PhD defence : Development and characterization of a THz photomixing source based on ultra-stable IR lasers : application to water vapor

The terahertz (THz) spectral range (0.1–10 THz) provides unique opportunities for accurate characterization of pure rotational transitions of light molecules as well as the ro-vibrational modes of heavier ones. THz absorption lines exhibit intensities comparable to those in the infrared domain, they benefit from significantly reduced Doppler broadening, resulting in sharp, well-defined spectral signatures that are particularly pronounced and exploitable for analysis.

In the gas phase, THz spectroscopy gives precious information about molecular energy levels. Beyond their fundamental interest, these data are also crucial for fields such as astrophysics and atmospheric chemistry. A precise understanding of absorption line profiles, including their position, pressure broadening and shift, are essential.

This thesis describes the development of a THz source that achieves Hz-level linewidth and stability of a few mHz/s, combined with the agility to finely scan transitions and investigate sub-Doppler structures. This source is based on robust optical methods for narrowing DFB lasers using optical feedback technique, onto a common highly stable V-shaped cavity. This thesis details the technical aspects of the instrument, its performances, and the series of characterizations conducted to ensure the reliability of scientific results. An application on water vapour is also present, in particular the analysis of absorption profiles between 600 and 1300 GHz, as well as the implementation of experimental setups for dispersive studies and measurements in the saturated regime.

Physics PhD, under the supervision of Samir KASSI.

PhD committee :

• Gaël MOURET, PROFESSEUR DES UNIVERSITES, UNIVERSITE DU LITTORAL COTE D'OPALE

• François BONDU, DIRECTEUR DE RECHERCHE, CNRS DELEGATION BRETAGNE ET PAYS DE LOIRE

• Arnaud GARNACHE, DIRECTEUR DE RECHERCHE, CNRS DELEGATION OCCITANIE EST

• Emilie HERAULT, MAITRESSE DE CONFERENCES, UNIVERSITE SAVOIE MONT BLANC

• Mathieu BARTHELEMY, PROFESSEUR DES UNIVERSITES, UNIVERSITE GRENOBLE ALPES