Terbium doped glasses: their optical properties and potential applications

Abstract

The optical properties of inorganic glasses doped with terbium have been investigated with particular emphasis on silicate glass systems. The effect of increasing terbium concentration on the refractive index of silicate glasses has been investigated and the optical absorption from 0.2 to 40 microns has also been studied. The energy levels of the trivalent terbium ions involved in the various optical processes have been identified by reference to theoretical predictions and spectra in other media. Glasses doped with terbium show intense series of blue and green luminescence emissions when excited by ultra-violet or X-ray radiation. Emissions from the (^5) D (_3) level of the Tb (^3+) ions produce the blue luminescence while the green emission results from transitions from the (^5) D (_4) level. At terbium concentrations above 0.3 mole % the blue emissions are quenched by multipolar transitions from the (^5) D (_3) level to the (^5) d (_4) level. The green emissions are quenched at concentrations above 6 mole % by an exchange-dipole mechanism. The effect of temperature on the emission characteristics has been determined. Intense luminescence persists to temperatures above 500 C in silicate glasses. The reduction in temperature does not greatly change the emission intensity. Inhomogeneous broadening, due to the random nature of the glass matrix, persists even at liquid helium temperatures. Decay rates have been measured at various temperatures with both ultra-violet and X-ray excitation. The effect of other rare earths on the photoluminescence has also been investigated, and a model for the lanthanide ion site is proposed. The thermoluminescence characteristics of terbium doped silicate glasses have also been measured. Increased terbium concentration reduces the glow peak intensity. A model of the mechanism producing thermoluminescence is proposed. Differences between binary (sodium silicate) and ternary (lithium aluminosilicate) glasses, observed in both photo- and thermoluminescence, are discussed. Other optical properties, such as the Faraday Effect and cathodoluminescence, are reviewed in a survey of the literature.