Thermomechanical ablation under plasmonic field excited by ultrashort laser pulse. Part I: Plasmonics

Abstract

Ultrashort laser pulse with duration τL ~ 1 ps, e.g. acting in Kretschmann geometry, supports a standing plasmonic wave in a thin metal film deposited onto a surface of transparent dielectric prism during the pulse. It is very important that the standing plasmonic wave spatially redistributes energy of the 1st laser beam which excites a surface plasmon polariton (SPP), where SPP traps this energy, and laser energy incident on SPP (laser 2, region where standing wave locates). Thus a spatially periodic heating pattern is formed with dissipative hot spots around the antinodes and cold spots around the nodes of the standing wave. Distance between two subsequent nodes equals to plasmonic wavelength ~ 0.7 μ m. Another important circumstance is the ultrashort duration of the non-homogeneous heating. It is shorter than the acoustic time scale ts = df/cs defining mechanical response of the film to heating; here df is film thickness and cs is a sound speed in metal. ts is about a few tens of picoseconds for our range of thicknesses. This means that mechanical response of a film to the heating is retarded in time relative to the heating stage, which leads to the subsonic motion of material and large generated stresses because they are directly proportional to volume density of absorbed energy. Fast freezing of moving molten film fixes its deformed configuration. This opens a new way for fabrication of desirable structures at nanoscales. In the first part of our work we present a picture of standing wave. The second part given separately is devoted to the thermal and hydrodynamic response to the fast heating by the standing wave.