# Terahertz gas phase spectroscopy using a high finesse Fabry-P\'erot cavity

## TL;DR

This paper demonstrates a high finesse Fabry-Pérot cavity for THz gas phase spectroscopy, significantly enhancing sensitivity and enabling detailed molecular line studies.

## Contribution

It introduces a novel THz resonator based on a low-loss waveguide with high reflectivity, achieving a finesse above 3000 and enabling advanced spectroscopic measurements.

## Key findings

- Achieved a finesse above 3000 at 620 GHz.
- Enabled detection of extremely weak molecular lines with high S/N ratio.
- Studied Lamb-Dip effects with low-power THz emitter.

## Abstract

The achievable instrument sensitivity is a critical parameter for the continued development of THz applications. Techniques such as Cavity-Enhanced Techniques and Cavity Ring Down Spectroscopy have not yet been employed at THz frequency due to the difficulties to construct a high finesse Fabry-P\'erot cavity. Here, we describe such a THz resonator based on a low-loss oversized corrugated waveguide with highly reflective photonic mirrors obtaining a finesse above 3000 around 620 GHz. These components enable a Fabry-Perot THz Absorption Spectrometer with an equivalent interaction length of one kilometer giving access to line intensities as low as 10-27cm- 1/(molecule/cm2) with a S/N ratio of 3. In addition, the intracavity optical power have allowed the Lamb-Dip effect to be studied with a low-power emitter, an absolute frequency accuracy better than 5 kHz can be easily obtained providing an additional solution for rotational spectroscopy.

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Source: https://tomesphere.com/paper/1906.10367