Transmittance in a thin aluminum layer at nanosecond pulsed laser ablation

Since the beginning of the 1960s and up to the present time, laser ablation from metal surfaces under the action of short light pulses has been the topic of many researches. One of the first objects in the early ablation experiments presented thin metal films evaporated by radiation, which were used in lasers with nanosecond pulses as optical gates for Q-switching in resonators or for decoupling in amplifiers. Bleaching of the gates based on metal layers with a time constant τt ≈ 10−8 s observed in a number of experiments, was usually considered as a result of a simple evaporation of matter. We analyze the data of the experiments with a mylar tape [≈0.05 µm aluminum (Al) layer on ≈20 µm lavsan substrate] used as a gate for optical isolation in one of the first Nd: glass laser facilities with a power of ≈1 GW. That gate was irradiated by pulses of Q-switched oscillators: pulse duration 10−7–10−8 s, intensity 107–108 W/cm2. A jump in the transmittance of the expanding Al layer was registered (from ≈0.1 to 50% at τt ≈ 10−9 s). The present study shows that one should consider the metal–insulator phase transition in a superheated liquid metal layer as the mechanism of the fast (up to 10–10–10–11 s) increase in the transmittance of the Al film gate at nanosecond-pulsed laser ablation.