Origin of photostructural changes in chalcogenide glasses : negative-U centres excitations

The origin of the photostructural changes in chalcogenide glasses is related to the existence in the glasses of negative-U centres as basic charge carriers so that gap-light generated excited states of the centres generate metastable defects in the original structure. The theory gives rise to a consistent description of, and some prediction for, dependencies of the steady-state photodarkening on gap-light intensity and frequency, pressure and temperature.