Theory and simulation in nanophotonics : non-locality in photonic nanostructures

Ce manuscrit s’intéresse principalement à l'influence de la répulsion entre électrons libres sur la réponse optique des métaux. Les modèles de matériaux classiques considèrent que la réponse d'un métal est locale -- c'est à dire que la réponse en un point dépend exclusivement des champs en ce point. La prise en compte de la répulsion entre électrons conduit à adopter une description dite non locale de la réponse métallique. Cette thèse explore de façon théorique et numérique les effets de la non-localité sur les propriétés optiques de nanostructures métallo-diélectriques dans le visible et le proche infra-rouge. A l'aide d'un modèle hydrodynamique il est montré que, de façon surprenante, les modes d'interstices plasmoniques peuvent être sensible à la non-localité pour des épaisseurs de plusieurs dizaines de nanomètres. Il est également montré que le plasmon de surface lui même peut être sensible à la non-localité à condition de considérer une interface entre le métal et un diélectrique d'indice suffisamment élevé. Nous proposons et étudions (théoriquement) ici plusieurs configurations simples et réalistes (coupleurs à prisme et à réseaux) pour la mise en évidence expérimentale de la non-localité sur des structures dont les échelles caractéristiques sont de l'ordre de plusieurs dizaines ou centaines de nanomètres. Enfin, dans une seconde partie du manuscrit, le formalisme et les considérations numériques nécessaires à l'étude du rayonnement d'un dipôle dans une structure multi-couche sont présentés en détail puis validés grâce à des comparaisons de dyadiques de Green, diagrammes de rayonnement, et taux d'émission avec des cas disponibles dans la littérature.This manuscript is mainly focused on the influence of repulsion between free electrons on the optical response of metals. Classical material models consider that the metallic response is local -- i.e. that the response at a given point only depends on the fields at this point. Taking into account the repulsion between electrons leads to a so-called non-local description of the metalic response. This thesis explores in a theoritical and numerical way the effects of non-locality on the optical properties of metallo-dielectric nanostructures in the visible and near infrared. Using a hydrodynamical model it is shown that, suprisingly, the modes of plasmonic gaps can be sensitive to non-locality for thicknesses of several tens of nanometers. It is also shown that the surface plasmon itself can be sensitive to non-locality provided that an interface between a metal and a sufficiently high refractive index dielectric is considered. We propose and study here several simple and realictic setups (prism and grating couplers) which would allow to experimentally observe the impact of non-locality and which have characteristic scales of tens or even hundreds of nanometers. Finally, in a second part of the manuscript, the formalism and numerical considerations necessary for the study of a dipole radiation in a multi-layered structure are presented in detail and then validated thanks to comparisons of Green dyadics, radiation diagrams, and emission rates with cases avaible in the literature

Théorie et simulation en nanophotonique : non-localité dans les nanostructures métalliques

This manuscript is mainly focused on the influence of repulsion between free electrons on the optical response of metals. Classical material models consider that the metallic response is local -- i.e. that the response at a given point only depends on the fields at this point. Taking into account the repulsion between electrons leads to a so-called non-local description of the metalic response. This thesis explores in a theoritical and numerical way the effects of non-locality on the optical properties of metallo-dielectric nanostructures in the visible and near infrared. Using a hydrodynamical model it is shown that, suprisingly, the modes of plasmonic gaps can be sensitive to non-locality for thicknesses of several tens of nanometers. It is also shown that the surface plasmon itself can be sensitive to non-locality provided that an interface between a metal and a sufficiently high refractive index dielectric is considered. We propose and study here several simple and realictic setups (prism and grating couplers) which would allow to experimentally observe the impact of non-locality and which have characteristic scales of tens or even hundreds of nanometers. Finally, in a second part of the manuscript, the formalism and numerical considerations necessary for the study of a dipole radiation in a multi-layered structure are presented in detail and then validated thanks to comparisons of Green dyadics, radiation diagrams, and emission rates with cases avaible in the literature.Ce manuscrit s’intéresse principalement à l'influence de la répulsion entre électrons libres sur la réponse optique des métaux. Les modèles de matériaux classiques considèrent que la réponse d'un métal est locale -- c'est à dire que la réponse en un point dépend exclusivement des champs en ce point. La prise en compte de la répulsion entre électrons conduit à adopter une description dite non locale de la réponse métallique. Cette thèse explore de façon théorique et numérique les effets de la non-localité sur les propriétés optiques de nanostructures métallo-diélectriques dans le visible et le proche infra-rouge. A l'aide d'un modèle hydrodynamique il est montré que, de façon surprenante, les modes d'interstices plasmoniques peuvent être sensible à la non-localité pour des épaisseurs de plusieurs dizaines de nanomètres. Il est également montré que le plasmon de surface lui même peut être sensible à la non-localité à condition de considérer une interface entre le métal et un diélectrique d'indice suffisamment élevé. Nous proposons et étudions (théoriquement) ici plusieurs configurations simples et réalistes (coupleurs à prisme et à réseaux) pour la mise en évidence expérimentale de la non-localité sur des structures dont les échelles caractéristiques sont de l'ordre de plusieurs dizaines ou centaines de nanomètres. Enfin, dans une seconde partie du manuscrit, le formalisme et les considérations numériques nécessaires à l'étude du rayonnement d'un dipôle dans une structure multi-couche sont présentés en détail puis validés grâce à des comparaisons de dyadiques de Green, diagrammes de rayonnement, et taux d'émission avec des cas disponibles dans la littérature

Specular reflection and transmission of electromagnetic waves by disordered metasurfaces

Nanophotonique des milieux complexes : nouveaux outils de modélisation vers de nouveaux phénomènes optiquesSurfaces nanostructurées complexes pour la conception de l'apparence visuell

