ABSTRACT
Phase transitions at the solid-liquid interface, exposed to short
laser pulses at an energy density sufficient to melt the solid, can
be traced to modifications induced at the solid surface by the laser
pulses. In case of a metallic target, the laser radiation is absorbed
by free electrons, and the lattice temperature starts to increase due
to the electron-phonon relaxation process. If the absorbed energy is
sufficiently high, the metal target melts, so that a layer of the liquid
that surrounds it is heated up due to heat transfer from themetal. As
a result of the high pressure of the adjacent medium that contacts
the melt, the latter can be modified. Such modification is caused
when the generated liquid vapors induce viscous flows within the
molten layer, leading to the formation of various surface structures.
The characteristic thickness of the modified layer of the solid target
strongly depends on the melt thickness and therefore, on both the
laser fluence and its duration. The thickness of the molten layer hm, can be estimated by the heat diffusion length during the laser pulse
as follows: hm ∼ (atp)1/2, where tp stands for pulse duration, and a stands for the heat diffusion coefficient of the solid. This estimation
is valid only for laser fluence close to the melting threshold of the
solid. If the duration of the laser pulse is less than the time of
electron-phonon relaxation, then heating of the lattice occurswithin
the depth of the absorption of laser radiation. For typical metals the
mean free path of excited electrons during the relaxation process
is too short, and the melt thickness does not exceed a fraction of a
micrometer even for nanosecond (ns) laser pulses. As a result this
layer of material may be redistributed into one or another kind of
NS due to the recoil pressure of the liquid medium adjacent to it.
