Observation of higher subband emission from PbSe two-dimensional layers

Energy relaxation of an optically excited two-dimensional electron-hole gas has been investigated in PbSe as an example for dimensional quantization in a semiconductor with nearly symmetric valence and conduction band extrema. As compared with bulk-like reference samples the interband emission exhibits broadening and splitting into up to 5 clearly resolved lines. A systematic variation of the spectra with excitation density was observed: With increasing pump intensity the width of the total spectrum and the number of subbands increase due to both carrier heating and band filling. The emission bands are attributed to transitions between conduction and valence subbands with equal quantum number. Above a threshold well pronounced for each subband transition, the emission becomes stimulated. That indicates a potential for laser applications