Laser densification of medium pore size gel-silica glass

Type VI sol-gel silica monoliths with 32 Å and 45 Å pore radii were made using tetramethylsiloxane (TMOS) with HF as a catalyst. The pore texture was characterized using nitrogen absorption/desorption isotherms and BET analysis. Monoliths thermally stabilized to specific bulk densities of 1.67 g/cc to 2.00 g/cc were irradiated with a CO₂ laser to create microlenses. The densified spots were characterized using microscopic Fourier transform infrared (FTIR) spectroscopy. Full density microlenses were achieved; the peak position of the Si-O-Si vibrational mode was at 1122 cm^–1, equivalent to commercial optical silicas. The refractive index in the center of the densified spots was 1.46 at a wavelength of 632.8 nm with a gradient in refractive index of 0.07–0.08. The combinations of laser power and substrate bulk density that yield full density microlenses is expanded when compared with conditions previously established for small pore, 12 Å, gel-silica substrates. Dimensions of the laser densified region, magnitude of the index gradient, and shape of the GRIN profile are easier to control with the medium pore size gel-silica substrates.