Niels Verellen

Fano Resonances in Individual Coherent Plasmonic Nanocavities

We observe the appearance of Fano resonances in the optical response of plasmonic nanocavities due to the coherent coupling between their superradiant and subradiant plasmon modes. Two reduced-symmetry nanostructures probed via confocal spectroscopy, a dolmen-style slab arrangement and a ring/disk dimer, clearly exhibit the strong polarization and geometry dependence expected for this behavior at the individual nanostructure level, confirmed by full-field electrodynamic analysis of each structure. In each case, multiple Fano resonances occur as structure size is increased.

Plasmon Line Shaping Using Nanocrosses for High Sensitivity Localized Surface Plasmon Resonance Sensing

The detection of small changes in the wavelength position of localized surface plasmon resonances in metal nanostructures has been used successfully in applications such as label-free detection of biomarkers. Practical implementations, however, often suffer from the large spectral width of the plasmon resonances induced by large radiative damping in the metal nanocavities. By means of a tailored design and using a reproducible nanofabrication process, high quality planar gold plasmonic nanocavities are fabricated with strongly reduced radiative damping. Moreover, additional substrate etching results in a large enhancement of the sensing volume and a subsequent increase of the sensitivity. Coherent coupling of bright and dark plasmon modes in a nanocross and nanobar is used to generate high quality factor subradiant Fano resonances. Experimental sensitivities for these modes exceeding 1000 nm/RIU with a Figure of Merit reaching 5 are demonstrated in microfluidic ensemble spectroscopy.

Experimental Realization of Subradiant, Superradiant, and Fano Resonances in Ring/Disk Plasmonic Nanocavities

Subradiant and superradiant plasmon modes in concentric ring/disk nanocavities are experimentally observed. The subradiance is obtained through an overall reduction of the total dipole moment of the hybridized mode due to antisymmetric coupling of the dipole moments of the parent plasmons. Multiple Fano resonances appear within the superradiant continuum when structural symmetry is broken via a nanometric displacement of the disk, due to coupling with higher order ring modes. Both subradiant modes and Fano resonances exhibit substantial reductions in line width compared to the parent plasmon resonances, opening up possibilities in optical and near IR sensing via plasmon line shape design.

Unidirectional Side Scattering of Light by a Single-Element Nanoantenna

Unidirectional side scattering of light by a single-element plasmonic nanoantenna is demonstrated using full-field simulations and back focal plane measurements. We show that the phase and amplitude matching that occurs at the Fano interference between two localized surface plasmon modes in a V-shaped nanoparticle lies at the origin of this effect. A detailed analysis of the V-antenna modeled as a system of two coherent point-dipole sources elucidates the mechanisms that give rise to a tunable experimental directivity as large as 15 dB. The understanding of Fano-based directional scattering opens a way to develop new directional optical antennas for subwavelength color routing and self-referenced directional sensing. In addition, the directionality of these nanoantennas can increase the detection efficiency of fluorescence and surface enhanced Raman scattering.

Plasmonic Modes of Metallic Semishells in a Polymer Film

The symmetry-broken geometry and variation of metal composition of semishells induce new plasmonic properties. A system of separated metallic semishells embedded in a poly(dimethylsiloxane) polymer film provides an ideal platform to investigate the localized surface plasmon resonance modes of semishells. We demonstrate experimentally that silver, gold, copper, and aluminum semishells can offer distinct plasmonic responses due to the wide range of their material parameters. Numerical calculations combined with the plasmon hybridization theory render us a clear understanding and assignment of the plasmonic modes of the semishells.

Dark and bright localized surface plasmons in nanocrosses

A metallic nanocross geometry sustaining broad dipole and sharp higher order localized surface plasmon resonances is investigated. Spectral tunability is achieved by changing the cross arm length and the angle between the arms. The degree of rotational symmetry of the nanocross is varied by adding extra arms, changing the arm angle and shifting the arm intersection point. The particles symmetry is shown to have a crucial influence on the plasmon coupling to incident radiation. Pronounced dipole, quadrupole, octupole and Fano resonances are observed in individual cross structures. Furthermore, the nanocross geometry proves to be a useful building block for coherently coupled plasmonic dimers and trimers where the reduced symmetry results in hybridized subradiant and superradiant modes and multiple Fano interferences. Finite difference time domain calculations of absorption and scattering cross-sections as well as charge density profiles are used to reveal the nature of the different plasmon modes. Experimental spectra for the discussed geometries support the calculations.

Engineering the Input Impedance of Optical Nano Dipole Antennas: Materials, Geometry and Excitation Effect

An optical nano dipole antenna is analyzed by means of its input impedance as well as the matching properties of the antenna topology and material configuration. A comparison of this classical microwave driving method with plane wave excitation is accomplished, contrasting the resonances in the input impedance and optical cross sections for several setups, and analyzing the spectral response shape. It is found that for all structures analyzed, a simple linear expression can be defined characterizing the relation between total dipole length and resonant wavelength. The fact that this linear relationship remains valid for different excitation models, for most widely used antenna materials (Au, Ag, Cu, and Al) and even in the presence of substrates is important with respect to practical designs. To our knowledge, such an extensive study has not been performed before.

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

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.

Mapping magnetic near-field distributions of plasmonic nanoantennas

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

On the use of the method of moments in plasmonic applications

In the last 40 years the Method of Moments has been a cornerstone in the field of computational electromagnetics for antennas and other components in the microwave frequency range. In this paper, the applicability, advantages and disadvantages of this method are discussed in view of applying it at much higher frequencies, namely in the field of plasmonics.