Origin of refractive index change in photo-thermo-refractive glass

Abstract

There are still some uncertainties in the origin of photo-thermo-induced refractive index change in PTR glass. On one side, according to Nikonorov N.V. it is assumed to be due to difference of refractive indexes of sodium fluoride nanocrystals (n ~ 1.32) and glass (n ~ 1.49). Later Glebov L.B. et al. suggested another mechanism. They assumed that the transformation of Na and F distributed in PTR glass matrix into crystalline NaF (chemical changes) are not the main causes of photo- thermo-induced refractive index change and accounted it for high residual stresses around the NaF crystals. So present experimental research is focused on revealing of the refractive index change origin in fluoride PTR glass. It is shown experimentally that residual stresses have no measurable effect on the refractive index change in PTR glass, that is in agreement with the estimations made using the Maxwell-Garnet theory.