Light-Induced Coalescence of Plasmonic Dimers and Clusters.

The properties of nanoplasmonic structures depend strongly on their geometry, creating the need for high-precision control and characterization. Here, by exploiting the low activation energy of gold atoms on nanoparticle surfaces, we show how laser irradiation reshapes nanoparticle dimers. Time-course dark-field microspectroscopy allows this process to be studied in detail for individual nanostructures. Three regimes are identified: facet growth, formation of a conductive bridge between particles, and bridge growth. Electromagnetic simulations confirm the growth dynamics and allow measurement of bridge diameter, found to be highly reproducible and also self-limiting. Correlations in spectral resonances for the initial and final states give insight into the energy barriers for bridge growth. Dark-field microscopy shows that coalescence of multiple gaps in nanoparticle clusters can be digitally triggered, with each gap closing after discrete increases in irradiation power. Such control is important for light-induced nanowire formation or trimming of electronic and optoelectronic devices.

Inhibiting Analyte Theft in Surface-Enhanced Raman Spectroscopy Substrates: Subnanomolar Quantitative Drug Detection.

Quantitative applications of surface-enhanced Raman spectroscopy (SERS) often rely on surface partition layers grafted to SERS substrates to collect and trap-solvated analytes that would not otherwise adsorb onto metals. Such binding layers drastically broaden the scope of analytes that can be probed. However, excess binding sites introduced by this partition layer also trap analytes outside the plasmonic "hotspots". We show that by eliminating these binding sites, limits of detection (LODs) can effectively be lowered by more than an order of magnitude. We highlight the effectiveness of this approach by demonstrating quantitative detection of controlled drugs down to subnanomolar concentrations in aqueous media. Such LODs are low enough to screen, for example, urine at clinically relevant levels. These findings provide unique insights into the binding behavior of analytes, which are essential when designing high-performance SERS substrates.

Strong-coupling of WSe2 in ultra-compact plasmonic nanocavities at room temperature.

Strong coupling of monolayer metal dichalcogenide semiconductors with light offers encouraging prospects for realistic exciton devices at room temperature. However, the nature of this coupling depends extremely sensitively on the optical confinement and the orientation of electronic dipoles and fields. Here, we show how plasmon strong coupling can be achieved in compact, robust, and easily assembled gold nano-gap resonators at room temperature. We prove that strong-coupling is impossible with monolayers due to the large exciton coherence size, but resolve clear anti-crossings for greater than 7 layer devices with Rabi splittings exceeding 135 meV. We show that such structures improve on prospects for nonlinear exciton functionalities by at least 104, while retaining quantum efficiencies above 50%, and demonstrate evidence for superlinear light emission.

Interrogating Nanojunctions Using Ultraconfined Acoustoplasmonic Coupling.

Single nanoparticles are shown to develop a localized acoustic resonance, the bouncing mode, when placed on a substrate. If both substrate and nanoparticle are noble metals, plasmonic coupling of the nanoparticle to its image charges in the film induces tight light confinement in the nanogap. This yields ultrastrong "acoustoplasmonic" coupling with a figure of merit 7 orders of magnitude higher than conventional acousto-optic modulators. The plasmons thus act as a local vibrational probe of the contact geometry. A simple analytical mechanical model is found to describe the bouncing mode in terms of the nanoscale structure, allowing transient pump-probe spectroscopy to directly measure the contact area for individual nanoparticles.

How Light Is Emitted by Plasmonic Metals.

The mechanism by which light is emitted from plasmonic metals such as gold and silver has been contentious, particularly at photon energies below direct interband transitions. Using nanoscale plasmonic cavities, blue-pumped light emission is found to directly track dark-field scattering on individual nanoconstructs. By exploiting slow atomic-scale restructuring of the nanocavity facets to spectrally tune the dominant gap plasmons, this correlation can be measured from 600 to 900 nm in gold, silver, and mixed constructs ranging from spherical to cube nanoparticles-on-mirror. We show that prompt electronic Raman scattering is responsible and confirm that "photoluminescence", which implies phase and energy relaxation, is not the right description. Our model suggests how to maximize light emission from metals.

Revealing Nanostructures through Plasmon Polarimetry.

Polarized optical dark-field spectroscopy is shown to be a versatile noninvasive probe of plasmonic structures that trap light to the nanoscale. Clear spectral polarization splittings are found to be directly related to the asymmetric morphology of nanocavities formed between faceted gold nanoparticles and an underlying gold substrate. Both experiment and simulation show the influence of geometry on the coupled system, with spectral shifts Δλ = 3 nm from single atoms. Analytical models allow us to identify the split resonances as transverse cavity modes, tightly confined to the nanogap. The direct correlation of resonance splitting with atomistic morphology allows mapping of subnanometre structures, which is crucial for progress in extreme nano-optics involving chemistry, nanophotonics, and quantum devices.

The dual annihilation of a surface plasmon and a photon by virtue of a three-wave mixing interaction.

The enhanced nonlinear interactions that are driven by surface-plasmon resonances have readily been exploited for the purpose of optical frequency conversion in metallic structures. As of yet, however, little attention has been payed to the exact particulate nature of the conversion process. We show evidence that a surface plasmon and photon can annihilate simultaneously to generate a photon having the sum frequency. The signature for this nonlinear interaction is revealed by probing the condition for momentum conservation using a two-beam k-space spectroscopic method that is applied to a gold film in the Kretschmann geometry. The inverse of the observed nonlinear interaction-an exotic form of parametric down-conversion-would act as a source of surface plasmons in the near-field that are quantum correlated with photons in the far-field.