Specular reflection and transmission of electromagnetic waves by disordered metasurfaces

Nanophotonique des milieux complexes : nouveaux outils de modélisation vers de nouveaux phénomènes optiquesSurfaces nanostructurées complexes pour la conception de l'apparence visuell

Plasmonic enhancement of spatial dispersion effects in prism coupler experiments

Recent experiments with film-coupled nanoparticles suggest that the impact of spatial dispersion is enhanced in plasmonic structures where high wavevector guided modes are excited. More advanced descriptions of the optical response of metals than Drude's are thus probably necessary in plasmonics. We show that even in classical prism coupler experiments, the plasmonic enhancement of spatial dispersion can be leveraged to make such experiments two orders of magnitude more sensitive. The realistic multilayered structures involved rely on layers that are thick enough to rule our any other phenomenon as the spill-out. Optical evanescent excitation of plasmonic waveguides using prism couplers thus constitutes an ideal platform to study spatial dispersion

The energy point of view in plasmonics

International audienceThe group velocity of a plasmonic guided mode can be written as the ratio of the flux of the Poynting to the integral of the energy density along the profile of the mode. This theorem, linking the way energy propagates in metals to the properties of guided modes and Bloch modes in a multilayer, provides a unique physical insight in plasmonics. It allows to better understand the link between the negative permittivity of metals and the wide diversity of exotic phenomenon that occur in plasmonics-like the slowing down of guided modes, the high wavevector and the negative refraction. Deeply subwavelength metallic structures give us an unprecedented control of visible light, allowing to focus, concentrate, absorb, scatter light very efficiently, or to even enhance the emission of light by fluorophores[1] at totally new levels. Metals actually present a very peculiar optical response that dielectrics are totally incapable of-which can be linked to the presence of a free electron gas, a plasma, inside even the tiniest metallic nanoparticles[2]. Plasmonic resonators are the smallest optical resonators possible and their resonances can always be linked to the excitation of some kind of plasmonic guided mode. There is thus a large diversity of these modes, from the well known surface plasmon[3] to long and short-range surface plasmons[4], gap-plasmons[5] or modes supported by hyperbolic metallo-dielectric multilayers[6]. Most of them present very high wavevectors which explains the reduced size of the plasmonic resonators[7, 8]: they are essentially cavities for guided modes with very small effective wavelength. One must finally underline that exotic phenomena like negative refraction may also occur in metallo-dielectric multilayers[9-12]. All these features lack a unified view that would enable to give a physical insight into the reasons why large wavevector guided modes and negative refraction are common in plasmon-ics and very exotic in dielectric structures-requiring the careful tailoring of photonic crystals, for instance[13, 14]. We think that considering the way that energy flows when such modes propagate provides this kind of insight. The average flux of the Poynting vector has actually been used in the context of metamaterial and negative index materials as a useful tool to predict in which direction a mode really propagates (i.e. the sign of its group velocity). Such a approach relies on a theorem showing that the energy velocity is equal to the group velocity for modes propagating in non-dispersive, dielectric media[15, 16]. This theorem has been mostly ignored because, except in a few cases like when a mode approaches a cutoff condition, the group velocity does not present any exotic behaviour. Assuming this link holds even in the case of plasmonic or metamaterial waveguides, it can prove very useful to determine the sign of the group velocity by simply using the profile of the guided modes at a given frequency without having to actually compute the dispersion relation[17-19]. However, the optical response of metals is linked to the presence of free electrons, that transport a part of the guided mode energy and whose kinetic energy can not be neglected. For a plane wave propagating in a plasma, provided the energy of the electrons is taken into account both in the energy flux and in the energy density, it has been shown by Bers[20] that the energy and flux velocity are the same. This underlines that Yariv and Yeh's theorem[15] can not be applied to plasmonic waveguides. Since metals are highly dispersive and can be considered as boxes containing a real plasma, it is even surprising that computing the average flux of the Poynting vector could be successful in predicting the sign of the group velocity. Here we show that it is actually possible to generalize Yariv and Yeh's theorem in the context of dispersive media, including metals. This means that there is no need to modify the expression of the energy flux and only to adapt the energy density expression, to make the theorem valid-despite what has been established for plasma[20]. Said otherwise, the group velocity of a guided mode in a plasmonic multilayer is equal to the energy velocity of the electromagnetic field alone, and the energy transported by the free electrons, while not negligible[20], can be completely ignored. We then show using several examples how considering the energy velocity can provide a unifying vision of the optical response of plasmonic multilayers. We consider a multilayered structure invariant in the x and y directions, and a guided mode, solution of Maxwell's equations presenting a e i(k x x−ω t) dependency in x and t. We will assume the mode is p-polarized, because nothing exotic occurs for the s polarization in metallo-dielectric structures. Maxwell'

Influence of spatial dispersion on surface plasmons, nanoparticles and grating couplers

International audienceRecent experiments have shown that spatial dispersion may have a conspicuous impact on the response of plasmonic structures. This suggests that in some cases the Drude model should be replaced by more advanced descriptions that take spatial dispersion into account, like the hydro-dynamic model. Here we show that nonlocality in the metallic response affects surface plasmons propagating at the interface between a metal and a dielectric with high permittivity. As a direct consequence, any nanoparticle with a radius larger than 20 nm can be expected to be sensitive to spatial dispersion whatever its size. The same behavior is expected for a simple metallic grating allowing the excitation of surface plasmons, just as in Woods famous experiment. Finally, we carefully set up a procedure to measure the signature of spatial dispersion precisely, leading the way for future experiments. Importantly, our work suggests that for any plasmonic structure in a high permittivity dielectric, nonlocality should be taken into account

Fabrication of complex metasurfaces to control the visual appearance

International audienc