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Title: A tuneable THz source using excitonic non-linear effects and application in absorption spectroscopy
Author: Majeed, Avan
ISNI:       0000 0004 7428 1968
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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This thesis reports the generation of tuneable THz radiation (0.2-6 THz) through second order nonlinear effects in the excitation of excitons in GaAs/AlAs multiquantum wells (MQWs). A MQW GaAs/AlAs sample was designed to have excitonic resonances at a wavelength accessible by commercially available lasers (850 nm, 300 mW), and have E1HH1-E1LH1 splitting of 9.1 meV. The sample was grown by MBE on a heavily n-doped GaAs substrate to allow biasing of future devices. Following growth, an optical access mesa-diode structure was fabricated to assess the structure through photocurrent (PC) spectroscopy. The sample was then prepared prior to the preliminary THz measurements, with the substrate and cap layer being removed. The sample was then capillary bonded to a diamond heat spreader. Two collimated lasers were used to excite the excitonic resonances. Both lasers were normally incident to the sample surface. A clear THz signal was observed in the case of collinear excitation, which scaled with the density of states of the excitons. Power dependence measurements confirm this as a second order non-linear effect. Hence the THz wave is realized by difference frequency generation, utilizing an enhancement to χ³ obtained via resonant excitation of III-V semiconductor quantum well excitons. The symmetry of the quantum wells is broken by utilizing the built-in electric field across a p-i-n junction to produce effective χ² processes, derived from the high χ³ . This χ² media shows an onset of non-linear processes at ~ 4W/cm2 , allowing area (and hence power) scaling of the THz emitter. Phase matching was achieved laterally through excitation akin to two-colour interference. Using a simple interferometer, and fitting the measured power with the expected transmission of a Fabry-Perot etalon, frequency measurements confirm the ability to tune the THz radiation from 0.75-3 THz, with linewidths of ~20 GHz and efficiencies of ~2x10-5 allowing ~ μW power levels to be demonstrated. This was achieved without the use of plasmonic effects, nor any kind of an antenna, nor an applied E-field to the structure. Tuneable THz emission from 0.2-6 THz is assumed from the width of the excitonic transitions. This THz wave source is then used successfully to demonstrate transmission spectroscopy of atmospheric features at 750 GHz. This work opens prospects for new future swept-wavelength THz spectroscopy systems in the THz region.
Supervisor: Jewell, Geraint Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available