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.