Cr films of controlled thickness deposited on glass substrates were irradiated by a high number of low-intensity femtosecond laser pulses below the Cr single-pulse damage threshold, producing periodic surface structures (LIPSS) of very high regularity via metal oxidation. To address the multiplicity of electromagnetic modes allowed for thin films, a rigorous numerical approach for modeling surface plasmon polaritons (SPP) in thin-film geometry has been developed. Three types of modes are predicted: the classical SPP with periodicities Λ ~ λ (λ Is laser wavelength) at air-film interface, and ? ~ ?/n at film-substrate interface (n is refractive index of substrate), and a propagation mode ? ~ ?/nCr where nCr is refractive index of film material. Experimentally observed LIPSS periods match well the predicted modes for the extreme cases, ? ~ ?/n at film thickness h = 30 nm and ? ~ ? At h = 200 nm, indicating respectively the dominance of SPP excited at the film-substrate interface and at the metal surface. For 30 nm < h < 200 nm, decreasing of the film thickness results in transition from the pure surface mode to its coupling with and finally domination of the electromagnetic wave excited at the film-substrate interface, providing evidence of the SPP mechanism of LIPSS formation under laser irradiation of thin metallic films.
|Journal||Journal of Physics: Conference Series|
|Publication status||Published - 1 Jan 2018|
|Event||3rd International Conference on Metamaterials and Nanophotonics, METANANO 2018 - Sochi, Russian Federation|
Duration: 17 Sep 2018 → 21 Sep 2018