The evidence of the role of surface plasmon polaritons in formation of femtosecond highly-regular laser-induced periodic structures on Cr films

Abstract

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 Λ ~ λ/n at film-substrate interface (n is refractive index of substrate), and a propagation mode Λ ~ λ/nCr where nCr is refractive index of film material. Experimentally observed LIPSS periods match well the predicted modes for the extreme cases, Λ ~ λ/n at film thickness h ≤ 30 nm and Λ ~ λ 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.