Development of an intense THz source using tilted pulse front method in LiNbO3 crystal and its application in non-linear THz spectroscopy.
Year: 2013
Type: dissertation
Abstract: The aim of this thesis was to develop an intense THz sources using tilted pulse \nfront technique and its application in nonlinear THz spectroscopy of \nsemiconductors and graphene. \nAt the Advanced Laser Light Source, an intense THz source based on large \naperture ZnTe crystal had already been available. This source uses 50mJ laser \nenergy to generate THz pulses with 1µJ energy. The main limitation of the ZnTe \nbased source is its low signal to noise ratio, as it uses the 100Hz laser system, \nwhere the pulse to pulse fluctuation may vary up to 10%. \nTo overcome the limited signal to noise ratio and to increase the efficiency of \nTHz generation, a new intense THz source based on the tilted pulse front \ntechnique in LiNbO₃ crystal is designed and developed as a first part of this thesis. \nAt the time of source characterization it was realized that the measurement of \nintense THz electric field is a big challenge, as the conventional electro-optic \nsampling technique suffers from the limitation of over-rotation. In this thesis work \na new detection technique has been demonstrated based on spectral domain \ninterferometry, which not only overcomes the limitation of over-rotation but also \nsignificantly simplifies the detection system. \nUsing the developed intense THz sources, the nonlinear response of \nsemiconductors like GaAs are investigated. Optical pump-THz probe technique is \nused to investigate the carrier dynamics in GaAs sample. When the THz electric \nfield is intense enough, it not only probes but also pumps the semiconductor \nwhich is observed as absorption bleaching of the intense THz pulse in the GaAs \nsample. The first experiment was done at fixed optical pump fluence. For further \ninvestigation, the optical pump fluence is increased and as a result THz induced \nabsorption bleaching is decreased. The results are modeled using intervalley \nscattering Drude model. This simple model explains the experiment results quite \nwell. \nNext, using the available THz sources, the linear and nonlinear properties of \ngraphene have been investigated. The sheet conductivity of the graphene sample \nis investigated using a low energy THz source. The sheet conductivity varies from \nsample to sample and can be used to test the quality of the graphene sample. After \nperforming linear characterization, the nonlinear properties of graphene sample \nare also investigated using intense THz sources available at ALLS. The results \nshow that when the graphene sample is pumped with an intense THz electric field, \nthe frequency multiplication effect takes place. This is the first experimental \ndemonstration of the nonlinear effects at THz frequencies. We are in collaboration \nwith Tokyo University for theoretical modeling of these experimental \nobservations. \n
Author: Gargi Sharma
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W25799112