Subwavelength periodic nanostructures fabricated by femtosecond laser in metal, dielectric and metal-dielectric-metal coating

Subwavelength periodic nanostructures on metal, dielectric or metal-dielectric-metal coating were fabricated by femtosecond laser. Circular apertures with diameter of ∼0.6μm and a lattice period of 1.0 and 2.0 um were fabricated by single laser pulse irradiation. Surface geometries of nanostructure unit on the three kinds of coatings were compared using the scanning electron microscope and atomic force microscope. Moreover, the nanostructure units fabricated by front-side ablation technique and back-side ablation technique were compared. The minimum diameter of the circular aperture fabricated both by front-side ablation technique and back-side ablation technique could reach approximate 300 nm. By studying mechanisms of femtosecond laser interaction with metal films and dielectric films, it's found that metal layers dominate femtosecond laser interaction with metal-dielectric-metal sandwich type coatings. It's also found that the edges of apertures fabricated by back-side ablation technique in metal coatings and metal-dielectric-metal coatings are more regular and round than those fabricated by front-side ablation technique. Femtosecond laser fabrication therefore provides possibilities for maskless and speedy fabrication of fishnet structures which exhibit negative refractive index at optical frequencies.