F. López-Tejeira

Efficient unidirectional nanoslit couplers for surface plasmons

The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components1,2. But in order that such potential can be realized, it is vital that the relatively poor light–SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3,4,5,6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system’s size. Back-side illumination of subwavelength apertures in optically thick metal films7,8,9,10,11,12,13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.

Synthesis and photonic bandgap characterization of polymer inverse opals

Monodisperse silica colloids with diameters ranging from 200–500 nm aresynthesized following the Stober–Fink–Bohn method [47]. The as-synthesizedsilica sols are purified and redispersed in 200 proof ethanol by at least six centri-fugation/redispersion cycles. The methods described in our previous paper [36]are used to fabricate three-dimensionally ordered planar colloidal crystals withthickness ranging from one monolayer to 50 monolayers. In short, a glass slideis immersed vertically into ~15 mL purified silica sol (1% particle volume frac-tion) contained in a glass scintillation vial. After ethanol slowly evaporates, aniridescent film is formed on top of the glass slide. A large area (1 cm fl 3 cm)sample can be made over 3–5 days. After each single coating is deposited, thefilm is taken out of the silica sol and air-dried for 10 min and then dipped againinto another purified silica sol with differing particle size. This coating–drying–coating cycle can be repeated many times and each time the particle size can bearbitrary selected. The thickness of each crystalline sub-unit can be easily tunedby changing the concentration of the silica sol [36]. In this way, a layered struc-ture with an arbitrary pattern of sphere sizes can be assembled. Macroporouspolystyrene films are made by templating the colloidal crystal as describedbefore [18].SEM is carried out on a Philips XL30 ESEM. A CrC-100 sputtering systemhas been used to coat a thin layer of gold on the samples before SEM analysis.To reveal an edge appropriate for cross-sectional SEM analysis, the samples arescraped using a sharp razor blade and tilted 30–40˚. Transmission spectra areobtained by using an Ocean Optics ST2000 fiber optic UV–near-IR spectrome-ter. An Oriel model 6000 UV lamp with 68806 basic power supply is used topolymerize styrene.Received: October 5, 2000

Metallo-dielectric core-shell nanospheres as building blocks for optical three-dimensional isotropic negative-index metamaterials

Materials showing electromagnetic properties that are not attainable in naturally occurring media, the so called metamaterials, have been lately, and still are, among the most active fields in optical and materials physics and engineering. Among those properties, one of the most attractive is the sub-diffraction resolving capability predicted for media having index of refraction of -1. Here we propose a fully 3D, isotropic metamaterial with strong electric and magnetic responses in the optical regime, based on spherical metallo-dielectric core-shell nanospheres. The magnetic response stems from the lowest, magnetic-dipole resonance of the dielectric shell with high refractive index, and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Since the response does not originate from coupling between structures, no particular periodic arrangement needs to be imposed. Moreover, due to the geometry of the constituents, the metamaterial is intrinsically isotropic and polarization independent. It could be realized with current fabrication techniques with materials such as Silver (core) and Silicon or Germanium (shell). For these particular realistic designs, the metamaterials present negative index in the range within 1.2-1.55 microns.Materials showing electromagnetic properties that are not attainable in naturally occurring media, so-called metamaterials, have been lately, and still are, among the most active topics in optical and materials physics and engineering. Among these properties, one of the most attractive ones is the subdiffraction resolving capability predicted for media having an index of refraction of −1. Here, we propose a fully three-dimensional, isotropic metamaterial with strong electric and magnetic responses in the optical regime, based on spherical metallo-dielectric core–shell nanospheres. The magnetic response stems from the lowest, magnetic-dipole resonance of the dielectric shell with a high refractive index, and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Since the response does not originate from coupling between structures, no particular periodic arrangement needs to be imposed. Moreover, due to the geometry of the constituents, the metamaterial is intrinsically isotropic and polarization independent. It could be realized with current fabrication techniques with materials such as silver (core) and silicon or germanium (shell). For these particular realistic designs, the metamaterials present a negative index in the range of 1.2–1.55μm. 3 Author to whom any correspondence should be addressed. New Journal of Physics 13 (2011) 123017 1367-2630/11/123017+15

