Graduate Thesis Or Dissertation

Generation of narrowband THz pulses and THz studies of ultrafast phenomena in semiconductor quantum wells

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  • This work comprises two main parts: creating and shaping narrowband, pulsed THz radiation in a table-top optical setup; and applying THz pulses to semiconductor nanostructures to study electron dynamics. I developed a scheme to shape the THz output of a fanned-out periodically poled lithium niobate (PPLN) crystal. The pulses are generated by optical rectification of 800nm pump pulses. The periodicity of the PPLN determines the exact THz frequency, and the PPLN crystal was grown in such a way that different regions of the crystal generated different THz frequencies. Spatial filtering controls the power spectrum of the output pulses, which we measured by electrooptic detection in a nonlinear crystal. I created an optical arrangement to generate tunable, narrowband THz radiation by difference-frequency generation in zinc telluide (ZnTe). A single, chirped pump pulse was used for the optical source, and the difference-frequency was obtained by mixing two chirped optical pulses with a relative time delay in a ZnTe crystal. The generated THz pulse energy was measured using a silicon bolometer, revealing conversion efficiencies as high as 4 x 10⁻⁶. Using a Michelson interferometer, the THz field autocorrelation was also measured, showing tunability of the emitted field with a spectral range of 0.5 - 2.2 THz. I used THz radiation as a tool for examining excitonic states in GaAs quantum wells. The optical transmission spectra of these quantum wells were observed near the light-hole and heavy-hole excitonic 1s resonance lines around 800 nm. The spectral modulation of the exciton resonances was measured as intense singlecycle THz radiation was applied, reaching field strengths as high as 10 kV/cm. By varying the delay between the IR probe pulse and the THz driving pulse, I observed coherent, transient extreme-nonlinear effects in the transmission spectra.
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