单发太赫兹时域光谱技术

doi:10.3964/j.issn.1000-0593(2019)05-1377-09

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Abstract 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.

Key words：THz time domain spectroscopy technique; Single-shot THz time domain spectroscopy technique; Electro-optic effect

Received: 2018-02-13 Accepted: 2018-06-16

ZTFLH:

O433

Corresponding Authors: PENG Xiao-yu, LUO Chun-hua E-mail: lch@cust.edu.cn；xypeng@cigit.ac.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)05-1377-09 OR https://www.gpxygpfx.com/EN/Y2019/V39/I05/1377

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