Yannick Sonnefraud

Symmetry Breaking in Plasmonic Nanocavities : Subradiant LSPR Sensing and a Tunable Fano Resonance

A metallic nanostructure consisting of a disk inside a thin ring supports superradiant and very narrow subradiant modes. Symmetry breaking in this structure enables a coupling between plasmon modes of differing multipolar order, resulting in a tunable Fano resonance. The LSPR sensitivities of the subradiant and Fano resonances are predicted to be among the largest yet for individual nanostructures.

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.

Tunability of Subradiant Dipolar and Fano-Type Plasmon Resonances in Metallic Ring/Disk Cavities: Implications for Nanoscale Optical Sensing

Plasmonic nanocavities consisting of the concentric arrangement of a disk and a ring sustain both subradiant and superradiant dipolar plasmon modes with large associated field enhancements and high refractive index sensitivities. In structures with broken symmetry, additionally a highly tunable Fano interference feature appears, which can be explained with a simple analytical harmonic oscillator model. The spectral tunability of these resonances from the visible to the mid-infrared is investigated, highlighting a potential for applications in surface enhanced spectroscopies.

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.

Plasmonic light-harvesting devices over the whole visible spectrum.

On the basis of conformal transformation, a general strategy is proposed to design plasmonic nanostructures capable of an efficient harvesting of light over a broadband spectrum. The surface plasmon modes propagate toward the singularity of these structures where the group velocity vanishes and energy accumulates. A considerable field enhancement and confinement is thus expected. Radiation losses are also investigated when the structure dimension becomes comparable to the wavelength.

Controlling Light Localization and Light-Matter Interactions with Nanoplasmonics

Nanoplasmonics is the emerging research field that studies light-matter interactions mediated by resonant excitations of surface plasmons in metallic nanostructures. It allows the manipulation of the flow of light and its interaction with matter at the nanoscale (10(-9) m). One of the most promising characteristics of plasmonic resonances is that they occur at frequencies corresponding to typical electronic excitations in matter. This leads to the appearance of strong interactions between localized surface plasmons and light emitters (such as molecules, dyes, or quantum dots) placed in the vicinity of metals. Recent advances in nanofabrication and the development of novel concepts in theoretical nanophotonics have opened the way to the design of structures aimed to reduce the lifetime and enhance the decay rate and quantum efficiency of available emitters. In this article, some of the most relevant experimental and theoretical achievements accomplished over the last several years are presented and analyzed.

Role of Defects in the Phase Transition of VO2 Nanoparticles Probed by Plasmon Resonance Spectroscopy

Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO(2) has remained elusive. By combining plasmon resonance nanospectroscopy with density functional calculations, we correlate decreased phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically defined VO(2) nanoparticles, we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures.

Revealing Plasmonic Gap Modes in Particle-on-Film Systems Using Dark-Field Spectroscopy

Polarization-controlled excitation of plasmonic modes in nanometric Au particle-on-film gaps is investigated experimentally using single-particle dark-field spectroscopy. Two distinct geometries are explored: nanospheres on top of and inserted in a thin gold film. Numerical simulations reveal that the three resonances arising in the scattering spectra measured for particles on top of a film originate from highly confined gap modes at the interface. These modes feature different azimuthal characteristics, which are consistent with recent theoretical transformation optics studies. On the other hand, the scattering maxima of embedded particles are linked to dipolar modes having different orientations and damping rates. Finally, the radiation properties of the particle-film gap modes are studied through the mapping of the scattered power within different solid angle ranges.

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.

Near-field optical microscopy with a nanodiamond-based single-photon tip.

We introduce a point-like scanning single-photon source that operates at room temperature and offers an exceptional photostability (no blinking, no bleaching). This is obtained by grafting in a controlled way a diamond nanocrystal (size around 20 nm) with single nitrogen-vacancy color-center occupancy at the apex of an optical probe. As an application, we image metallic nanostructures in the near-field, thereby achieving a near-field scanning single-photon microscopy working at room temperature on the long term. Our work may be of importance to various emerging fields of nanoscience where an accurate positioning of a quantum emitter is required such as for example quantum plasmonics.