PhD Defense of Dongwei ZHAI

Keywords  :  
UV PUMP,terahertz,carrier dynamics,optical rectification

Abstarct :
In this thesis, we investigate the possibility to enhance terahertz (THz) generation by nonlinearity resonance in optical rectification (OR). Firstly, we develop a multifunctional time-domain THz system that combines a classic THz time-domain spectroscopy setup fed by an optical parametric oscillator, the optical/pump-THz/probe technique, and OR generation configuration. Then, we perform OR THz generation in ZnTe, GaSe, KTP/RKTP by pumping these crystals from UV to IR, that is from below to above their bandgap energies.
We observe a clear peak of THz emission when the crystals are pumped around their bandgap. We show that these peaks are due to an enhancement of the crystal nonlinearity at the bandgap. Moreover, the THz signal exhibits a second maximum at half-the-bandgap in KTP/RKTP. We demonstrate that it is related to a two-photon enhanced nonlinearity by performing a two-photon absorption study of these crystals. To the best of our knowledge, this is the fisrt time such a phenomenon is reported. During this study, we also demonstrate that, contrary to what was published, KTP is a indirect bandgap crystal. Meantime, we modeled OR THz generation by taking into account the second-order nonlinear susceptibility dispersion and phase-matching effect, which is validated by fitting the experimental results in the transparent range of ZnTe and GaSe crystals. This leads us to optimize the THz spectral bandwidth delivered by ZnTe an GaSe by adjusting the sample thickness. Finally, we perform preliminary measurements of THz generation in a OR reflection scheme. Here again, this result is original. Finaly, we start a related investigation on ITO nano-thin films. Because of the small film thickness, we apply integrated optics techniques adapted to the THz range, by depositing the films over a silicon or quartz THz waveguide. We retrieve the known THz characteristics of ITO and we estimate the experimental parameters requested to perform a photo-carrier dynamics study in doped ITO films.

Jury members :

• Jean-louis COUTAZ, Professor emeritus, University of Savoie Mont-Blanc : Supervisor
• Frederic GARET, Professor of universities, University of Savoie Mont-Blanc : CoSupervisor
• Patricia SEGONDS, Professor of universities, University Grenoble Alpes : Examiner
• Carlo SIRTORI, Professor, Ecole Normale Supérieure : Examiner
• Valdas PASISKEVICIUS, Professor, Royal Institute of Technology : Reviewer
• Eric FREYSZ, Research Director, University of Bordeaux  : Reviewer