One- and two-photon excited luminescence and band-gap assignment inCaWO4

Abstract

Luminescence properties of ${\mathrm{CaWO}}_{4}$ have been investigated using complementary one- and two-photon excitation techniques. Analysis of the thermal changes in the luminescence spectra indicates that the observed high-energy shift of the ${\mathrm{CaWO}}_{4}$ emission maximum is mainly caused by a change of position of the intrinsic blue band. The thermal broadening and shift of this band can be interpreted satisfactorily in terms of a model of the luminescence center interacting with the vibrating crystalline environment. The characteristic parameters of the phonon system obtained from the experiment agree with those from earlier independent studies. Intense laser stimulation of ${\mathrm{CaWO}}_{4}$ in the spectral region $<505\mathrm{nm}$ $(>2.45\mathrm{eV})$ results in emission with spectral and kinetics features that are characteristic of the radiative decay of a ${\mathrm{WO}}_{4}^{2\ensuremath{-}}$ oxyanion complex in this crystal. The kinetics of luminescence decay under two-photon excitation changes with increasing excitation density due to exchange interaction of the elementary excitations. From a comparative analysis of the optical properties of ${\mathrm{CaWO}}_{4}$ obtained in the course of two-photon and one-photon spectroscopic studies (absorption, reflection, one- and two-photon excitation spectra) it is concluded that the energy gap of the crystal is $5.2\ifmmode\pm\else\textpm\fi{}0.3\mathrm{eV}.$