  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.
