Benjamin Gallinet

Influence of Electromagnetic Interactions on the Line Shape of Plasmonic Fano Resonances

The optical properties of plasmonic nanostructures supporting Fano resonances are investigated with an electromagnetic theory. Contrary to the original work of Fano, this theory includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn. These predictions are verified with surface integral numerical calculations in a broad variety of plasmonic nanostructures including dolmens, oligomers, and gratings. This work presents a robust and consistent analysis of plasmonic Fano resonances and enables the control of their line shape based on Maxwells equations. The insights into the physical understanding of Fano resonances gained this way will be of great interest for the design of plasmonic systems with specific spectral responses for applications such as sensing and optical metamaterials.

Mechanisms of Fano Resonances in Coupled Plasmonic Systems

Fano resonances in hybridized systems formed from the interaction of bright modes only are reported. Despite precedent works, we demonstrate theoretically and experimentally that Fano resonances can be obtained by destructive interference between two bright dipolar modes out of phase. A simple oscillator model is provided to predict and fit the far-field scattering. The predictions are verified with numerical calculations using a surface integral equation method for a wide range of geometrical parameters. The validity of the model is then further demonstrated with experimental dark-field scattering measurements on actual nanostructures in the visible range. A remarkable set of properties like crossings, avoided crossings, inversion of subradiant and superradiant modes and a plasmonic equivalent of a bound state in the continuum are presented. The nanostructure, that takes advantage of the combination of Fano resonance and nanogap effects, also shows high tunability and strong near-field enhancement. Our study provides a general understanding of Fano resonances as well as a simple tool for engineering their spectral features.

Ab initio theory of Fano resonances in plasmonic nanostructures and metamaterials

An ab initio theory for Fano resonances in plasmonic nanostructures and metamaterials is de- veloped using Feshbach formalism. It reveals the role played by the electromagnetic modes and material losses in the system, and enables the engineering of Fano resonances in arbitrary geome- tries. A general formula for the asymmetric resonance in a non-conservative system is derived. The influence of the electromagnetic interactions on the resonance line shape is discussed and it is shown that intrinsic losses drive the resonance contrast, while its width is mostly determined by the coupling strength between the non-radiative mode and the continuum. The analytical model is in perfect agreement with numerical simulations.

Plasmonic Radiance: Probing Structure at the Ångström Scale with Visible Light

Plasmonic modes with long radiative lifetimes combine strong nanoscale light confinement with a narrow spectral line width carrying the signature of Fano resonances, making them very promising for nanophotonic applications such as sensing, lasing, and switching. Their coupling to incident radiation, also known as radiance, determines their optical properties and optimal use in applications. In this work, we theoretically and experimentally demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime, the line shape exhibits remarkable sensitivity to the dielectric environment. We show that geometrical displacements and deformations at the Ångström scale can be detected optically by measuring the radiance. In the intermediate regime, the electromagnetic energy stored in the mode is maximal, with large electric field enhancements that can be exploited in surface enhanced spectroscopy applications. In the strong coupling regime, the interaction can result in hybridized modes with tunable energies.

Relation between near–field and far–field properties of plasmonic Fano resonances

The relation between the near-field and far-field properties of plasmonic nanostructures that exhibit Fano resonances is investigated in detail. We show that specific features visible in the asymmetric lineshape far-field response of such structures originate from particular polarization distributions in their near-field. In particular we extract the central frequency and width of plasmonic Fano resonances and show that they cannot be directly found from far-field spectra. We also address the effect of the modes coupling onto the frequency, width, asymmetry and modulation depth of the Fano resonance. The methodology described in this article should be useful to analyze and design a broad variety of Fano plasmonic systems with tailored near-field and far-field spectral properties.

