Surface and volume contributions to total diffractional efficiency in As2S3 thin film glasses

Chalcogenide glasses such as As2S3 illuminated by near bandgap light (Eg∼2.4 eV) exhibit photodarkening and photoexpansion. An exposition by a sinusoidal intensity holographic pattern thus will create a combination of two superposed volume (δn) and surface (δL) gratings [J. Lightwave Technol. 15 (8) (1997) 1343; IEEE Photon Technol. Lett. 8 (8) (1996) 1041]. We present studies of contribution of each grating to the total diffractional efficiency (DE) of these photoinduced gratings, recorded on vapor-deposited As2S3 thin films. An index matching technique is used to achieve a situation where mainly δn contributes to the total DE. Also, measuring the dynamic evolution of both δL and total DE allows to calculate δn (up to 5×10−2) contribution via the theoretical formula. Both techniques show that the main contribution to the total DE for our films (of various thickness, i.e. 1.4, 3.0 and 5.0 μm) under weak illumination (under 2.2 mW/mm2), is due to the grating of the refractive index modulation δn.