Composition dependences of refractive index and thermo-optic coefficient in Ge-As-Se chalcogenide glasses

The refractive indices (n) and thermo-optic coefficients (ζ) of Ge₁₄As_xSe_86 − x and Ge_xAs₁₂Se_88 − x glasses are measured in the 2–12 μm mid-infrared range. The composition dependences of the two parameters are investigated and strategies for tailoring them are proposed. The density (d) shows a maximum at the stoichiometric composition when Ge content is fixed; while it exhibits a minimum at the mean coordination number (< r >) of 2.67 when As concentration is fixed. The volume thermal expansion coefficient (β) broadly decreases with increasing < r >. Two semi-empirical relations are used to predict n and ζ from the glass composition. The n of a Ge-As-Se glass could be predicted with an error of < 1% from the d of the glass and the molar refractivity (R_i) of the constituent elements. The estimated values of R_i for Ge, As and Se between 2 and 12 μm range from 10.12–10.52, 11.71–11.91, and 11.17–11.25 cm³/mol, respectively. The ζ of a Ge-As-Se glass could be forecasted with an error of < 6 ppm/K from the β of the glass and the thermal coefficients of the molar refractivity (φ_i) of the constituent elements. The estimated values of φ_i for Ge, As and Se between 2 and 12 μm range from 26.2–26.7, 54.9–55.7 and 85.4–86.8 ppm/K, respectively.