz-TDS技术原则上仅需要一个激光脉冲就可以获取一个完整的太赫兹时域脉冲波形。介绍几种主要的单发THz-TDS探测技术，这些技术都利用了电光晶体的泡克尔斯效应，通过测量探测光的某个物理量的变化来提取太赫兹信号。根据探测方法不同可分为光谱编码、空间编码和互相关等技术。在光谱编码技术中，探测光不同频率成分在时间上发生分离，不同时间成分分别被太赫兹脉冲不同时刻电场调制，通过测量探测光各个频率被太赫兹脉冲调制前后的光谱的变化提取太赫兹脉冲波形。该方法光路简单，测量结果直观，有较高的信噪比，但其时间分辨率较低，且被测太赫兹信号容易产生失真。为提高被测信号的时间分辨率，有人提出了空间编码技术，即不同位置探测光分别被太赫兹脉冲不同时刻电场调制，通过测量探测光各个位置太赫兹脉冲调制前后的光强变化提取太赫兹脉冲波形。根据不同空间展开方法可分为一维空间编码技术和二维空间编码技术。空间编码技术中虽然有较高的时间分辨率，但由于探测光在空间展开能量分散使得其信噪比相对较低。此外，还有一种较高时间分辨率的技术即互相关技术，可分为共线互相关和非共线互相关技术。在非共线互相关技术中，被太赫兹脉冲调制的激光啁啾脉冲与短脉冲互相关作用产生二次谐波，通过太赫兹脉冲调制前后二次谐波空间分布变化来提取太赫兹信号；在共线互相关技术中被太赫兹脉冲调制的啁啾脉冲与短脉冲共线入射到光谱仪，通过干涉条纹提取太赫兹信号，该技术提高了时间分辨率和信噪比，但光路布置复杂，不能进行实时监测。回顾了这几种单发THz-TDS探测技术的发展历程，综述探测技术的原理、实验方案和测量结果，并讨论了这些探测技术的优势和不足。

Single-Shot Terahertz Time Domain Spectroscopy Techniques

Terahertz time-domain spectroscopy (THz-TDS) is widely used in materials, biomedicine, chemistry, pharmacy, security and other fields. Traditional scanning THz-TDS technologies need to scan point by point by changing the time delay between the probe pulse and the THz pulse so as to reconstruct the time domain signals, only suitable for sample detection in THz radiation source with high repetition rate and high stability. However, in the cases of THz radiation source with low repetition rate, large fluctuation, or in irreversible processes, scanning THz-TDS detection technique is not applicable. In these cases, single-shot THz-TDS techniques are desirable. In principle, single-shot THz-TDS technologies require only one shot probe laser pulse to obtain a complete THz temporal waveform. In this article, the main detection techniques in single-shot THz-TDS are introduced. These techniques utilize the Pockel effect of the electro-optic crystal to retrieve the terahertz signal by measuring a physical quantity change of the probe pulse. According to the different single-shot methods, these techniques may be classified into the spectral-encoding technique, spatial-encoding technique and cross-correlation technique. In spectral-encoding technique, different frequency components of probe pulse are separated in time, and different temporal components are modulated by electric fields at different times of THz pulse. The THz waveform can be extracted from the difference between the spectral distributions of the probe pulse with and without THz pulse modulation. This technique has shown its advantages with simple optical path, visual measurement results and high signal-to-noise ratio (SNR), but also shown its disadvantages with low time resolution and distortion of the measured THz signals. In order to improve the time resolution, the spatial-encoding technique was proposed. In this technique, different positions of probe pulse are modulated by electric fields at different times of THz pulse. The THz waveform can be retrieved by measuring the difference between intensity of the probe pulse with and without THz pulse modulation. There are two methods of this technique: one-dimensional spatial-encoding and two-dimensional spatial-encoding technique. Although the technique has shown high time resolution, the SNR of detected signal is relatively low because of the dispersive energy of the probe beam. Another technique to improve the time resolution is cross-correlation technique, which can be classified into the collinear cross-correlation and non-collinear cross-correlation technique. In the non-collinear cross-correlation technique, the second-harmonic generation from the cross-correlation between the short readout probe pulse and the chirped probe pulse is modulated by the terahertz pulse. The THz waveform can be extracted from the difference between the second-harmonic distribution with and without THz pulse modulation. In the collinear cross-correlation technique, the chirped probe pulse is modulated by the THz pulse and a short readout probe pulse with collinear incidence to the spectrometer. The THz waveform can be extracted from the difference between the interference fringes with and without THz pulse modulation. The method has shown high time resolution and SNR, but the optical path is complex, and the signal cannot be real-time monitored. In this article, the development of the above mentioned main single-shot THz-TDS detection techniques are introduced. The principles, the application and some measurement results of these techniques are reviewed in detail. The advantages and disadvantages of them are also discussed.