Robin Ogier

Ultrafast spinning of gold nanoparticles in water using circularly polarized light

Controlling the position and movement of small objects with light is an appealing way to manipulate delicate samples, such as living cells or nanoparticles. It is well-known that optical gradient and radiation pressure forces caused by a focused laser beam enables trapping and manipulation of objects with strength that is dependent on the particles optical properties. Furthermore, by utilizing transfer of photon spin angular momentum, it is also possible to set objects into rotational motion simply by targeting them with a beam of circularly polarized light. Here we show that this effect can set ∼200 nm radii gold particles trapped in water in 2D by a laser tweezers into rotation at frequencies that reach several kilohertz, much higher than any previously reported light driven rotation of a microscopic object. We derive a theory for the fluctuations in light scattering from a rotating particle, and we argue that the high rotation frequencies observed experimentally is the combined result of favorable optical particle properties and a low local viscosity due to substantial heating of the particles surface layer. The high rotation speed suggests possible applications in nanofluidics, optical sensing, and microtooling of soft matter.

Plasmon Enhanced Internal Photoemission in Antenna-Spacer-Mirror Based Au/TiO2 Nanostructures

Emission of photoexcited hot electrons from plasmonic metal nanostructures to semiconductors is key to a number of proposed nanophotonics technologies for solar harvesting, water splitting, photocatalysis, and a variety of optical sensing and photodetector applications. Favorable materials and catalytic properties make systems based on gold and TiO2 particularly interesting, but the internal photoemission efficiency for visible light is low because of the wide bandgap of the semiconductor. We investigated the incident photon-to-electron conversion efficiency of thin TiO2 films decorated with Au nanodisk antennas in an electrochemical circuit and found that incorporation of a Au mirror beneath the semiconductor amplified the photoresponse for light with wavelength λ = 500-950 nm by a factor 2-10 compared to identical structures lacking the mirror component. Classical electrodynamics simulations showed that the enhancement effect is caused by a favorable interplay between localized surface plasmon excitations and cavity modes that together amplify the light absorption in the Au/TiO2 interface. The experimentally determined internal quantum efficiency for hot electron transfer decreases monotonically with wavelength, similar to the probability for interband excitations with energy higher than the Schottky barrier obtained from a density functional theory band structure simulation of a thin Au/TiO2 slab.

Dimer-on-mirror SERS substrates with attogram sensitivity fabricated by colloidal lithography

Nanoplasmonic substrates with optimized field-enhancement properties are a key component in the continued development of surface-enhanced Raman scattering (SERS) molecular analysis but are challenging to produce inexpensively in large scale. We used a facile and cost-effective bottom-up technique, colloidal hole-mask lithography, to produce macroscopic dimer-on-mirror gold nanostructures. The optimized structures exhibit excellent SERS performance, as exemplified by detection of 2.5 and 50 attograms of BPE, a common SERS probe, using Raman microscopy and a simple handheld device, respectively. The corresponding Raman enhancement factor is of the order 10(11), which compares favourably to previously reported record performance values.

Continuous-Gradient Plasmonic Nanostructures Fabricated by Evaporation on a Partially Exposed Rotating Substrate.

A continuous-gradient approach of material evaporation is employed to fabricate nanostructures with varying geometric parameters, such as thickness, lateral positioning, and orientation on a single substrate. The method developed for mask lithography allows continuous tuning of the physical properties of a sample. The technique is highly valuable in simplifying the overall optimization process for constructing metasurfaces.

Fabrication of continuous gradient plasmonic nanostructures

Metallic nanostructures constitute one of the most important building blocks of contemporary nanoscience and nanotechnology because they support localized surface plasmon resonances (LSPRs) that dramatically enhance light-matter interactions. However, LSPRs are strongly dependent on nanostructure sizes, shapes, orientations and interparticle distances. The plasmonic properties of individual metal nanostructures therefore need to be finely tuned by optimizing their geometrical parameters. Conventional techniques, including both bottom-up wet-chemistry growth and top-down lithography, are not perfect and manufacturing uncertainties can result in considerable deviations from the desired optical behaviour. The preparation of specific structures therefore often requires sequential optimization processes that are repetitive, time consuming and costly.