The role of van der Waals bonding in photosensitivity of chalcogenide glasses

We report on a comparative study of photosensitivity (scalar and vectoral photoinduced effects and photoexpansion) in Ga-La-S and conventional, As 2 S 3 , chalcogenide glasses. Both scalar (photorefraction and photoexpansion) and vectoral (photoinduced anisotropy) effects are 2-3 orders of magnitude weaker in Ga-La-S glasses, such as Ga 1.4 La 0.6 S 3 , than in As 2 S 3 glass. We relate the suppression of photosensitivity in Ga-La-S glass to the lack of van der Waals bonding in its structure, while this bonding accounts for photosensitivity in conventional chalcogenide glasses, such as As 2 S 3 . We propose that photosensitivity in chalcogenide glasses is accounted for by photoinduced change in polarizability of van der Waals type hyperpolarizable bonds. The presence of van der Waals bonding in the glass structure may affect quantitative predictions of the theory of glass-network rigidity/constraints, which relates glass stability/fragility to the average co-ordination number of constituent atoms that is entirely due to covalent bonding.