Densification of fused silica under 193-nm excitation

The experimental fact that fused silica undergoes densification upon prolonged exposure to high-energy radiation is well documented. About 25 years ago, Primak Kampwirth [J. Appl. Phys.39, 5651 (1968)] reported extensive densification results in SiO2 obtained with neutron, electron, and γ-ray exposures. More recently, results from experiments with pulsed deep-ultraviolet exposures have been reported. We report here our experimental results and analysis of the densification of Corning HPFS™ materials under 193-nm exposure. The densification δρ/ρ induced by the radiation was obtained from interferometric and birefringence measurements with the aid of a finite-element analysis. The use of such analysis is necessary to obtain the laser-induced densification independent of sample size, geometry, irradiation pattern, and intensity profile of the exposure beam. In our case the sample was 10 mm × 15 mm × 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 distinct values of fluence per pulse. We found that the derived densification follows a universal function of the dose, defined 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 previously determined by Primak in his high-energy-exposure study. This strongly suggests that the laser-induced densification mechanism involves the optically induced weakening of bonds and subsequent relaxation.