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

摘要

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 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 single-cycle 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.;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 800-nm 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 electro-optic detection in a nonlinear crystal.