Optical Properties of Activated and Unactivated Hexagonal ZnS Single Crystals

Single crystals of hexagonal ZnS, unactivated and activated with Cu, Al, or Mn, have been examined. The polarization effects in the transmission spectrum of unactivated ZnS were measured further into the fundamental absorption than were previously measured. The theoretical prediction that, for direct transitions at k=0. light polarized perpendicular to the $c$ axis is more strongly absorbed than light polarized parallel to the $c$ axis is born out throughout the fundamental absorption region except in the wavelength region between 290 and 325 mμ. The wavelength dependence and the polarization of the excitation and fluorescence spectra of the activated and unactivated crystals were measured at room temperature and at 77°K. The excitation spectra showed an agreement with the selection rule at the edge of the fundamental absorption, but there was a reversal of the selection rule deep in the absorption region. There was also an impurity absorption exhibiting the same polarization properties as the edge. Some fluorescent emissions were polarized perpendicular and some were polarized parallel to the $c$ axis. Speculations are made on the reversal of the polarization deep in the fundamental absorption and on the symmetry and the nature of the sites causing the various fluorescence bands.