Á. Sipos, E. Tóth, A. Török, O. Fekete, G. Szabó, M. Csete
University of Szeged,
Keywords: integrated lithography, plasmonic spectral engineering
Summary:Comparative study has been performed on the spectral and near-field properties of complex concave and convex nano-object patterns that can be fabricated via interferometric illumination of colloid sphere monolayers (IICSM) by applying circularly polarized light [1-4]. In IICSM mini-arrays composed of a central ring and satellite nano-crescents can be fabricated via illuminating hexagonal colloid sphere monolayers by two interfering circularly polarized beams. Compared to conventional colloid sphere lithography larger degrees of freedom is achievable in plasmonic spectral engineering due to ten geometrical parameters that can be tuned independently (Fig. 1a/a,b). When colloid sphere monolayers are aligned on thin metal films, various rounded shaped nanoholes can be directly fabricated, while a lift-off procedure makes it possible to transfer the pattern into analogous convex metal nano-objects (Fig. 1a/c). In present study geometrical parameters of the complex patterns were determined based on the near-field distribution, which develops on the substrate under a gold colloid sphere monolayer illuminated by two 400 nm wavelength circularly polarized beams incident at teta=41.81°, 19.47°, 12.83°, in alfa=60° azimuthal orientation of the incidence plane (Fig. 1a/a,b). The concave and convex rectangular patterns with three different (p=300 nm, 600 nm, 900 nm) periods but composed of identical mini-arrays were re-illuminated by p-polarized light in complementary azimuthal orientations to demonstrate their spectral engineering capabilities. On all inspected patterns the convex reflectance in gamma=16°/106° (nominated as U/C) azimuthal orientation corresponds to the concave transmittance in gamma=106°/16° azimuthal orientation, proving that in complementary patterns illuminated by complementary beams the reflectance and transmittance are interchanged, according to the Babinet principle (Fig. 1b-e). All convex patterns exhibit localized plasmonic resonances on individual nano-objects and surface lattice resonances on their arrays, while the optical response of the concave patterns is more structured due to Fano modulations originating from coupled localized and propagating plasmonic modes. By tuning the pattern periodicity analogous extrema are observable on all complex patterns at small wavelengths. In C-orientation both of the convex and concave patterns only the FWHM of the peaks modifies (Fig. 1b). In contrast, in U-orientation additional local maxima appear on the patterns having a period commensurate with the wavelength caused by Wood-Rayleigh anomalies. Moreover, on the concave patterns the Fano peaks with their neighboring dips shift noticeably to smaller wavelengths, when the period is increased (Fig. 1c). By tuning the gap and thickness of nanocrescents simultaneously causes that the complementary local and global maxima shift to larger wavelengths on the convex and concave patterns in C-orientation (Fig. 1d). In contrast, in U-orientation the spectra on either pattern only slightly modify (Fig. 1e). According to the relative orientation of nanocrescents in the miniarrays, the charge and near-field distributions indicate C/U resonances on the convex (concave) rectangular patterns in gamma=106° (16°)/16° (106°) azimuthal orientations (Fig. 1f, top (bottom)), moreover on the concave patterns localized and propagating plasmonic modes are also observable (Fig 1f, bottom). These results prove that precise plasmonic spectral engineering is realizable via patterns that can be fabricated exclusively via IICSM methodology.