DENSIFICATION OF FUSED-SILICA UNDER 193-NM EXCITATION

The experimental fact that fused silica undergoes densification upon p rolonged exposure to high-energy radiation is well documented. About 2 5 pears ago, Primak and Kampwirth [J. Appl. Phys. 39, 5651 (1968)] rep orted extensive densification results in SiO2 obtained with neutron, e lectron, and gamma-ray exposures. More recently, results from experime nts with pulsed deep-ultraviolet exposures have been reported. We repo rt here our experimental results and analysis of the densification of Coming HPFS(TM) materials under 193-nm exposure. The densification del ta rho/rho induced by the radiation was obtained from interferometric and birefringence measurements with the aid of a finite-element analys is. The use of such analysis is necessary to obtain the laser-induced densification independent of sample size, geometry, irradiation patter n, and intensity profile of the exposure beam. In our case the sample was 10 mm x 15 mm x 20 mm irradiated across the 10-mm face with a 5-mm apertured Gaussian beam in the 15-mm direction. The birefringence and wave-front distortion were measured off line as a function of number of pulses for district values of fluence per pulse. We found that the derived densification follows a universal function of the dose, define d as the product of the number of pulses and the square of the fluence per pulse. In fact, it follows the same functional form as that previ ously determined by Primak in his high-energy-exposure study. This str ongly suggests that the laser-induced densification mechanism involves the optically induced weakening of bonds and subsequent relaxation. ( C) 1997 Optical Society of America.