Sheng successfully defended his PhD proposal on June 23, 2010.

TITLE:
Measurements of Carrier Dynamics in Quantum Cascade Lasers and Quantum Wells using Femtosecond Mid-IR Pulses

ABSTRACT:
Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions and resonant tunneling, emitting mid- to far-infrared light. This research will use femtosecond Mid-IR pulses generated by difference frequency generation (DFG) to investigate the carrier dynamics in QCLs and quantum well (QW) structures grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) in Princeton University, UMBC and CCNY.

Due to the ultrafast carrier dynamics such as tunneling, electron-electron scattering, and electron-phonon scattering, which occurs on the timescale of femtoseconds to picoseconds, femtosecond pulses are necessary to time resolve these phenomenon. We utilize Mid-IR resonant pumping of the intersubband transitions to eliminate interband electron-hole generation and heavy and light-hole effects, in this unipolar device.The spectra and pulse duration of the fs Mid-IR pulses are measured using a Fourier transform infrared (FTIR) spectrometer and two-photon absorption autocorrelation, respectively.

In the pump-probe technique a strong pump beam is coupled into the QCL to change the population difference between upper and lower lasing level so as to perturb the steady-state gain from equilibrium. A second weaker time-delayed probe beam is used to monitor the evolution of the electron population – this pump-probe signal is directly related to the occupation probability of the carriers in each energy level. Additionally, effects such as coherent tunneling, Bloch oscillations, Rabi oscillations, homogeneous and inhomogeneous scattering, and dephasing effects in two dimensional nanostructures may also be studied with this measurement technique. The carrier dynamics in different designed QCLs at room temperature and low temperature will be investigated. Using transition rate equations for the QCLs, photon-driven transport, phonon-assisted relaxation, resonant tunneling, and superlattice transport and relaxation will be ascertained. These results will provide us a better understanding of the physics of QCLs and QW structures, which would further help us to improve the design and performance of QCLs.