Origin of high-speed modification of refractive index in fused quartz by vacuum ultraviolet laser irradiation

We report on high-speed modification and a large change of the refractive index on the order of 10^-2 of fused quartz upon vacuum ultraviolet (VUV) laser irradiation. The origin of the large refractive index change is discussed based on the laser-induced color center and surface morphology changes. The VUV-UV absorption spectrum of modified samples 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 laser-induced electron excitation. On the other hand, simultaneous UV laser irradiation in our present experimental scheme is responsible for the generation of a surface damage at the large number of pulses, which causes scattering and deterioration of the optical properties of the irradiated regions