High-resolution spectroscopy of free induction decay from gas molecules excited by broadband terahertz pulses

Abstract

The advantage of terahertz time-domain spectroscopy (THz-TDS) is that absorption spectra can be obtained over a wide frequency range of about 0.1–4 THz. On the other hand, one of the issues with THz-TDS is its low frequency resolution. In this study, the broadband terahertz pulses were used as a light source to excite gas molecules, and the free-induction decay emitted from the gas molecules was measured by using a subharmonic mixer instead of a photoconductive antenna or electro-optic sampling. The THz free-induction decay signal was down-converted by the mixer to several hundred MHz signals and detected by a sampling oscilloscope. The resulting frequency resolution was more than one order of magnitude better than the typical time-domain spectroscopy, although the measurable frequency range in a single scan was about 1 GHz, depending on the bandwidth of the mixer used and the measurement system.