Ultrashort pulse action onto thin film on substrate: Qualitative model of shock propagation in substrate explaining phenomenon of fast growth of a hole with increase of absorbed energy
Thin films on substrate are important class of targets for surface nanomodification for plasmonic or sensoric applications. There are many papers devoted to this problem. But all of them are concentrated on dynamics of a film, paying small attention to substrate. In these papers the substrate is just an object absorbing the first shock. Here we present another point of view directed onto dynamics of a substrate. We consider (i) generation of a shock wave (SW) in a support by impact of a contact; (ii) transition from one-dimensional to two-dimensional (2D) propagation of SW; (iii) we analyze lateral propagation of the SW along a film-support contact; and (iv) we calculate pressure in the compressed layer behind the decaying SW. This positive pressure acting from substrate to 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 circle of the focal laser spot on film surface. Absorbed laser heat exponentially decays around an irradiated spot $F=F_{\rm c} \exp(-r^2/R_{\rm L}^2)$, where $R_{\rm L}$ is radius of a Gaussian beam, $F$ and $F_{\rm c}$ are local and central fluences, $r$ is a radius from the axis. While the law of decay for the 2D SW in substrate is the power law. Therefore in our case of powerful laser action the edge of a separation circle is defined by propagation of the SW in the support.