José A. Sánchez-Gil

Light scattering from random rough dielectric surfaces

A theoretical and numerical study is made of the scattering of light and other electromagnetic waves from rough surfaces separating vacuum from a dielectric. The extinction theorem, both above and below the surface, is used to obtain the boundary values of the field and its normal derivative. Then we calculate the angular distribution of the ensemble average of intensity of the reflected and transmitted fields. The scattering equations are solved numerically by generating one-dimensional surface profiles through a Monte Carlo method. The effect of roughness σ and correlation distance T on the aforementioned angular distribution, as well as on the reflectance, is analyzed. Enhanced backscattering and new transmission effects are observed, also depending on the permittivity. The ratio σ/T is large in all cases studied, and thus no analytical approximation, such as the Kirchhoff approximation (KA) and small perturbation methods, could a priori be expected to hold. We find, however, that the range of validity of the KA can be much broader than that previously found in perfect conductors.

Optical scattering resonances of single and coupled dimer plasmonic nanoantennas

The optical resonances of individual plasmonic dimer antennas are investigated using confocal darkfield spectroscopy. Experiments on an array of antennas with varying arm lengths and interparticle gap sizes show large spectral shifts of the plasmon modes due to a combination of geometrical resonances and plasmon hybridization. The resonances of the coupled-dimer antennas are considerably broadened compared to those of single nanorods, which is attributed to a superradiant damping of the coupled antenna modes. The scattering spectra are compared with electrodynamic model calculations that demonstrate both the near-field and far-field characteristics of a half-wave antenna.

Low-frequency active surface plasmon optics on semiconductors

A major challenge in the development of surface plasmon optics or plasmonics is the active control of the propagation of surface plasmon polaritons (SPPs). Here, we demonstrate the feasibility of low-frequency active plasmonics using semiconductors. We show experimentally that the Bragg scattering of terahertz SPPs on a semiconductor grating can be modified by thermal excitation of free carriers. The transmission of SPPs through the grating at certain frequencies can be switched completely by changing the temperature less than 100°C. This semiconductor switch provides a basis for the development of low-frequency surface-plasmon optical devices.

Nanowire Antenna Emission

We experimentally demonstrate the directional emission of polarized light from single semiconductor nanowires. The directionality of this emission has been directly determined with Fourier microphotoluminescence measurements of vertically oriented InP nanowires. Nanowires behave as efficient optical nanoantennas, with emission characteristics that are not only given by the material but also by their geometry and dimensions. By means of finite element simulations, we show that the radiated power can be enhanced for frequencies and diameters at which leaky modes in the structure are present. These leaky modes can be associated to Mie resonances in the cylindrical structure. The radiated power can be also inhibited at other frequencies or when the coupling of the emission to the resonances is not favored. We anticipate the relevance of these results for the development of nanowire photon sources with optimized efficiency and/or controlled emission by the geometry.

Gold nanostars as thermoplasmonic nanoparticles for optical heating

Gold nanostars are theoretically studied as efficient thermal heaters at their corresponding localized surface-plasmon resonances (LSPRs). Numerical calculations are performed through the 3D Greens Theorem method to obtain the absorption and scattering cross sections for Au nanoparticles with star-like shape of varying symmetry and tip number. Their unique thermoplasmonic properties, with regard to their (red-shifted) LSPR wavelentgh, (∼ 30-fold increase) steady-state temperature, and scattering/absorption cross section ratios, make them specially suitable for optical heating and in turn for cancer thermal therapy.

Scattering efficiency and near field enhancement of active semiconductor plasmonic antennas at terahertz frequencies

Terahertz plasmonic resonances in semiconductor (indium antimonide, InSb) dimer antennas are investigated theoretically. The antennas are formed by two rods separated by a small gap. We demonstrate that, with an appropriate choice of the shape and dimension of the semiconductor antennas, it is possible to obtain large electromagnetic field enhancement inside the gap. Unlike metallic antennas, the enhancement around the semiconductor plasmonics antenna can be easily adjusted by varying the concentration of free carriers, which can be achieved by optical or thermal excitation of carriers or electrical carrier injection. Such active plasmonic antennas are interesting structures for THz applications such as modulators and sensors.

Near-field and far-field scattering of surface plasmon polaritons by one-dimensional surface defects

A rigorous formulation for the scattering of surface plasmon polaritons (SPPs) from a one-dimensional surface defect of any shape that yields the electromagnetic field in the vacuum half-space above the vacuum-metal interface is developed by the use of an impedance boundary condition. The electric and magnetic near fields, the angular distribution of the far-field radiation into vacuum due to SPP-photon coupling, and the SPP reflection and transmission coefficients are calculated by numerically solving the k-space integral equation upon which the formulation is based. In particular, we consider Gaussian-shaped defects (either protuberances or indentations) and study the dependence of the above-mentioned physical quantities on their

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

1/e

Surface defect scattering of surface plasmon polaritons: Mirrors and light emitters

half-width a and height h. SPP reflection is significant for narrow defects

Enhanced and directional emission of semiconductor nanowires tailored through leaky/guided modes

(a\ensuremath{\lesssim}\ensuremath{\lambda}/5,