Optical non-linearities in degenerate CdS:In

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Abstract
The nonlinear optical behaviour of heavily In-doped CdS crystals is demonstrated by measuring the optical gain. The observed spectra reveal distinct similarities to gain spectra of stimulated electron-hole plasma (EHP) recombinations in unintentionally doped crystals. This is an indication that the gain in the authors heavily doped CdS:In samples is also caused by EHP recombinations. However, due to the high doping level (NIn>1019 cm-3) several properties of the crystals differ from those of undoped samples. As the conduction band is partly filled even without excitation, the gain occurs at considerably higher energies than in undoped CdS. Additionally, enhanced photon scattering at free electrons and at the crystal imperfections, which are introduced by doping, reduces the net gain and distributes the stimulated emission into all spatial directions. Due to strong intraband absorption in the conduction band the spectra of net gain are shifted to negative values at lower energies.