Formation of micro- and nanostructures on the surface of laser-created molten layer with inverted normal temperature gradient

A nonlinear two-dimensional hydrodynamic (HD) equation of Kuramoto-Sivashinsky(KS) type for the thickness of the laser pulse-induced viscous molten layer on solid base is derived in the long wavelength and weak nonlinearity approximation. Linear stability analysis shows that under the condition that the temperature gradient in the surface laser-melted layer is directed from the surface to the bulk, the thermocapillar or evaporative hydrodynamic instability sets in, that leads to the formation of surface relief structures with dimensions proportional to the thickness of the molten layer. Computer simulations predict the successive formation, in linear and nonlinear regimes, of extended lamellar-like, disordered and hexagonal periodic structures of the surface relief when the time of irradiation is increased. Under tight laser light focusing, in the quasi-nonlinear regime, phase synchronization of Fourier harmonics of the surface relief, occurring due to the three HD mode interactions, is shown to lead to formation of holes or nanobumps (nanojets). Crown-like computer solutions of the HDKS equation are obtained in the case of Gaussian intensity distribution in the laser beam.