New plasmonic materials in visible spectrum through electrical charging

Abstract

Due to their negative permittivity, plasmonic materials have found increasing number of applications in advanced photonic devices and metamaterials, ranging from visible wavelength through microwave spectrum. In terms of intrinsic loss and permittivity dispersion, however, limitations on available plasmonic materials remain a serious bottleneck preventing practical applications of a few novel nano-photonic device and metamaterial concepts in visible and nearinfrared spectra.To overcome this obstacle, efforts have been made and reported in literature to engineer new plasmonic materials exploring metal alloys, superconductors, graphene, and heavily doped oxide semiconductors. Though promising progress in heavily doped oxide semiconductors was shown in the near-infrared spectrum, there is still no clear path to engineer new plasmonic materials in the visible spectrum that can outperform existing choices noble metals, e.g. gold and silver, due to extremely high free electron density required for high frequency plasma response.This study demonstrates a path to engineer new plasmonic materials in the visible spectrum by significantly altering the electronic properties in existing noble metals through high density charging/discharging and its associated strong local bias effects. A density functional theory model revealed that the optical properties of thin gold films (up to 7 nm thick) can be altered significantly in the visible, in terms of both plasma frequency (up to 12%) and optical permittivity (more than 50%). These corresponding effects were observed in our experiments on surface plasmon resonance of a gold film electrically charged via a high density double layer capacitor induced by a chemically non-reacting electrolyte.

Anisotropic permittivity of ultra-thin crystalline Au films: Impacts on the plasmonic response of metasurfaces

“…In this study, we present plasmonic enabled frequency selective surfaces, i.e., optical filters, based on metasurface (FSS) structures that are constructed from ultra-thin gold (Au) films whose permittivities have been determined through density functional theory (DFT) simulations 3 and confirmed experimentally. 4 These ultra-thin Au films possess large anisotropies in their DFT-based permittivities which are not predicted by classical models. We contrast the resonance behaviors of metasurface (FSS) structures using the DFT-based permittivities, 3 a commonly used Drude model, the experimentally obtained Johnson-Christy (JC) values, 5 and a size-dependent permittivity (SDP) model based on the JC data which incorporates a size-dependent dampening term and inter-band transitions.…”