Development of an intense THz source using tilted pulse front method in LiNbO3 crystal and its application in non-linear THz spectroscopy.

Sharma, Gargi (2013). Development of an intense THz source using tilted pulse front method in LiNbO3 crystal and its application in non-linear THz spectroscopy. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 159 p.

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Résumé

The aim of this thesis was to develop an intense THz sources using tilted pulse front technique and its application in nonlinear THz spectroscopy of semiconductors and graphene. At the Advanced Laser Light Source, an intense THz source based on large aperture ZnTe crystal had already been available. This source uses 50mJ laser energy to generate THz pulses with 1µJ energy. The main limitation of the ZnTe based source is its low signal to noise ratio, as it uses the 100Hz laser system, where the pulse to pulse fluctuation may vary up to 10%. To overcome the limited signal to noise ratio and to increase the efficiency of THz generation, a new intense THz source based on the tilted pulse front technique in LiNbO₃ crystal is designed and developed as a first part of this thesis. At the time of source characterization it was realized that the measurement of intense THz electric field is a big challenge, as the conventional electro-optic sampling technique suffers from the limitation of over-rotation. In this thesis work a new detection technique has been demonstrated based on spectral domain interferometry, which not only overcomes the limitation of over-rotation but also significantly simplifies the detection system. Using the developed intense THz sources, the nonlinear response of semiconductors like GaAs are investigated. Optical pump-THz probe technique is used to investigate the carrier dynamics in GaAs sample. When the THz electric field is intense enough, it not only probes but also pumps the semiconductor which is observed as absorption bleaching of the intense THz pulse in the GaAs sample. The first experiment was done at fixed optical pump fluence. For further investigation, the optical pump fluence is increased and as a result THz induced absorption bleaching is decreased. The results are modeled using intervalley scattering Drude model. This simple model explains the experiment results quite well. Next, using the available THz sources, the linear and nonlinear properties of graphene have been investigated. The sheet conductivity of the graphene sample is investigated using a low energy THz source. The sheet conductivity varies from sample to sample and can be used to test the quality of the graphene sample. After performing linear characterization, the nonlinear properties of graphene sample are also investigated using intense THz sources available at ALLS. The results show that when the graphene sample is pumped with an intense THz electric field, the frequency multiplication effect takes place. This is the first experimental demonstration of the nonlinear effects at THz frequencies. We are in collaboration with Tokyo University for theoretical modeling of these experimental observations.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Ozaki, Tsuneyuki
Co-directeurs de mémoire/thèse:	Morandotti, Roberto
Mots-clés libres:	THz spectroscopy; intense THz sources; signal to noise ratio
Centre:	Centre Énergie Matériaux Télécommunications
Date de dépôt:	09 juill. 2014 20:18
Dernière modification:	16 mars 2016 14:53
URI:	https://espace.inrs.ca/id/eprint/2165

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