Photoinduced changes in the linear and non-linear optical properties of chalcogenide glasses K. Petkov a , P.J.S. Ewen b, * a Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bontchev str., Bl. 109, 1113 So®a, Bulgaria b Department of Electronics and Electrical Engineering, University of Edinburgh, The King's Buildings, May®eld Road, Edinburgh EH9 3JL, UK Received 20 August 1998; received in revised form 5 April 1999 Abstract The linear refractive index, n, and the position of the absorption edge for thin chalcogenide/chalcohalide ®lms from the systems As±S, As±S±X (X  Bi, Tl 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 Ge, the absorption edge was shifted to longer wavelengths (i.e. photodarkening occurred) for As-containing ®lms, and to shorter wavelengths (i.e. pho- tobleaching occurred) for Ge-containing ®lms. As±S±Ge ®lms exhibit either photodarkening or photobleaching, de- pending on Ge content. The largest values of these edge shifts ranged from 20 up to 60 nm, in the case of Ge 31 S 63 Tl 6 . 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 n 2 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 As 42 S 58 or As 2 S 3 Tl 0:13 may, after UV exposure, exhibit signi®cantly larger values of n 2 than as-deposited As 40 S 60 . Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 42.70.Ce; 42.70.Nq; 78.20.Ci 1. Introduction Chalcogenide ®lms have been the subject of systematic investigation for many years because of the changes in physical and chemical properties which occur in these samples after exposure and annealing [1±7], and because of the potential applications of these eects in areas such as sub- micron lithography, optical memories, holography and imaging [2,5]. For example, a number of pa- pers [3±5,7] have shown that the changes in the dissolution rates of the ®lms after exposure enable them to be used as high-resolution inorganic photoresists, with the added advantage that they are resistant to chromium etchant and can there- fore be used in the production of photomasks. All these photoinduced changes are strongly depen- dent on the method of glass synthesis and ®lm preparation. More recently, signi®cant interest has arisen in the non-linear optical properties of chalcogenide glasses because of their potential use in optical- Journal of Non-Crystalline Solids 249 (1999) 150±159 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: 44-131 650 5651; fax: +44-131 650 6554; e-mail: pjse@ee.ed.ac.uk 0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 3 0 - 0