Effect of Ge concentration on optical properties of films synthesized by vacuum-thermal evaporation of glassy Ge–Se alloys

•non-crystalline GexSe100-x (x = 15−25 at. %) films were obtained by evaporation in vacuum.•optical parameters (OPs) of the films depend on the Ge concentration.•abnormal dispersion of refractive indices n (λ) with nmax was detected in ultraviolet.•the change in the position of nmax is affected by the Ge concentration in the films.•nonlinear OPs χ(3) and n2 of the films increase as Ge concentration decreases. Four non-crystalline GexSe100−x (x = 15−25 at. %) films were obtained by vacuum-thermal deposition of glassy Ge–Se alloys. With the use of spectroscopic ellipsometry, it was determined that the films are semiconductors with indirect band gap energy Eg = 2.0−2.07 eV. An anomalous dispersion of the refractive index n (λ) with nmax at λmax depending on x was revealed in the films at λ = 240−500 nm. It was established that as the concentration of Ge decreases, nmax increases, and λmax red shifts. Based on the Wemple-DiDomenico model, the nonlinear coefficients χ(3) and n2 characterizing the synthesized films were calculated as a function of x. The highest χ(3) and n2 values are achieved in the films at x = 15 at. %. The results obtained can be used for the development and creation of Ge–Se optical filters with an adjustable blue absorption band edge.