Linear and nonlinear optical properties change in Ag/GeS heterostructure thin films by thermal annealing and laser irradiation

The photo and thermal annealing induced linear and nonlinear optical properties, structural properties and surface morphology of thermally evaporated Ag/GeS bilayer thin films are reported in the present study. The related changes due to Ag diffusion into GeS layer by annealing at 200 °C and 532 nm laser irradiation have been probed. The X-ray diffraction revealed no structural change whereas the surface structure was viewed by field emission scanning electron microscopy. The optical data recorded by UV–visible spectrophotometer in the range of 500–1200 nm shows the decrease in refractive index (n) and extinction coefficient (k). The Ag diffusion into GeS layer increased the optical band gap due to the decrease of width of the localised states. The single oscillator Wemple–Di Domenico model was used to study the dispersion relation. The third order nonlinear susceptibility (χ(3)) and nonlinear refractive index (n2) were calculated by using suitable semi-empirical relation. The change in density of defects and localised states explained the observed linear and nonlinear changes with annealing and laser irradiation. The homopolar bond to heteropolar bond transformation is noticed from X-ray photoelectron spectroscopy. The observed optical changes can be used in various optical applications.