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PhD Proposal Defense: Hong Cai

Date: Monday, April 30, 2012
Time: 3:00pm
Location: PHYS 401

Optical Measurement on Quantum Cascade Lasers and Mid-IR Semiconductor Materials Using Femtosecond Pulses

Ultrafast time-resolved optical technique provides an insight into the carrier dynamics and light-matter interactions in quantum cascade lasers (QCLs) and mid-IR semiconductor materials. The proposed research will use the mid-infrared (mid-IR) fs pulses from a difference frequency generator (DFG) to investigate the carrier dynamics and nonlinearities of QCLs provided by Princeton University, grown by metal-organic chemical vapor deposition (MOCVD). Additionally, fs near and mid-IR pulses are used to excite photoluminescence (PL) to investigate the quality of novel mid-IR semiconductor materials.

QCLs are unipolar devices based on intersubband transitions instead of interband transitions as in most semiconductor lasers. To investigate their carrier dynamics such as resonant tunneling, stimulated emission, and superlattice transport, we need to eliminate electron-hole generation, light and heavy-hole effects, and other interband processes. fs mid-IR pulses provide the resonant photon energy for pump-probe techniques in QCLs. In the pump-probe technique, a strong pump beam is coupled into the active core of a working QCL to perturb the population inversion and the gain from equilibrium. A weak probe beam is used to monitor the gain recovery after the carrier distribution is perturbed by the pump. The dependence of the probe signal on the time delay between the pump and probe gives the time-resolved information on the intersubband transition related carrier mechanisms mentioned above. The pump-probe signal indicates the dephasing mechanisms such as electron-electron (e-e) scattering, electron longitudinal optical (LO) phonon scattering, and carrier heating etc., decreasing the intersubband tunneling rate. Among all the dephasing mechanisms, the LO phonon scattering plays a dominant role in QCLs. This proposed research will apply pump-probe technique in QCLs at different temperatures, because the phonons are largely suppressed at cryogenic temperatures. In addition, QCLs have giant inherent nonlinearities, which give the QCLs a potential in ultrafast pulse generation. The Kerr nonlinearities will be studied by coupling the mid-IR fs pulses into a QCL waveguide.

The second part of this research is to investigate novel mid-IR semiconductor materials using fs pulse excited photoluminescence (PL) and time-resolved photoluminescence (TRPL). Preliminary experimental measurements have been conducted on an InAs/GaSb type-II superlattice sample grown by MOCVD at UMBC.

The results of this research will provide a better understanding of carrier dynamics in QCLs and novel mid-IR materials and would further help the designers and growers to improve the quality and performance of mid-IR devices.