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The optical parameters of optically uniaxial γ1‐(GaxIn1−x)2Se3 single crystals are systematically measured and analyzed. The optical rotatory power along the optical c‐axis is determined in visible spectral region at 77 K and at room temperature and complemented with theoretical estimates for other crystallographic directions. The g33 component of the gyration tensor is calculated from the experimental data. The dichroism is studied in the vicinity of the absorption edges. The dispersion of ordinary and extraordinary refraction indices is determined at room temperature for the transparency window 0.57 to 10 μ. Up to 5 μm the dispersion can be interpreted in terms of noninteracting harmonic oscillators (single‐term Sellmeier relation).

Thermal evaporation technique is used to deposite (Ga0,2In0,8)2Se3 thin films. The refractive index and extinction coefficient dispersions are obtained from the spectral ellipsometry measurements. The dispersion of the refractive index is described in the framework of the Wemple–Di Domenico model. The optical transmission spectra of a (Ga0,2In0,8)2Se3 thin film are studied in the temperature range 77–300 K. The temperature behavior of the Urbach absorption edge, as well as the temperature dependences of the energy pseudogap and Urbach energy, are investigated. The influence of different types of disordering on the optical absorption edge of a (Ga0,2In0,8)2Se3 thin film is discussed. Optical parameters of a (Ga0,2In0,8)2Se3 thin film and a single crystal are compared.

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