In this project a trace gas detection system has been developed and constructed, which was based on direct molecular absorption in the terahertz regime. To strongly enhance the absorption signal the gas was enclosed in a spherical cavity. Specifically, we have achieved to enhance the optical path length inside the cavity by a factor of more than hundred. The factors limiting this enhancement have been investigated and attributed mainly to the structure of the terahertz modes in the cavity. In the course of the project the enhanced path length did not lead to the targeted range of the trace gas sensitivity, which can be rationalized by the structure of the excited cavity modes. The use of this characteristic structure as a diagnostics tool of the radiation is envisioned.
