Multiscale patterning of plasmonic metamaterials

Abstract

The interaction of light with surface plasmons—collective oscillations of free electrons—in metallic nanostructures has resulted in demonstrations of enhanced optical transmission¹, collimation of light through a subwavelength aperture², negative permeability and refraction at visible wavelengths^3,4, and second-harmonic generation from magnetic metamaterials⁵. The structures that display these plasmonic phenomena typically consist of ordered arrays of particles or holes with sizes of the order of 100 nm. However, surface plasmons can interact with each other over much longer distances, so the ability to organize nanoscale particles or holes over multiple length scales could lead to new plasmonic metamaterials with novel optical properties⁶. Here, we present a high-throughput nanofabrication technique—soft interference lithography—that combines the ability of interference lithography⁷ to produce wafer-scale nanopatterns with the versatility of soft lithography⁸, and use it to create such plasmonic metamaterials. Metal films perforated with quasi-infinite arrays of 100-nm holes were generated over areas greater than 10 cm², exhibiting sharp spectral features that changed in relative amplitude and shifted to longer wavelengths when exposed to increased refractive index environments. Moreover, gold nanohole arrays patterned into microscale patches exhibited strikingly different transmission properties; for instance, patches of nanoholes displayed narrow resonances (<14.5 nm full-width-at-half-maximum) that resulted in high refractive index sensitivities far exceeding those reported previously⁹. Soft interference lithography was also used to produce various infinite and finite-area arrays of nanoparticles, including patterns that contained optically distinct particles side by side and arrays that contained both metallic and dielectric materials.