10 . Na2O-x . Nb2O5-(90 - x). TeO2 (x = 0, 5, 10 and 20) glasses have been prepared and some of their physical and optical properties are investigated. The refractive index variation with wavelength is measured using a novel technique, white light interferometry. Optical hand gaps and Urbach energies are estimated from the optical absorption spectra. Optical parameters viz., dispersion energy, average oscillator energy, Abbe's number and third order nonlinear susceptibility values are estimated from the dispersion of the refractive index. Respective refractive index and estimated third order nonlinear optical susceptibility values are larger than for the tellurite glasses reported earlier. Relatively large refractive index values obtained for the present glasses are attributed to the hyperpolarisability of the Nb-O bands. The present study of the optical band gap and dispersion energies of these glasses suggests the decrease of covalency of the glass with the increase of niobium (Nb) content. (C) 2001 Elsevier Science Ltd. All rights reserved. References: 26
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