Optical gain in ZnS epilayers

The variable stripe length method is used to study optical gain in MOVPE-grown ZnS GaAs epitaxial layers. Optical gain and spontaneous emission spectra are extracted from experimental results. A net optical gain of about 30 cm −1 under excitation by a XeCl laser having a power density of 100 kW cm −2 is observed at very low temperatures (T < 20 K) and it is rapidly quenched for higher temperatures. Gain is interpreted to be due to stimulated emission from the bound exciton state and from the biexciton ground state towards free exciton levels. The spectral shape of the gain spectrum is well fitted by the sum of these two contributions. Smaller gain at lower photon energies is obtained due to exciton-excition collisions.