Thermally stable grating comprised of silsesquioxane film fabricated with hot-embossing

Thermally stable grating was fabricated with hot-embossing with the application of sol-gel derived silsesquioxane film. Planar films consisted of phenylsilsesquioxane and methylsilsesquioxane showed thermal stability with respect to the low birefringence, less than 1×10^-3, and no intrinsic absorption from 400 to 1700nm for the prolonged heating at 200°C for 10hours. The refractive index of the film was controllable from 1.49 to 1. 56 at 632.8nm wavelength with increasing a molar ratio of the phenyl contents from 0.3 to 1.0, where the index controllability was ±0.0005. A short embossing process, less than 15 minutes, was demonstrated. In-situ mode-line measurement for the embossed grating showed change in refractive index under heating and cooling cycle was linear and reversible. A high thermooptic coefficient (dn/dT) of the films from room temperature to 150°C, which was approximately -2×10^-4 despite of its composition, was stable after subsequent nine thermal cycles. The measurement at the temperature up to 300°C showed streak propagation in the film and the prolonged heating at 150°C for 700 hours indicated no change in refractive index. This material is considered to be suitable for optical device application, since it showed the combined advantages of the both organic and inorganic materials such as embossing capability and high TO coefficient with thermal stability.