Development of a Herriott multi-pass cell for gas phase terahertz spectroscopy

Abstract

Terahertz spectroscopy has emerged as an important technique for gas detection, however, its sensitivity in detecting specific gases remains inadequate, limiting its wider application. To address this issue, this paper presents a design methodology for a Herriott cell specifically optimised for the low THz frequency range. By using multiple reflections for focusing, this method overcomes the problem of large divergence angles of THz beams, effectively suppresses beam distortion, and maximises the optical path length, thereby significantly improving the detection sensitivity for trace gases. Based on this design, a Herriott cell suitable for the 260–400 GHz frequency range has been developed. A Herriott cell with a 3.3 m optical path was fabricated and a THz gas spectroscopy test system based on this cell was established. Experimental results show that the local noise of the THz spectra for pure trace gases is in the order of 10^-3, while the noise for the acetonitrile −air mixture is in the order of 10^-2. The system is capable of detecting pure acetonitrile and methanol gases at pressures below 0.0119 Pa and 0.0269 Pa, respectively, as well as acetonitrile at a partial pressure of 0.00935 in a 10 Pa acetonitrile/air mixture.