Ultrafast carrier recombination and plasma expansion via stimulated emission in II-VI semiconductors

Abstract

Time-resolved beam deflection and four-wave mixing techniques have been used to investigate the spatial and temporal evolution of degenerate electron-hole plasmas in CdS, ${\mathrm{CdS}}_{0.75}$${\mathrm{Se}}_{0.25}$, and ZnSe following two-photon absorption of 120 fs, 620 nm pulses at temperatures between 80 and 295 K. The decay times for carrier densities larger than ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ are found to be less than 10 ps, and are independent of the local density gradient. We suggest that stimulated emission recombination and reabsorption occur until the entire plasma becomes nondegenerate. This fast carrier transfer process may account for other reports in the literature of rapid expansion in degenerate ambipolar plasmas.