Abstract Periodically corrugated waveguides consisting of a ZnS layer clad on one side by Al have been made free standing by deposition on to photoresist followed by removal of the photoresist using a solvent. Coupling of laser light into guided modes causes heating of the sample which leads to a new nonlinear effect, namely, a surface distortion thermally induced by differences in expansion between the ZnS and the Al layers. A temperature rise of 5·4°C is sufficient to give strong effects in the intensity of the reflected beam. This phenomenon has been studied in detail for two samples with time resolved experiments allowing the speed of response to be found.