Resonance enhancement of nonlinear photoluminescence in gallium selenide and related compounds

Maker fringe experiments on the layered chalcogenide semiconductor gallium selenide (GaSe) with weak cw diode lasers are presented. It is demonstrated that nonlinear photoluminescence emitted by this material and by the similar compound GaSe_0.9S_0.1 under illumination with a 632.8-nm He — Ne laser shows very strong resonance enhancement upon heating when the absorption edge and exciton levels are shifted towards the laser line. The photoluminescence appears to be strongest when the energy level of the direct exciton, which emits it, is resonant with the photon energy of the laser. The previously observed enhancement of the photoluminescence by electric fields is interpreted in this context.