Photoinduced changes in the linear and non-linear optical properties of chalcogenide glasses

The linear refractive index, n, and the position of the absorption edge for thin chalcogenide/chalcohalide films from the systems As-S, As-S-X (X = Bi, Ti or I), and Ge-S-Me (Me = Bi, Tl, As) have been measured before and after exposure to above-bandgap illumination. For compositions containing either As or Ce, the absorption edge was shifted to longer wavelengths (i.e. photodarkening occurred) for As-containing films, and to shorter wavelengths (i.e. photobleaching occurred) for Ge-containing films. As-S-Ge films exhibit either photodarkening or photobleaching, depending on Ge content. The largest values of these edge shifts ranged from 20 up to 60 nm, hi the case of Ge31S63Tl6. The conditions for synthesizing the glasses studied are reported. A formula for predicting the non-linear refractive index, n(2), for chalcogenide/chalcohalide glasses from the dispersion of n, and which enables nz to be related to structural parameters, has been developed and compared with existing formulae for other types of glass and crystals, Using the various formulae and the measured values of n, n(2) for these materials has been predicted. The results indicate that glasses with compositions near As42S58 or As2S3Tl0.13 may, after UV exposure, exhibit significantly larger values of nz than as-deposited As40S60. (C) 1999 Elsevier Science B.V. All rights reserved.