Self-organization of surfaces on the nanoscale by topography-mediated selection of quasi-cylindrical and plasmonic waves

Abstract : Using coupled electromagnetic and hydrody-namic calculations, we elucidate theoretically the topo-graphic transition from a random metallic surface to a periodic sub-wavelength grating by ultrashort laser abla-tion. The origin of this transition lies in the successive selection of hybrid surface waves scattered by random nanoholes. Contrary to the common belief that surface plasmon polaritons play the dominant role in the process and define the grating periodicity, we show that both quasi-cylindrical and surface plasmon waves are involved, whereas the diversity in the resulting spacings λ/2-λ (λ is the laser wavelength) is the manifestation of a broad frequency overlap of these waves, controlled by their relative phase shifts with respect to the plasmonic counterparts. The topography evolution imposes the dominant contribution to the surface sub-wavelength pattern by selecting the appropriate wave character from plasmonic modes to evanescent cylindrical waves. With the radiation dose, the grating periodicity exhibits a pronounced blue shift due to reinforced dipole-dipole coupling between the nano-holes and surface curvatures in the laser-processed area. This allows the creation of regular patterns with tunable periodicity.