Carrier Relaxation Dynamics of an InGaAs/InAs/GaAs/InAlAs Quantum Dot Cascade Laser Structure

Quantum dot cascade lasers (QDCLs) are a class of unipolar semiconductor light sources that integrate three-dimensional quantum confinement with cascade transport mechanisms. In this study, time-resolved femtosecond pump−probe reflectivity spectroscopy is employed to investigate the carrier dynamics of an epitaxial InGaAs/InAs/GaAs/InAlAs QDCL structure over a temperature range of 78−300 K, under optical excitation conditions both above and below the band edge. The carrier lifetime as long as a few nanoseconds was revealed under interband excitation, which is an order of magnitude longer than that of conventional cascade laser structure composed of quantum wells (QWs). On the other hand, under intersubband excitation, we observed shorter carrier lifetime for QDCL in comparison with a QW-based cascade laser structure. In addition, the distinct temperature-dependent carrier relaxation dynamics was observed for QDCL compared with the QW-based cascade laser structure, under both the interband and intersubband transitions, and the corresponding physical mechanisms governing the carrier relaxation in QDCL structure are discussed. The findings could deepen the understanding of carrier relaxation dynamics in QDCL structures and provide valuable insights into the strategies for advancing quantum efficiency and thermal management, as well as all-optical modulation schemes of high-performance QDCLs.