Ultrafast Mid-Infrared Dynamics in Quantum Cascade Lasers

摘要

This report summarizes the entire research period. In this program, we performed experiments using a femtosecond mid-infrared pump-probe system implemented for QCL samples operating at 4.6 and 5.3 m. We employed femtosecond time-resolved pump-probe measurements to probe the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery was determined by the time-dependent transport of electrons through the cascade heterostructure; as the laser approaches and exceeds threshold, the gain recovery shows a dramatic reduction due to the onset of quantum stimulated emission. Since the electron transport through each state in the cascade is determined by the state lifetime, the transport in a cascade laser is driven by the photon density in the cavity. The gain recovery is qualitatively different from that in conventional atomic, molecular and interband semiconductor lasers due to the superlattice transport in the cascade. We also studied the effects of pulse propagation in the laser, including group velocity dispersion and coherent pulse reshaping due to ultrafast Rabi flopping of the gain medium.