Tuning linear and nonlinear optical properties of Se₇₀Te₂₀Sb₁₀ thin films via 3 MeV proton irradiation for photonic applications

This study aims to investigate how 3 MeV proton irradiation influences the structural, linear, and nonlinear optical properties of Se₇₀Te₂₀Sb₁₀ thin films, with the goal of assessing their potential for photonic device applications. Thin films were thermally deposited by electron-beam evaporation and exposed to proton fluences ranging from 5 × 10¹³ to 5 × 10¹⁶ ions/cm². X-ray diffraction and Field Emission Scanning Electron Microscope (FESEM) confirmed the amorphous structure of the films across all fluences. while AFM revealed a substantial increase in surface roughness from 40.5 nm (as-grown) to 386.7 nm at the highest fluence, indicating irradiation-induced morphological reorganization. Optical characterization showed that transmittance and reflectance decreased with increasing fluence, while the absorption coefficient (α) in the 1.5–2.5 eV range increased. The optical band gap (E_g) exhibited a non-linear dependence, decreasing from 1.298 eV (as-grown) to 1.281 eV at 5 × 10¹⁴ ions/cm² then rising to 1.389 eV at 5 × 10¹⁵ ions/cm², and then reducing again at the highest fluence. Urbach energy (Eu) varied complementarily, reflecting disorder–recovery dynamics. The third-order nonlinear susceptibility χ³ peaked at 1.803 × 10⁻¹¹ esu, with corresponding enhancement in the nonlinear refractive index n₂. Dielectric functions and energy loss spectra further confirmed fluence-dependent changes in electronic response. These findings demonstrate that Sb-stabilized Se–Te alloys exhibit tunable optical properties under proton irradiation, making them promising candidates for radiation-hardened optoelectronic and photonic devices.