On the linearity of holographic recording in amorphous $As2S3$ films

Optical recording in amorphous arsenic trisulfide was investigated for amplitude to phase conversion to be exploited in applications such as holographic recording and the fabrication of diffractive optical elements. The main result of this study is the demonstration that it is possible to obtain a linear relation between the spatial variation of the recording intensity and the recorded phase modulation. The extent of the linear recording region depends strongly on the recording wavelength. At short wavelength (458 nm in these experiments), where the rate of the photoinduced structural changes is strong, the recording is practically linear from zero exposure up to about 25% of the maximum value of the refractive index change. With higher exposure, saturation effects start to play an important role and degrade the linearity of the response. Recording with longer wavelength (488 and 514.5 nm) is less effective (absorption becomes weaker) and then relaxation processes in the material become significant. If the competition between photoinduced processes and relaxation processes is strong, the linear relationship between exposure and refractive index change can no longer be linear. Nevertheless, even for recording with the longer wavelengths, a region of exposures can be found within which variations of intensity can be converted into variations of phase with a linear relationship.