CC BY
33THz-I-12
Cavity Assisted High-Resolution THz Spectroscopy
Francis Hindle1, Coralie Elmaleh1, Marc Fourmentin1, Fabien Simon2, Anastasiia Pienkina1, Robin Bocquet1, Arnaud Cuisset1, and Gaël Mouret1
1Laboratoire de Physico-Chimie de l'Atmosphère, Université du Littoral-Côte d'Opale, 189 A Ave. Maurice
Schumann 59140 Dunkerque, France 2SATT-Nord, 25, Avenue Charles Saint-Venant - 59800 Lille, France
francis.hindle@univ-littoral.fr
The development of cavity-based techniques in the IR has proved to be a particularly successful approach to increase spectrometer sensitivity using a high finesse cavity. Unfortunately, the construction of similar cavities operating at THz frequencies is not possible due to the lack of high-reflectivity spherical mirrors. To overcome this limitation, we have developed a cavity using a low loss corrugated waveguide and photonic mirrors. The cavity modes have been measured around 600 GHz using a standard frequency multiplier source yielding a finesse of around 3500. Under these conditions we expect an equivalent gas interaction length of around 1 km for a table-top setup. Currently typical THz spectrometer setups use path lengths limited to around 2 m of a standard cells and up to around 100 m for a multiple pass cells.
We have constructed a cavity assisted THz spectrometer for sensitive high-resolution spectroscopy using this cavity. The mirrors are mounted on piezo-electric actuators so the overall cavity length can be finely adjusted and locked to the THz frequency of the source. The THz frequency and cavity mode are simultaneously sweep across the region of interest and a molecular signal recorded. Two different signals can be employed. The first uses a frequency modulation (FM) and is sensitive to the amplitude and cavity finesse including any molecular absorption. Alternatively, the photon residency time can also be measured after the source is removed. In this case the additional losses caused by the presence of a molecular transition can be evaluated providing a quantitative measurement of the absorption coefficient. Line strengths as weak as 10-27 cm-1/(molecule/cm2) have been measured using this instrument.
