Photoluminescence frequency up-conversion in GaSe single crystals as studied by confocal microscopy

The photoluminescence spectrum of melt-grown GaSe single crystals was investigated with a confocal Raman microscope equipped with a HeNe laser. Three luminescence bands of different intensity were observed, which are mainly located to the blue of the laser line. The luminescence signals show a quadratic dependence on excitation intensity. The effect is interpreted as second-harmonic generation in the strongly optically nonlinear material followed by the excitation of electrons into the conduction band and luminescence emission from direct-gap Wannier excitons. The relative intensities of the three luminescence peaks exhibit a spatial variation on the crystal surface, which was mapped with the confocal microscope. Possible explanations of this effect are discussed. In an external electric field the luminescence shows a strong increase and a quadratic redshift, whose magnitude is consistent with the Franz–Keldysh mechanism.