An effective modulation of a He–Ne laser beam at a wavelength of 6328 Å guided in a semiconductor ZnS thin film has been obtained when the guided wave travels transversely across the region irradiated by intense uv light. This seems to be caused by the absorption effect by means of the optically excited free carriers. The modulation ratio has shown monotonic increase with the excitation intensity. At maximum excitation, a modulation ratio of 35% was obtained. For normal transmission of a laser beam through the excited shallow region near the surface, measurements of reflectance change and phase retardation have been also carried out under uv light and pulsed electron excitations, are discussed in connection with the absorption effects by the free carriers generated by these excitations. In the case of excitation by an electron beam of 15 mA and 30 kV, the fractional increase of the absorption was1×10−3.