Spatial tailoring of the refractive index in infrared glass-ceramic films enabled by direct laser writing

•A photothermal process was employed to tune Ge-As-Pb-Se films’ refractive indices.•The index change was studied as a function of laser and heat treatment conditions.•The index change is repeatable and scalable for films with thicknesses up to 40 µm.•The study provides crucial guidance towards a gradient refractive index structure. The development of infrared gradient refractive index (GRIN) components relies on the ability to modify the refractive index and dispersion properties of suitable host materials with a high spatial selectivity and a sufficient magnitude of change. We present a novel multi-step approach to induce local refractive index changes in chalcogenide optical materials. Films with thicknesses between 1 and 40 µm fabricated from multi-component GeSe2-As2Se3-PbSe (GAP–Se) glass-ceramic materials were irradiated with continuous-wave and nanosecond-pulsed laser light, and post-processed with heat-treatments. A maximum local refractive index change of Δn = 0.088 across a broad spectral range in the infrared was realized. Spatial control of the refractive index variation was achieved through thermally-induced crystallization of a Pb-rich crystal phase. The magnitude of the index change scaled with the laser power and the exposure dose while the material maintained the required optical quality. The material performance validated in this study for thick films (20–40 µm films) reconfirms our ability to extend results from thin GAP-Se films towards novel optical designs.