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.
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.
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