Action of a femtosecond laser pulse on thin metal film supported by glass substrate

Numerous papers are devoted to the problem of irradiating thin films on the substrate for surface nano-modification. But all of them concern the dynamics of the dynamics, paying almost no attention to substrate just as object of the first shockwave absorption. A different point of view with an emphasis on the dynamics of a substrate is presented. Under powerful laser action upon a thin metal film a hole arises. Its radius depends on the absorbed laser energy. Experimental results, quantitative theoretical model and numerical research are presented and show that the hole formation is influenced by propagation of the shock wave in the substrate, but not in the film itself. Four stages are considered. (i) Shockwave generation in a support because of an impact of a contact. (ii) Transition from one-dimensional to two-dimensional propagation of the shockwave. (iii) Lateral propagation of the shockwave along a film-support contact. And (iv) calculating pressure in the compressed layer behind the decaying shockwave. This positive pressure acting from substrate on the film accelerates the film in direction to vacuum. Above some threshold, velocity of accelerated film is enough to separate the film from support. In these cases the circle of separation is significantly wider than the focal laser spot on film surface.