Photoluminescence excitation spectroscopy of the dense exciton gasinvolved in the lasing transition in ZnSe epitaxial layers

The lasing transition in ZnSe epitaxial layers has been investigated at 77 K. The lowest lasing threshold (I_th) was achieved when the layers were resonantly excited at the photon energy of the exciton level. It was found that the exciton level at the excitation intensity just above the I_th was red-shifted by about 16 meV compared with the free exciton line (2.792 eV) under weak excitation condition. The energy difference between the exciton line and the lasing peak was about 19 meV at I=I_th and increased with increasing excitation intensity up to I=8×I_th. This suggests that the stimulated emission occurs due to the inelastic exciton-exciton scattering process at this temperature