Scattering of surface plasmons by one-dimensional periodic nanoindented surfaces

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Fano-like interference of plasmon resonances at a single rod-shaped nanoantenna

In this work, the scattering of surface plasmons by a finite periodic array of one-dimensional grooves is theoretically analyzed by means of a modal expansion technique. We have found that the geometrical parameters of the array can be properly tuned to achieve optimal performance of the structure either as a Bragg reflector or as a converter of surface plasmons into light. In this last case, the emitted light is collimated within a few degrees cone. Importantly, we also show that a small number of indentations in the array are sufficient to fully achieve its functional capabilities.

Experimental evidence of polarization dependence in the optical response of opal-based photonic crystals

Single metallic nanorods acting as half-wave antennas in the optical range exhibit an asymmetric, multi-resonant scattering spectrum that strongly depends on both their length and dielectric properties. Here we show that such spectral features can be easily understood in terms of Fano-like interference between adjacent plasmon resonances. On the basis of analytical and numerical results for different geometries, we demonstrate that Fano resonances may appear for such single-particle nanoantennas provided that interacting resonances overlap in both spatial and frequency domains.

Scattering of surface plasmon polaritons by one-dimensional inhomogeneities.

We report angle resolved reflectivity measurements showing the polarization dependence of the pseudogap in artificial opals. This kind of photonic crystals consist of silica spheres ordered in a face-centered-cubic lattice. The analyzed gap originates from the (111) family of planes. It is shown that the width of the Bragg peak observed in the reflectance spectra follows the bands determining the pseudogap, which are selectively excited according to the polarization (s or p) of the light impinging on the opal. Moreover, it is found that the coupling of light with the photonic bands occurs according to their predicted symmetry, which was assigned by arguments based on group theory.

High-performance nanosensors based on plasmonic Fano-like interference: probing refractive index with individual nanorice and nanobelts.

The scattering of surface plasmons polaritons by a one-dimensional defect of the surface is theoretically studied by means of both Rayleigh and modal expansions. The defects considered are either relief perturbations or variations in the permittivity of the metal. The dependence of transmission, reflection, and out-of-plane scattering on parameters defining the defect is presented. We find that the radiated energy is forwardly directed with respect to the surface plasmon propagation in the case of an impedance defect. However, for relief defects, the radiated energy may be directed into the backward or forward or both direction, depending on the defect width.

Mode parity-controlled fano- and lorentz-like line shapes arising in plasmonic nanorods

We propose two different configurations for which the Fano-like interference of longitudinal plasmon resonances occurring at individual metallic nanoparticles can be easily employed in refractive index sensing: a colloidal suspension of nanospheroids (nanorice) and a single nanowire with rectangular cross section (nanobelt) on top of a dielectric substrate. We numerically study the performance of the two in terms of their figures of merit, which are calculated under realistic conditions. For the case of nanorice, we explicitly incorporate the effect of size dispersity into the simulations. Our obtained results show that the application of the proposed configurations seems to be not only feasible but also very promising.

Symmetry characterization of eigenstates in opal-based photonic crystals

We present the experimental observation of spectral lines of distinctly different shapes in the optical extinction cross-section of metallic nanorod antennas under near-normal plane wave illumination. Surface plasmon resonances of odd mode parity present Fano interference in the scattering cross-section, resulting in asymmetric spectral lines. Contrarily, modes with even parity appear as symmetric Lorentzian lines. Finite element simulations are used to verify the experimental results. The emergence of either constructive or destructive mode interference is explained with a semianalytical 1D line current model. This simple model directly explains the mode-parity dependence of the Fano-like interference. Plasmonic nanorods are widely used as half-wave optical dipole antennas. Our findings offer a perspective and theoretical framework for operating these antennas at higher-order modes.