Gaëtan Lévêque

Optical interactions in a plasmonic particle coupled to a metallic film.

The interplay between localized surface plasmon (LSP) and surface plasmon-polariton (SPP) is studied in detail in a system composed of a three-dimensional gold particle located at a short distance from a gold thin film. Important frequency shifts of the LSP associated with the particle are observed for spacing distances between 0 and 50 nm. Beyond this distance the LSP and SPP resonances overlap, although some cavity effects between the particle and the film can still be observed. In particular, when the spacing increases the field in the cavity decreases more slowly than one would expect from a simple image dipole interpretation. For short separations the coupling between the particle and the film can produce a dramatic enhancement of the electromagnetic field in the space between them, where the electric field intensity can reach 5000 times that of the illumination field. Several movies show the spectral and time evolutions of the field distribution in the system both in and out of resonance. The character of the different modes excited in the system is studied. They include dipolar and quadrupolar modes, the latter exhibiting essentially a magnetic response.

Tunable composite nanoparticle for plasmonics

We present a numerical study of the tunability properties of a plasmonic subwavelength particle deposited on a metallic slab. The particle is composed of a metallic part, supporting a localized plasmon mode, separated from the slab by a dielectric spacer. It is shown that the position of the resonance wavelength can be modified over a large spectral range by changing either the spacer thickness by a few tens of nanometers or its susceptibility within the range of usual dielectrics. A linear relation is observed between the resonance wavelength and the spacer permittivity.

Acousto-optic couplings in two-dimensional phoxonic crystal cavities

We investigate the acousto-optic coupling, based on both photo-elastic and opto-mechanical mechanisms, in periodic structures with simultaneous photonic and phononic band gaps. The investigations are focused on a cavity defect in which the strong confinement of acoustic and optic waves enhances the interaction. We calculate the modulation of each photonic mode frequency by each phononic mode confined in the cavity. We compare the strength for the photo-elastic and opto-mechanical effects in the different cases. Both mechanisms can be in phase or out of phase and produce additive or subtractive effects in the total acousto-optic coupling.

Transient behavior of surface plasmon polaritons scattered at a subwavelength groove

We present a numerical study and analytical model of the optical near field diffracted in the vicinity of subwavelength grooves milled in silver surfaces. The Green’s tensor approach permits the computation of the phase and amplitude dependence of the diffracted wave as a function of the groove geometry. It is shown that the field diffracted along the interface by the groove is equivalent to replacing the groove by an oscillating dipolar line source. An analytic expression is derived from the Green’s function formalism, which reproduces well the asymptotic surface plasmon polariton SPP wave as well as the transient surface wave in the near zone close to the groove. The agreement between this model and the full simulation is very good, showing that the transient “near-zone” regime does not depend on the precise shape of the groove. Finally, it is shown that a composite diffractive evanescent wave model that includes the asymptotic SPP can describe the wavelength evolution in this transient near zone. Such a semianalytical model may be useful for the design and optimization of more elaborate photonic circuits, whose behavior in a large part will be controlled by surface waves.

Optimization of finite diffraction gratings for the excitation of surface plasmons

The excitation of a surface plasmon polariton (SPP) wave on a metal-air interface by a diffraction grating under monochromatic normal illumination is investigated numerically. The influence of the different experimental parameters (grating thickness, period, and duty cycle) is discussed in detail for a semi-infinite metal and a thin film. Both engraved (grooves) and deposited (protrusions) gratings are considered. The most efficient coupling to the SPP is obtained for a groove grating which duty cycle is about 0.5. Furthermore a small grating depth of some tens of nanometers is sufficient to excite a SPP mode with a coupling efficiency higher than 16% in each direction. Implications for practical SPP experiments are discussed.

