POLARIZED PHOTOLUMINESCENCE OF SINGLE CRYSTAL PHOSPHORS.

摘要

Polarization characteristics of the luminescence of three non-cubic crystals were examined and mechanisms for the luminescence processes were proposed. The following crystals were studied: natural kunzite (LiAlSi(,2)O(,6):Mn)-monoclinic; synthetic gadolinium molybdate containing rare earth impurities (GdMoO(,4):Tb,Eu)-orthorhombic at room temperature; and synthetic calcium molybdate (CaMO(,4)) which exhibits intrinsic luminescence-orthorhombic.;The behavior of rare earth impurities, Tb('3+) and Eu('3+), in gadolinium molybdate was investigated using 254 nm ultraviolet excitation as well as 488 nm plane-polarized argon laser excitation. Spectral assignments were made for the various transitions in both the cases. An hitherto unreported intense ('5)D(,1)-('7)F(,4) Eu('3+) transition was observed with the laser excitation. The effect of the phase transition at 433 K on the luminescence of gadolinium molybdate activated with Tb('3+) and Eu('3+) was also investigated. The shape and intensity of the ('5)D(,4)-('7)F(,5) and ('5)D(,0)-('7)F(,2) transitions in Tb('3+) and Eu('3+) were monitored as a function of temperature. The effect of the phase transition was found to be insignificant on the luminescence behavior, and the results were interpreted in terms of the lack of change in the local symmetry. The observations were consistent with the behavior of Nd('3+)-doped gadolinium molybdate reported earlier in the literature.;Polarization studies in the single domain crystal of gadolinium molybdate excited by an argon laser indicated that the polarization characteristics of the incident beam influence the polarization characteristics of the emitted beam. This was in contrast to results obtained from kunzite which indicated an erasure of the polarization memory. Luminescence measurements below liquid nitrogen temperature resulted in an unusual "low temperature quenching" effect observed in the rare earth-containing gadolinium molybdate crystal. Several models were developed to account for the experimental observations. It was concluded that the mechanism was a phonon-assisted resonance absorption.;Polarized measurements on calcium molybdate, isostructural to the more well known calcium tungstate, indicated strong polarization effects. The molybdate emission was intense when polarized along the Z direction which is the optic axis of the uniaxial crystal. There was no difference in polarization when the emission was polarized along the X or Y direction.;A detailed analysis of the polarized emission of manganese was investigated in the monoclinic host crystal, kunzite, LiAlSi(,2)O(,6), and an analytical expression was obtained for the shape of the emission spectrum by a non-linear technique. It was found to be a double Gaussian curve. Measurements of the emission spectra from 6K to room temperature for different crystal orientations confirmed the presence of only one site for the impurity ion. The orientation of the electric dipole oscillator in the unit cell was obtained and the local symmetry axis obtained by polarized luminescence measurements was in excellent agreement with the one obtained by EPR measurements.