  In this paper the formation mechanisms of the femtosecond laser-induced
periodic surface structures (LIPSS) are discussed. One of the most
frequently-used theories explains the structures by interference between the
incident laser beam and surface plasmon-polariton waves. The latter is most
commonly attributed to the coupling of the incident laser light to the surface
roughness. We demonstrate that this excitation mechanism of surface plasmons
contradicts to the results of laser-ablation experiments. As an alternative
approach to the excitation of LIPSS we analyse development of hydrodynamic
instabilities in the melt layer.
