Summary:

The aim of this thesis is to study the use of frequency combs for high-resolution molecular spectroscopy in the terahertz (THz) region of the electromagnetic spectrum. This approach is based on the conversion of a comb in the optical domain to the THz domain. However, the state of the art of these sources requires a very sensitive heterodyne THz receiver in order to measure the amplitude of each tooth of the comb separately. We demonstrate the effectiveness of this approach at frequencies below 1 THz using Schottky diode-based receivers. Commercial mixers exist for higher frequencies, but there is the problem of the local oscillator (LO). We explored the use of optically pumped molecular lasers (OPML). Several active media were considered: ammonia (NH3), hydrogen cyanide (HCN), water vapour (H2O) and heavy water vapour (D2O). Experimental results show significant gains, but these are limited by the power of the quantum cascade lasers (QCLs) available. Sufficient power to pump a mixer was nevertheless obtained, paving the way for more advanced OPML designs. Lastly, the OPML was used as a LO in a molecular spectroscopy experiment involving the use of a quantum cascade laser.gamesynchrotron radiation. The experimental set-up and the techniques used for signal acquisition enable high-precision measurements in a range (1 - 4 THz) where few solutions exist. This work should eventually lead to the development of a compact spectrometer with high performance throughout this range.

Abstract: