Overexcited CdSe quantum well lasers

Low-temperature studies of Cd(Zn)Se single quantum well lasers are reported. The MOVPE-grown samples lase in the true blue spectral region under pulsed nitrogen laser excitation. A microscopic technique allows accurate control of both the size and shape of the excited area. It has been possible to isolate high quality regions of a given sample for gain measurements. Thresholds of order 1 kW cm^-2 are routinely obtainable. The microscopic approach also allows a convincing display of temporally stable mode structure. In this presentation, we emphasise the stimulated emission spectra of samples selectively excited at very high intensity (up to 1000 times the laser threshold). At moderate levels of overstimulation the well emission broadens and its peak shifts to higher energy. The peak shift saturates at 25 meV. Upon saturation of the well, two distinct bands appear at higher energy. One of these corresponds in energy to the P-band of ZnSe, while the other shows the spectral characteristics of a 2D electron-hole plasma. We discuss these results in terms of the dynamics of carrier interchange between well and barrier.