Refractive Index Sensing with Subradiant Modes: A Framework To Reduce Losses in Plasmonic Nanostructures

Plasmonic modes with long radiative lifetimes, subradiant modes, combine strong confinement of the electromagnetic energy at the nanoscale with a steep spectral dispersion, which makes them promising for biochemical sensors or immunoassays. Subradiant modes have three decay channels: Ohmic losses, their extrinsic coupling to radiation, and possibly their intrinsic dipole moment. In this work, the performance of subradiant modes for refractive index sensing is studied with a general analytical and numerical approach. We introduce a model for the impact that has different decay channels of subradiant modes on the spectral resolution and contrast. It is shown analytically and verified numerically that there exists an optimal value of the mode coupling for which the spectral dispersion of the resonance line shape is maximal. The intrinsic width of subradiant modes determines the value of the dispersion maximum and depends on the penetration of the electric field in the metallic nanostructure. A figure of merit, given by the ratio of the sensitivity to the intrinsic width, which are both intrinsic properties of subradiant modes, is introduced. This figure of merit can be directly calculated from the line shape in the far-field optical spectrum and accounts for the fact that both the spectral resolution and contrast determine the limit of detection. An expression for the intrinsic width of a plasmonic mode is derived and calculated from the line shape parameters and using perturbation theory. The method of analysis introduced in this work is illustrated for dolmen and heptamer nanostructures. Fano-resonant systems have the potential to act as very efficient refractive index sensing platforms compared to Lorentz-resonant systems, due to control of their radiative losses. This study paves the way toward sensitive nanoscale biochemical sensors and immunoassays with a low limit of detection and, in general, any nano-optical device where Ohmic losses limit the performance.

Color Rendering Plasmonic Aluminum Substrates with Angular Symmetry Breaking

We fabricate and characterize large-area plasmonic substrates that feature asymmetric periodic nanostructures made of aluminum. Strong coupling between localized and propagating plasmon resonances leads to characteristic Fano line shapes with tunable spectral positions and widths. Distinctive colors spanning the entire visible spectrum are generated by tuning the system parameters, such as the period and the length of the aluminum structures. Moreover, the asymmetry of the aluminum structures gives rise to a strong symmetry broken color rendering effect, for which colors are observed only from one side of the surface normal. Using a combination of immersed laser interference lithography and nanoimprint lithography, our color rendering structures can be fabricated on areas many inches in size. We foresee applications in anticounterfeiting, photovoltaics, sensing, displays, and optical security.

Second-harmonic generation from periodic arrays of arbitrary shape plasmonic nanostructures: a surface integral approach

A surface integral formulation for the second-harmonic generation (SHG) from periodic metallic–dielectric nanostructures is described. This method requires the discretization of the scatterers’ surface in the unit cell only. All the physical quantities involved in this problem are derived in the unit cell by applying specific periodic boundary conditions both at the fundamental and the second-harmonic (SH) frequencies. Both the fundamental and the SH electric fields are computed using the method of moments and periodic Green’s function evaluated with the Ewald’s method. The accuracy of the method is carefully assessed using two specific cases, namely the surface plasmon enhancement of SHG from a gold film and the SHG from L-shaped nanoparticle arrays. These two examples emphasize the accuracy and versatility of the proposed method, which can be applied to a broad range of periodic metallic structures, including plasmonic arrays on arbitrary substrates and metamaterials.

Fano resonant plasmonic systems: Functioning principles and applications

Keywords: Nanophotonics ; Plasmonics Reference EPFL-CONF-187056 Record created on 2013-06-17, modified on 2017-05-10

Diffractive nanostructures for enhanced light-harvesting in organic photovoltaic devices.

We present in-coupling gratings for improving the performance of thin film organic solar cells. The impact of the grating on the absorption in the active layer is modeled and explained using a standard cell architecture. An increase in absorption of 14.8% is predicted and is shown to be independent from the active material. The structure is then applied on blade-coated devices and yields an efficiency improvement of 12%. The angular behavior of the structures is measured showing superior performance for two dimensional gratings. By simulating the current generation for different angles and illumination conditions, we predict a total yearly increase of the generated current of 12% using an optimized grating. The fabrication of these structures, moreover, is compatible with roll-to-roll production techniques, thus making them an optimal solution for printed photovoltaics.