Plasmon-induced nanoscale quantised conductance filaments.

Plasmon-induced phenomena have recently attracted considerable attention. At the same time, relatively little research has been conducted on electrochemistry mediated by plasmon excitations. Here we report plasmon-induced formation of nanoscale quantized conductance filaments within metal-insulator-metal heterostructures. Plasmon-enhanced electromagnetic fields in an array of gold nanodots provide a straightforward means of forming conductive CrOx bridges across a thin native chromium oxide barrier between the nanodots and an underlying metallic Cr layer. The existence of these nanoscale conducting filaments is verified by transmission electron microscopy and contact resistance measurements. Their conductance was interrogated optically, revealing quantised relative transmission of light through the heterostructures across a wavelength range of 1-12 µm. Such plasmon-induced electrochemical processes open up new possibilities for the development of scalable devices governed by light.

Polarized plasmonic enhancement by Au nanostructures probed through Raman scattering of suspended graphene.

We characterize plasmonic enhancement in a hotspot between two Au nanodisks using Raman scattering of graphene. Single layer graphene is suspended across the dimer cavity and provides an ideal two-dimensional test material for the local near-field distribution. We detect a Raman enhancement of the order of 10(3) originating from the cavity. Spatially resolved Raman measurements reveal a near-field localization one order of magnitude smaller than the wavelength of the excitation, which can be turned off by rotating the polarization of the excitation. The suspended graphene is under tensile strain. The resulting phonon mode softening allows for a clear identification of the enhanced signal compared to unperturbed graphene.

Electron tunneling through ultrathin boron nitride crystalline barriers.

We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.

Surface-enhanced Raman spectroscopy of graphene.

Surface-enhanced Raman scattering (SERS) exploits surface plasmons induced by the incident field in metallic nanostructures to significantly increase the Raman intensity. Graphene provides the ideal prototype two-dimensional (2d) test material to investigate SERS. Its Raman spectrum is well-known, graphene samples are entirely reproducible, height controllable down to the atomic scale, and can be made virtually defect-free. We report SERS from graphene, by depositing arrays of Au particles of well-defined dimensions on a graphene/SiO(2) (300 nm)/Si system. We detect significant enhancements at 633 nm. To elucidate the physics of SERS, we develop a quantitative analytical and numerical theory. The 2d nature of graphene allows for a closed-form description of the Raman enhancement, in agreement with experiments. We show that this scales with the nanoparticle cross section, the fourth power of the Mie enhancement, and is inversely proportional to the tenth power of the separation between graphene and the center of the nanoparticle. One important consequence is that metallic nanodisks are an ideal embodiment for SERS in 2d.

Plasmonic resonances in optomagnetic metamaterials based on double dot arrays.

We study optical properties of optomagnetic metamaterials produced by regular arrays of double gold dots (nanopillars). Using combined data of spectroscopic ellipsometry, transmission and reflection measurements, we identify localized plasmon resonances of a nanopillar pair and measure their dependences on dot sizes. We formulate the necessary condition at which an effective field theory can be applied to describe optical properties of a composite medium and employ interferometry to measure phase shifts for our samples. A negative phase shift for transmitted green light coupled to an antisymmetric magnetic mode of a double-dot array is observed.

Multiple path length dual polarization interferometry.

An optical sensor for quantitative analysis of ultrathin films and adsorbed layers is described. Quantification of both layer thickness and refractive index (density) can be made for in situ and ex-situ coated films. With the use of two polarizations, in situ measurements are made via one path length in a young's interferometer arrangement while ex-situ measurements use multiple path lengths. The multiple path length young's interferometer arrangement is embodied in a solid state waveguide configuration called the multiple path length dual polarization interferometer (MPL-DPI). The technique is demonstrated with ultrathin layers of poly(methylmethacrylate) and human serum albumin.

Graphene-based liquid crystal device.

Graphene is only one atom thick, optically transparent, chemically inert, and an excellent conductor. These properties seem to make this material an excellent candidate for applications in various photonic devices that require conducting but transparent thin films. In this letter, we demonstrate liquid crystal devices with electrodes made of graphene that show excellent performance with a high contrast ratio. We also discuss the advantages of graphene compared to conventionally used metal oxides in terms of low resistivity, high transparency and chemical stability.