Enhanced Optical Transmission Mediated by Localized Plasmons in Anisotropic, Three-Dimensional Nanohole Arrays

Abstract

This paper describes 3D nanohole arrays whose high optical transmission is mediated more by localized surface plasmon (LSP) excitations than by surface plasmon polaritons (SPPs). First, LSPs on 3D hole arrays lead to optical transmission an order of magnitude higher than 2D planar hole arrays. Second, LSP-mediated transmission is broadband and more tunable than SPPenhanced transmission which is restricted by Bragg coupling. Third, for the first time, two types of surface plasmons can be selectively excited and manipulated on the same plasmonic substrate. This new plasmonic substrate fabricated by high-throughput nanolithography techniques paves the way for cutting-edge optoelectronic and biomedical applications.Keywords nanohole array; enhanced optical transmission; plasmonics; molding; surface plasmon polariton; localized surface plasmon Surface plasmon polaritons (SPPs) mediate light-matter interactions in subwavelength hole arrays and are the dominant mechanism for enhanced optical transmission,1 , 2 plasmonic focusing,3 and the plasmon Talbot effect.4 , 5 Most work to date has focused on manipulating SPPs by tuning the hole shape,6 , 7 the array symmetry,8 , 9 or the superlattice geometry.10 , 11 However, the tailoring of only the in-plane, two-dimensional (2D) structure has limited the excitation of strong localized surface plasmons (LSPs) within individual holes. Here we report that LSPs from three-dimensional (3D) nanoholes enhance transmission more than SPPs from the same nanohole array. Subwavelength hole arrays with anisotropic hole shapes were fabricated by metal deposition at oblique angles on nanopyramidal templates. Localized resonances supported by 3D holes produced broadband optical transmission with intensities an order of magnitude higher than that from 2D planar nanohole arrays. Moreover, in contrast to SPP-mediated enhanced transmission,12 , 13 LSPenhanced optical transmission at different wavelengths and with different dispersion properties can be tuned by controlling the shape of the 3D hole. The discovery of broadband-tunable LSP and SPP resonances on the same plasmonic substrate provides new opportunities in optoelectronics and optical biosensing.* To whom correspondence should be addressed. todom@northwestern.edu. Figure 1 summarizes how metal films perforated with 3D nanoholes were fabricated from templates of nanopyramidal pits followed by metal deposition at oblique angles. Protruding 3D nanohole arrays (tapered ends pointing away from the substrate) were fabricated by deposition of gold (Au) at specific angles (α) on a silicon (Si) template followed by template-stripping14 using UV-curable polyurethane (PU) as the transfer material (Fig. 1a). PU replicas of the Si template were formed by first molding poly(dimethylsiloxane) (PDMS) against the Si nanopatterns and then molding the patterned PDMS against PU (Fig. 1b). Recessed 3D hole arrays (tapered ends pointing toward the substrate) were generated by the angled deposition of Au directly on the PU template (Fig. 1b). For bot...

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