Unified description on principles of fourier transform infrared spectroscopy and terahertz time-domain spectroscopy

摘要

Fourier transform and terahertz time-domain (TD) spectroscopic techniques are widely used in infrared and terahertz frequency bands, and they are usually considered as two different techniques. Here, however, we show that the two techniques are based on the same basic principle. In a Fourier transform infrared spectroscopy (FTIR), the infrared radiation from a continuous incoherent light source passes through a Michelson interferometer, and the interferogram as a function of optical path difference is recorded. The spectral information is decoded from the interferogram by using the Fourier transform process. In a terahertz TD spectrometer, the terahertz transient electrical pulse train from a femtosecond-laser-pumped photoconductive antenna (PCA) is sampled by another PCA (or an electro-optical crystal) gated by a probe femtosecond laser beam from the same femtosecond laser. In this paper, the principle of terahertz TD spectroscopy is described in Fourier domain. We show that the recorded TD terahertz "electrical waveform" can be regarded as an interferogram also. The FTIR and the terahertz TD spectroscopic techniques can be described within the same framework. The asynchronous optical sampling (dual comb) terahertz time-domain technique, a variant form of the traditional terahertz TD spectroscopic technique, can also be described by the same principle. Such a unified description on principles of FTIR and terahertz TD spectroscopies is useful for improving the performance of terahertz emitters and detectors, and developing new terahertz spectroscopic techniques based on femtosecond lasers.