ORIGIN OF HIGH-SPEED MODIFICATION OF REFRACTIVE-INDEX IN FUSED QUARTZBY VACUUM-ULTRAVIOLET LASER IRRADIATION

We report on high-speed modification and a large change of the refract ive index on the order of 10(-2) of fused quartz upon vacuum ultraviol et (VUV) laser irradiation. The origin of the large refractive index c hange is discussed based on the laser-induced color center and surface morphology changes. The VUV-UV absorption spectrum of modified sample s indicates the formation of a color center at around 163 nm (=Si-Si= defect), which is attributed to bond scission of fused quartz by VUV l aser-induced electron excitation. On the other hand, simultaneous UV l aser irradiation in our present experimental scheme is responsible for the generation of a surface damage at the large number of pulses, whi ch causes scattering and deterioration of the optical properties of th e irradiated regions.