Laser-induced modification of local refractive index in infrared glass-ceramic films

The ability to employ spatially-selective control of refractive index and dispersion variation with a high magnitude of change is essential for the realization of functional infrared graded-index (GRIN) components. Thin films fabricated from multi-component GAP-Se glass-ceramic materials were processed using nanosecond laser radiation at the wavelength lambda = 2 mu m. Various irradiation and post-processing protocols were implemented to maximize the magnitude of the local refractive index change, and to quantify the evolution of the glass to glass ceramic 'conversion' on optical material physical properties. Irradiation of films possessing various thicknesses from 1 to 25 mu m was performed using area-scan patterns, while the average laser power and the number of scans were varied. Irradiated materials were subsequently heat-treated, and the local refractive index was determined for different durations of the heat treatment. Depth-dependent composition and film morphology characterization of as-deposited films was evaluated, and surface morphology of the post laser-processed and heat-treated areas was studied to evaluate effects on the photo-thermal refractive index change associated with nanocrystal formation. Initial studies demonstrated a maximum positive refractive index change of Delta n approximate to 0.07 in a broad spectral range in the infrared which scales with film thickness and exposure dose while maintaining required optical quality.