Near-field-induced tunability of surface plasmon polaritons in composite metallic nanostructures

We numerically study near‐field–induced coupling effects in metal nanowire‐based composite nanostructures. Our multi‐layer system is composed of individual gold nanowires supporting localized particle plasmons at optical wavelengths, and a spatially separated homogeneous silver slab supporting delocalized surface plasmons. We show that the localized plasmon modes of the composite structure, forming so‐called magnetic atoms, can be controlled over a large spectral range by changing the thickness of the nearby metal slab. The optical response of single‐wire and array‐based metallic structures are compared. Spectral shifts due to wire–mirror interaction as well as the coupling between localized and delocalized surface plasmon modes in a magnetic photonic crystal are demonstrated. The presented effects are important for the optimization of metal‐based nanodevices and may lead to the realization of metamaterials with novel plasmonic functionalities.

Theory of molecular excitation and relaxation near a plasmonic device

The new optical concepts currently developed in the research field of plasmonics can have significant practical applications for integrated optical device miniaturization as well as for molecular sensing applications. Particularly, these new devices can offer interesting opportunities for optical addressing of quantum systems. In this article, we develop a realistic model able to explore the various functionalities of a plasmon device connected to a single fluorescing molecule. We show that this theoretical method provides a useful framework to understand how quantum and plasmonic entities interact in a small area. Thus, the fluorescence signal evolution from excitation control to relaxation control depending on the incident light power is clearly observed.

Optical forces in coupled plasmonic nanosystems: Near field and far field interaction regimes

We study the forces generated by an electromagnetic field on two coupled gold nanowires at the vicinity of the plasmon resonance wavelength. Two different regimes are observed, depending on the separation distance between the wires. In the near field coupling regime, both attractive and repulsive forces can be generated, depending on and the illumination wavelength. Furthermore, at the plasmon resonance, it is possible to create forces 100 times larger than the radiation pressure. In the far field coupling regime, both particles are pushed by the incident field. However, the force amplitude applied on each wire is modulated as a function of , even for large separations. This indicates that the system behaves like a cavity and pseudo Fabry-Perot modes can be excited between the particles. The interaction of these modes with the plasmon resonances of the nanowires, determines the forces on the particles. Around the plasmon resonance wavelength, when the cavity is tuned to the incident light, forces are close to the average value corresponding to the radiation pressure of the incident field. On the other hand, when the cavity is detuned, the particles are retained or pushed anti-symmetrically. We finally study the forces applied on these nanowires in the centre of mass reference frame (CMRF) for the far field coupling regime. For any separation distance we observe equilibrium positions in the CMRF for at least one illumination wavelength. The stability of these equilibrium positions is discussed in detail.

Coupling between plasmonic films and nanostructures: from basics to applications

Abstract Plasmonic film-nanoparticles coupled systems have had a renewed interest for the past 5 years both for the richness of the provided plasmonic modes and for their high technological potential. Many groups started to investigate the optical properties of film-nanoparticles coupled systems, as to whether the spacer layer thickness is tens of nanometers thick or goes down to a few nanometers or angstroms, even reaching contact. This article reviews the recent breakthroughs in the physical understanding of such coupled systems and the different systems where nanoparticles on top of the spacer layer are either isolated/random or form regular arrays. The potential for applications, especially as perfect absorbers or transmitters is also put into evidence.

Plasmonic mode interferences and Fano resonances in Metal-Insulator-Metal nanostructured interface.

Metal-insulator-metal systems exhibit a rich underlying physics leading to a high degree of tunability of their spectral properties. We performed a systematic study on a metal-insulator-nanostructured metal system with a thin 6 nm dielectric spacer and showed how the nanoparticle sizes and excitation conditions lead to the tunability and coupling/decoupling of localized and delocalized plasmonic modes. We also experimentally evidenced a tunable Fano resonance in a broad spectral window 600 to 800 nm resulting from the interference of gap modes with white light broad band transmitted waves at the interface playing the role of the continuum. By varying the incident illumination angle shifts in the resonances give the possibility to couple or decouple the localized and delocalized modes and to induce a strong change of the asymmetric Fano profile. All these results were confirmed with a crossed comparison between experimental and theoretical measurements, confirming the nature of different modes. The high degree of control and tunability of this plasmonically rich system paves the way for designing and engineering of similar systems with numerous applications. In particular, sensing measurements were performed and a figure of merit of 3.8 was recorded ranking this sensor among the highest sensitive in this wavelength range.