In-Plane Surface Lattice and Higher Order Resonances in Self-Assembled Plasmonic Monolayers: From Substrate-Supported to Free-Standing Thin Films.

Periodic arrays of plasmonic nanostructures are able to strongly confine light at the nanometer scale because of surface lattice resonances. These resonances are the result of electromagnetic coupling between single-particle localized surface plasmon resonances and Bragg resonances of the periodic lattice. Here, we investigate the effect of a finite size refractive index environment on the formation of surface lattice resonances by increasing the thickness of a polymer coating in nanometer-scale increments. Wet-chemically synthesized, spherical silver and gold nanoparticles with soft hydrogel shells are self-assembled into macroscopic, hexagonally ordered arrays on glass substrates using an interface-assisted approach. The resulting periodic plasmonic monolayers are subsequently coated by a polymer matching closely the refractive index of the glass support. The optical response of the plasmonic arrays is studied using far-field extinction spectroscopy and supported by numerical simulations. We show the formation of surface lattice resonances as well as higher order resonances in finite thickness polymer coatings. The resonance positions are determined by the interparticle spacing as well as the plasmonic material. Additionally, we demonstrate that a coating thickness of 450 nm is sufficient to support strong in-plane surface lattice resonances. This enables us to prepare macroscopic, free-standing polymer films with embedded plasmonic nanoparticle arrays, which feature strong surface lattice resonances.