Photoelastic properties of oxide and non-oxide glasses
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An unperturbed piece of glass is optically isotropic. Birefringence can be induced by breaking this symmetry, for example by applying a uniaxial stress to the glass. An empirical model exists which predicts when oxide glasses will exhibit positive, negative or zero birefringence under stress. This model has been used to determined new zero-stress optic oxide glass compositions; however, it has not been tested on non-oxide systems, nor does it take into account the wavelength of the incident light. The stress-optic response in chalcogenide glasses was investigated using stoichiometric GeS2-P2S5 and GeS2-Sb2S3, and non-stoichiometric Ge-P-S. The trends of the stress-optic response in stoichiometric non-oxide glasses correlated well with predictions based on the empirical model for oxide glasses. Small differences between the predictive parameter of the model, 〈d/Nc〉, were explained by expanding the treatment of metallicity. The wavelength dependence of the stress-optic coefficient for a large variety of oxide glass formers and modifiers was measured. Glasses composed of positive modifiers (e.g. BaO, CaO, Na2O) exhibited positive dispersion of the stress-optic coefficient, while those with negative modifiers (e.g. PbO, SnO) showed negative dispersion. Mixing positive and negative modifiers in a glass resulted in compositions exhibiting wavelength-independent stress-optic coefficients. The photoelastic response of a material can also be described by its elasto-optic tensor (pij). Isotropic materials have three elasto-optic tensor elements, two of which are independent. These two independent elements were measured for lead and barium borate, phosphate and silicate glasses using Brillouin spectroscopy. Both elements were found to correlate with the empirical parameter 〈d/Nc〉, while the third, dependent element showed no relationship to the empirical model. Finally, an alternative model of photoelasticity is discussed. The normalized polarizability of a material is found to correlate with both the sign and magnitude of the stress-optic coefficient.