Abstract
Ablation characteristics of chromium thin film on quartz by femtosecond laser pulses were investigated. Experiments were performed by femtosecond double-pulse irradiation. Double pulses were obtained using a Michelson interferometer and irradiated to a target sample by a couple of afocal systems. Delay time was controlled by adjustment of optical path. Intensity ratio and delay time between first and second pulse are important factors in the double-pulse ablation. Surround of ablation area was affected by the second pulse after evolution of ablation plume induced by the first pulse. This affected area is remarkable, when first pulse has sufficient intensity and delay time is adequate for evolution of laser ablation plume. The condition in which the intensity of the first pulse is lower than that of the second pulse brings smaller size of ablation particles. Through molecular dynamics simulations, we found two types of laser ablation for double pulse irradiation. One is the normal ablation involving formation of the ablation front. The other is the fracturing ablation, whose mechanism is considered as follows: The first pulse irradiation with energy close to the ablation threshold yields porous structure in the vicinity of surface. When the second pulse is irradiated to activated area with porous structure, the fracturing ablation occurs. This simulation result agreed well with experimental results.
