Fabrication of guiding structures in nanostructured tin-silicate glass ceramic by a focused femtosecond laser

A photoinduced decrease of refractive index has been demonstrated in SiO2:SnO2 (15 mol%) optical-grade glass ceramic, exposed to a 780 nm wavelength, focused femtosecond laser radiation at 1 kHz repetition rate. Exposition have been performed translating the sample perpendicularly to the laser beam at a constant velocity of 17 μm/s, with energies ranging between 2 and 5 μJ per pulse. The samples were fabricated by the sol-gel method, from 85 mol% tetraetoxysilane and 15 mol% dibutyl tin-diacetate, adding water to a solution of the precursors. Xerogel was produced by slowly evaporating the solvent for 2 weeks at 35 °C. The final material was obtained heating in O2 the xerogel samples by a thermal ramp (4 °C/h) up to 1050 °C. Samples were then cut from the blocks with a diamond saw to 10 × 10 × 1 mm3 and all the faces polished. A negative variation of the refractive index has been demonstrated in the irradiated zone (−3 × 10−4) measured by the prism coupling technique. Multimode waveguides at 633 nm have been obtained exploiting a positive refractive index change at the boundaries of the irradiated zone. Raman spectra have been measured showing several broad bands centered at about 430, 800 and 1100 cm−1, and with two peaks at 490 and 610 cm−1, when exciting at 488 nm. Strong red luminescence has also been observed when exciting at 633 nm suggesting the creation of non-bridging oxygen defects in the silica-based glass. We proposed that the negative refractive index change in the irradiated zone is due to the amorphization of the nanocrystals, while further investigation is needed to fully understand the origin of the positive index change at the boundaries.