Broadband terahertz gas spectroscopy through multimode self-mixing in a quantum cascade laser

In the terahertz (THz) frequency range, CBRN agents and explosives have unique spectral signatures, which can be used for their identification and analysis. Quantum cascade lasers (QCLs) are important sources for spectroscopy within the 2–5 THz range, and self-mixing (SM) interference in a laser cavity enables the laser device to act both as a radiation source and as a coherent detector. This technique removes the need for additional detectors. Here, we present a SM spectroscopy technique, with an integrated frequency monitoring system and demonstrate the measurement of methanol spectral features simultaneously from two modes of a multi-mode THz QCL over a 17 GHz bandwidth.