Population inversion in gapless semiconductors as result of interband pumping

Optical interband pumping of gapless semiconductor materials and the resulting nonequilibrium charge carrier distribution are analyzed, considering that excess photocarriers are produced by such pumping and after energy relaxation dip to the bottom of the respective band. Pumping sufficiently strong to produce a degenerate nonequilibrium carrier distribution can, therefore, also cause a population inversion at a band-to-band (conduction band-valence band) transition. Emission of optical phonons at temperatures far below that corresponding to the phonon frequency is the principal mechanism of photoelectron energy relaxation, photohole energy relaxation being much slower on account of the much larger mass. Computer simulation of the pumping kinetics on the basis of the corresponding transient-state equation, including recombination as well as electron-electron scattering, has yielded the dependence of amplification and internal absorption on the frequency of interband transitions for Cd(x)Hg(1-x)Te materials pumped by a CO2 laser with an intensity of 10,000 to 100,000 W/sq cm.