Dynamical Properties of Optical Excitations in II-VI Structures

Abstract

Ultrafast dynamical properties and lasing in two-dimensional II–VI heterostructures reveal a manifold of physical mechanisms which are partly unique and of great interest for the general understanding of the optical behavior of these structures and related devices. We review our investigations of polariton propagation effects strongly governing transmission of fs pulses perpendicular to II–VI waveguiding layers, similar to those in respective laser structures, for low to high excitation densities, being explained by the interference of simultaneously excited modes from four polariton branches involved. Second, exciton diffusion properties can be deduced from results of transient-grating experiments. Extremely large exciton diffusion constants could be measured in ultrapure ZnSe layers. For the first time, ultrafast time-resolved laser mode and gain dynamics is determined for II–VI laser diode structures. Coupling of longitudinal ground modes and higher-order transversal modes results in a density-dependent temporal beat pattern which is well described in a model of weakly phase-coupled modes in II–VI resonator cavities. Gain in laser diodes under optical pumping exhibits a complicated spectro-temporal shape with two subsequent stimulated-emission peaks and kinetic- as well as spectral-hole-burning features, being explained on base of carrier heating and cooling effects during lasing. Challenges for the development of a microscopic theory of lasing in II–VI diodes are discussed.