Excitons and phonons in processes of laser oscillation from II-VI semiconductors

The determination of the nature of optical laser transitions is one of the basic problems of semiconductor laser physics. It was shown earlier in certain mixed CdS
x
-CdSe
1-x
crystals that in the case of intense dipole-allowed exciton absorption, the indirect exciton transition provides the principal channel for laser action using free excitons with the assistance of optical phonons at relatively high temperatures and using bound excitons at relatively low temperatures. In the present work new evidence is reported on the above CdS
x
-CdSe
1-x
crystals as well as on other II-VI compounds (ZnS
x
-CdS
1-x
and ZnSe-CdSe), enabling conclusions about the characteristics of the oscillations produced by the complex phonon spectra of mixed crystals. Lasing of the above crystals was produced by optical two-photon pumping using a ruby laser. Experimental results showed that with two-photon pumping involving excitons in crystals of CdS, CdS-CdSe, ZnS-CdS, and ZnSe-CdSe, lasing can be obtained at temperatures no higher than
120-140\deg
K. When the temperature is not too low (
T \geq 60\deg
K), the laser action always preceeds by means of free excitons interacting with LO phonons. At lower temperatures than 50°K most of the crystals lase on bound excitons, with or without the involvement of optical phonons, depending on the spectral distribution of losses. In two cases (CdS and ZnS
0.09
-CdS
0.91
) at
T < 50\deg
K, the lasing occurred on transitions of the Auger recombination type involving excitions. The mechanism of laser action is a strong function of temperature. An important feature of mixed crystals is the dependence of the phonons involved on the composition of the samples. This dependence was studied in the CdS-CdSe crystals. These crystals have a complex phonon spectrum; they are characterized by two types of longitudinal optical phonons, LO
1
and LO
2
whose intensity ratio is dependent on the crystal composition. For crystals in which one of the components is dominant at 77°K, it is shown that the oscillation occurs via free excitons with either the LO
1
or LO
2
longitudinal optical phonon more intense. For crystals with comparable concentrations of their components, both LO
1
and LO
2
phonons take part simultaneously in the lasing process (two-phonon transitions).