Isabel Romero

Plasmons in nearly touching metallic nanoparticles: singular response in the limit of touching dimers.

The response of gold nanoparticle dimers is studied theoretically near and beyond the limit where the particles are touching. As the particles approach each other, a dominant dipole feature is observed that is pushed into the infrared due to interparticle coupling and that is associated with a large pileup of induced charge in the interparticle gap. The redshift becomes singular as the particle separation decreases. The response weakens for very small separation when the coupling across the interparticle gap becomes so strong that dipolar oscillations across the pair are inhibited. Lowerwavelength, higher-order modes show a similar separation dependence in nearly touching dimers. After touching, singular behavior is observed through the emergence of a new infrared absorption peak, also accompanied by huge charge pileup at the interparticle junction, if initial interparticle-contact is made at a single point. This new mode is distinctly different from the lowest mode of the separated dimer. When the junction is made by contact between flat surfaces, charge at the junction is neutralized and mode evolution is continuous through contact. The calculated singular response explains recent experiments on metallic nanoparticle dimers and is relevant in the design of nanoparticle-based sensors and plasmon circuits.

Plasmon tunability in metallodielectric metamaterials

The dielectric properties of metamaterials consisting of periodically arranged metallic nanoparticles of spherical shape are calculated by rigorously solving Maxwell’s equations. Effective dielectric functions are obtained by comparing the reflectivity of planar surfaces limiting these materials with Fresnel’s formulas for equivalent homogeneous media, showing mixing and splitting of individual-particle modes due to interparticle interaction. Detailed results for simple-cubic and fcc crystals of aluminum spheres in vacuum, silver spheres in vacuum, and silver spheres in a silicon matrix are presented. The filling fraction of the metal f is shown to determine the position of the plasmon modes of these metamaterials. Significant deviations are observed with respect to Maxwell-Garnett effective-medium theory for large f, and multiple plasmons are predicted to exist in contrast to Maxwell-Garnett theory.

Plasmon molecules in overlapping nanovoids

4 pp.-- PACS nrs.: 73.20.Mf, 78.67.-n, 84.40.Az.-- Pre-print versions available at: http://arxiv.org/abs/0711.2691.

Simulating electromagnetic response in coupled metallic nanoparticles for nanoscale optical microscopy and spectroscopy : nanorod-end effects

Collective oscillations of valence electrons in metallic materials determine their optical response. The energy and strength of these surface oscillations are a function of the shape, size and coupling of the nanoparticles. With the use of a boundary element method (BEM), we solve Maxwells equations to calculate light scattering and surface modes in nanorods that are commonly used as hosts and/or samples in different field-enhanced scanning-probe microscopies and spectroscopies. We calculate the near-field and far-field response of nanorods and show that different geometrical terminations of the rods give different optical response in the far field for short rod lengths. For longer lengths, the response of rods with different terminations becomes more similar. The near field features of the ends become most evident close to the rod structural features that define the end capping. We identify four regimes for the separation between nanorod pairs that provide different coupling between nanorods. We also show that the size dependence of the nanorod response is characterized by a rod radius that gives a minimum wavelength for the dipolar response. For thicker and thinner rods, the response redshifts.

Plasmons in coupled voids

Plasmons in structured metallic systems have attracted considerable attention over the last few years as promising candidates to realize plasmon guiding, plasmon amplification, and in general, optical components at the nanoscale. Examples of these systems are chains of nanoparticles, where <50 nm particles are coupled to guide their Mie-like plasmons and the small size of the particles is chosen to minimize radiative losses. Void cavities have been also investigated in the context of inverted opals. Here, we investigate the optical properties of systems formed by dielectric nanoinclusions in a metal. Radiative losses in this type of system are prevented by total confinement of the light inside the dielectric, from where it cannot propagate beyond the metal skin depth. This allows considering larger void sizes and achieving larger propagation distances. Our results are based upon solution of Maxwells equations using the boundary element method for systems consisting of two or more dielectric inclusions, including overlapping systems.