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PhD Defense - Sheng Liu

Sheng successfully defended his PhD dissertation on August 30, 2011.

Investigation of Carrier Dynamics and Nonlinear Effects in Quantum Cascade Lasers using Femtosecond Mid-Infrared Pulses

Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions and resonant tunneling, emitting mid- to far-infrared light. This dissertation used femtosecond (fs) mid-IR pulses generated by difference frequency generation (DFG) to investigate the ultrafast carrier dynamics and nonlinear effects in QCLs.

In this work, the carrier dynamics of high wall-plug efficiency designed QCLs were investigated utilizing fs mid-IR degenerate pump-probe technique. We observed ultrafast gain recovery within the first 200 fs due to the thin injector barrier inducing fast electron resonant tunneling, fast depletion from lower lasing level by two-phonon resonance design, and relaxation from the continuum region. The observed gain recovery oscillation is interpreted as electrons excited by incoming photons up to higher subbands or continuum region then falling into the next periods of active region so as to dramatically increase the gain on a short time scale. Later, slower gain recovery (2-3 picoseconds) is observed by electron transport through the injector region of the superlattice structure and tunneling into the next period of active region. Another ultraslow recovery component (hundreds of picoseconds) is likely caused by the electron heating and cooling effects as well as the collection of electrons into the lower subbands in real space and compensated by the external bias supply. The latter contribution is support by the observed positive photoconductivity with increased current and decreased voltage across the QCL.

We studied the SHG signal generated by focusing different polarizations (TE and TM modes) fs mid-IR pulses into the QCLs active core. SHG due to intersubband transitions is observed when the pump is TM polarized (SHGTM). The measured SHGTM spectrum narrows when the bias across the QCL increases due to the electron population re-distribution and subband realignment. The expected quadratic dependence of the SHGTM with mid-IR pump power is observed, but saturates at higher pump powers. The observed SHG pumped by TE polarized fs mid-IR pulses (SHGTE) is most likely generated by the bulk nonlinearity of the InP substrate. The second order nonlinearity for SHGTE is also confirmed, without observing the saturation. We estimated linear-to-nonlinear power conversion efficiency for both SHGTM and SHGTE.