Role of diffusive surface scattering in nonlocal plasmonics.

The recent generalised nonlocal optical response (GNOR) theory for plasmonics is analysed, and its main input parameter, namely the complex hydrodynamic convection-diffusion constant, is quantified in terms of enhanced Landau damping due to diffusive surface scattering of electrons at the surface of the metal. GNOR has been successful in describing plasmon damping effects, in addition to the frequency shifts originating from induced-charge screening, through a phenomenological electron diffusion term implemented into the traditional hydrodynamic Drude model of nonlocal plasmonics. Nevertheless, its microscopic derivation and justification is still missing. Here we discuss how the inclusion of a diffusion-like term in standard hydrodynamics can serve as an efficient vehicle to describe Landau damping without resorting to computationally demanding quantum-mechanical calculations, and establish a direct link between this term and the Feibelman d parameter for the centroid of charge. Our approach provides a recipe to connect the phenomenological fundamental GNOR parameter to a frequency-dependent microscopic surface-response function. We therefore tackle one of the principal limitations of the model, and further elucidate its range of validity and limitations, thus facilitating its proper application in the framework of nonclassical plasmonics.

Plasmonic response and SERS modulation in electrochemical applied potentials.

We study the optical response of individual nm-wide plasmonic nanocavities using a nanoparticle-on-mirror design utilised as an electrode in an electrochemical cell. In this geometry Au nanoparticles are separated from a bulk Au film by an ultrathin molecular spacer, giving intense and stable Raman amplification of 100 molecules. Modulation of the plasmonic spectra and the SERS response is observed with an applied voltage under a variety of electrolytes. Different scenarios are discussed to untangle the various mechanisms that can be involved in the electronic interaction between NPs and electrode surfaces.

Anisotropic enhancement of Yb3+ luminescence by disordered plasmonic networks self-assembled on RbTiOPO4 ferroelectric crystals.

Increasing Yb3+ absorption efficiency is currently desired in a number of applications including bio-imaging, photovoltaics, near infrared driven photocatalysis or ultra-short pulsed solid-state lasers. In this work, silver nanoparticles, which are connected forming disordered networks, have been self-assembled on Yb3+ doped RbTiOPO4 crystals to produce a remarkable enhancement of Yb3+ absorption, and hence in the photoluminescence of this ion. The results are interpreted taking into account the near-field response of the plasmonic networks, which display strong amplification of the electric field at the maximum of Yb3+ excitation at around 900 nm, together with the anisotropic character of the Yb3+ transitions in RbTiOPO4. We show that in the near field regime, the scattering of the plasmonic networks produces additional polarization field components to those of the incident field, which allows access to the largest transition dipolar moment of Yb3+ ions in RbTiOPO4. As a result, a much more efficient route for Yb3+ excitation takes place at the immediacy of the plasmonic networks. This work provides fundamental insights for improving the optical properties of rare earth ions by the suitable design of metallic nanoparticle arrangements, and constitutes a promising step towards the development of new multifunctional solid-state lasers.

Tracking Optical Welding through Groove Modes in Plasmonic Nanocavities.

We report the light-induced formation of conductive links across nanometer-wide insulating gaps. These are realized by incorporating spacers of molecules or 2D monolayers inside a gold plasmonic nanoparticle-on-mirror (NPoM) geometry. Laser irradiation of individual NPoMs controllably reshapes and tunes the plasmonic system, in some cases forming conductive bridges between particle and substrate, which shorts the nanometer-wide plasmonic gaps geometrically and electronically. Dark-field spectroscopy monitors the bridge formation in situ, revealing strong plasmonic mode mixing dominated by clear anticrossings. Finite difference time domain simulations confirm this spectral evolution, which gives insights into the metal filament formation. A simple analytic cavity model describes the observed plasmonic mode hybridization between tightly confined plasmonic cavity modes and a radiative antenna mode sustained in the NPoM. Our results show how optics can reveal the properties of electrical transport across well-defined metallic nanogaps to study and develop technologies such as resistive memory devices (memristors).

Diffractive chains of plasmonic nanolenses: combining near-field focusing and collective enhancement mechanisms.

We study, by means of full-electrodynamic calculations using the layer-multiple-scattering method, the effect of diffractive coupling on the enhancement of the local electromagnetic field in periodic arrays of nanolenses consisting of three silver spheres with progressively decreasing sizes and separations. The interaction between the hot-spot modes of an isolated nanolens with the Rayleigh-Wood anomalies of the periodic lattice leads to a further enhancement of the local field intensity, which can be controlled by an appropriate choice of the geometrical parameters involved.

Effective parameters for periodic photonic structures of resonant elements.

We report on the effective electromagnetic parameters of metamaterials consisting of resonant building units, through systematic full-electrodynamic calculations by the layer-multiple-scattering method on a model system: a photonic crystal of metallic nanoshells. The results obtained by the S-matrix retrieval procedure for single- and multi-layer slabs of ordered arrays of such nanoshells are analysed in conjunction with the complex band structure of the corresponding infinite crystal and the Maxwell-Garnett effective-medium approximation. We discuss conditions that must be fulfilled in order for an effective-medium description of a metamaterial to be valid and explain artefacts which often occur in numerical calculations of the effective parameters. In particular, we propose a method to resolve ambiguities in the determination of the effective refractive index, which become prominent for thick slabs, based on the complex band structure of the infinite crystal.