Vincenzo Giannini

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.

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.

Electrodynamic calculations of spontaneous emission coupled to metal nanostructures of arbitrary shape : nanoantenna-enhanced fluorescence

We present a theoretical study of the spontaneous emission of an optical emitter close to a metal nanostructure of arbitrary shape. The modification of the corresponding radiative and nonradiative decay rates and resulting quantum efficiencies, expressed on the basis of a semiclassical dipole model in terms of the local plasmonic mode density, is calculated by means of the rigorous formulation of the Greens theorem surface integral equations. Metal losses and the intrinsic nonradiative decay rate of the molecules are properly considered, presenting relationships valid in general for arbitrary intrinsic quantum yields. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent molecule is observed when coupled to an optical dimer nanoantenna. Upon varying the dipole position, it is possible to obtain a predominant enhancement of radiative decay rates over the nonradiative counterpart, resulting in an increase of the internal quantum efficiency. For emitters positioned in the gap, quantum efficiency enhancements from an intrinsic value of 1% to ~75% are possible.

Surface plasmon polariton-mediated enhancement of the emission of dye molecules on metallic gratings

We present measurements of iso-frequency dispersion surfaces of light scattered from metallic micro-gratings with different periods. The dispersion surfaces, obtained using an infinity-corrected microscope objective, exhibit maxima attributed to diffracted orders as they become evanescent and pronounced minima due to the resonant excitation of surface plasmon polaritons (SPPs). We have also measured the enhanced photoluminescence of a thin layer of dye molecules on top of the grating. This enhanced luminescence is attributed to the local field enhancement close to the surface due to the coupling of the excitation frequency to SPPs. Moreover, we obtain a directional emission of the luminescence, which is attributed to the grating-assisted coupling of SPPs excited by the dye to free space light. The technique used for the measurements presented in this paper can be extended to characterize the angular emission patterns of emitters coupled to micro- and nano-plasmonic structures.

Long-range surface polaritons in ultra-thin films of silicon

We present an experimental and theoretical study of the optical excitation of long-range surface polaritons supported by thin layers of amorphous silicon (a-Si). The large imaginary part of the dielectric constant of a-Si at visible and ultraviolet (UV) frequencies allows the excitation of surface polariton modes similar to long-range surface plasmon polaritons on metals. Propagation of these modes along considerable distances is possible because the electric field is largely excluded from the absorbing thin film. We show that by decreasing the thickness of the Si layer these excitations can be extended up to UV frequencies, opening the possibility to surface polariton UV optics compatible with standard Si technology.

Electromagnetic model and calculations of the surface-enhanced Raman- shifted emission from Langmuir-Blodgett films on metal nanostructures

We present a theoretical study of the electromagnetic contribution to surface-enhanced Raman scattering (SERS) from a Langmuir-Blodgett film close to a metal surface. This macroscopic dipolar model fully accounts for the Raman-shifted emission so that meaningful SERS (electromagnetic) enhancement factors that do not depend only on the local electromagnetic field enhancement at the pump frequency are defined. For a plane metal surface, analytical SERS enhancement factors that are consistent for all pump beam polarization and molecular orientation are obtained. In order to investigate SERS on complex nanostructured metal surfaces, we introduce this model into the formally exact, Greens theorem surface integral equation formulation of the scattered electromagnetic field. This formulation is thus employed to calculate numerically the near-field and far-field emissions at the Raman-shifted frequency for very rough, random nanostructured surfaces, with emphasis on the impact of collective processes for varying pump frequency and Raman shift. Our results reveal that the widely used |E|4 approximation tends to overestimate average SERS enhancement factors.

Long-range guided THz radiation by thin layers of water

We propose a novel method to guide THz radiation with low losses along thin layers of water. This approach is based on the coupling of evanescent surface fields at the opposite sides of the thin water layer surrounded by a dielectric material, which leads to a maximum field amplitude at the interfaces and a reduction of the energy density inside the water film. In spite of the strong absorption of water in this frequency range, calculations show that the field distribution can lead to propagation lengths of several centimeters. By means of attenuated total reflection measurements we demonstrate the coupling of incident THz radiation to the long-range surface guided modes across a layer of water with a thickness of 24 μm. This first demonstration paves the way for THz sensing in aqueous environments.

Giant Terahertz field enhancement with plasmonic antennas

We theoretically investigate the excitation of localized surface plasmon resonances (LSPRs) in semiconductor antennas at Terahertz (THz) frequencies. Such resonances induce considerable intensity enhancement of the highly localized electric field. At THz frequencies semiconductors show a Drude-like behavior, similarly to metals at optical frequencies. In fact, in the THz regime, semiconductors with narrow band gap (such as InSb) or doped semiconductor (e.g., Si) have a dielectric constant with a negative real component and a relatively small imaginary component. This is actually the dielectric signature of good metals in the visible and near infrared regimes. Therefore, in analogy to optical plasmonic metal antennas [1], THz plasmonic antennas can be made of semiconductors [2].

Long range surface polaritons supported by thin absorbing layers

Surface polaritons are two-dimensional electromagnetic waves propagating at the interface between two media. The electromagnetic field of surface polaritons decays exponentially with distance from the interface. When a thin film is embedded in between two similar dielectrics, the two surface polaritons excited on the opposite interfaces of the thin film can couple. This coupling hybridizes the surface polaritons into two modes called long range surface polariton (LRSPs) and short range surface polariton (SRSPs). LRSPs and SRSPs have been thoroughly investigated in thin metallic films [1]. There is, however, little work on surface polaritons supported by thin films of absorbing materials [2].

Scattering of Electromagnetic Waves from Nanostructured, Self-Affine Fractal Surfaces: Near-Field Enhancements

Since the early days of fractality1, the scattering of electromagnetic (EM) waves from fractal surfaces has been a field of intense activity. Physical fractals appear ubiquitously in nature, possessing fractal properties within a broad, however finite, range of scales. Therefore the study of classical wave scattering from fractals is a problem of interest not only from a fundamental point of view, but also from the practical knowledge that probing technologies, such as surface optical characterization, remote sensing, radar, and sonar, can yield about a wide variety of systems. In fact, it is now well understood that many naturally occurring surfaces exhibit scale invariance, particularly in the form of self-affinity.1-4