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Metamaterials are artificial composite media engineered to exhibit extraordinary properties of wave propagation resulting from non-conventional values of effective homogeneous optical parameters such as the electric permittivity and the magnetic permeability. The extreme properties generally result from the collective response of the constitutive structural elements which have to be of sub-wavelength dimensions to satisfy the requirement of optical homogeneity and which have to be highly polarizable to provide efficient optical functions. For visible light applications, sub-wavelength dimensions imply structuration at the nanoscale whereas high polarizability can be achieved by optical resonators such as plasmonic or Mie resonators. This review shows how the bottom-up approach based on nano-chemistry and the self-assembly methods of colloidal physical-chemistry can be used to produce nano-sized tunable magneto-electric resonators which can subsequently be assembled in bulk nanostructured meta-materials as well as in optically thin metasurfaces. We review some of the optical properties observed in visible light from the fabricated systems. Specific optical experiments and numerical simulations are of crucial importance for the design of the most efficient structures and the extraction of the effective optical parameters.

International audience; The generation of artificial optical magnetism (AOM) in engineered composites has been the major concept that led to the emergence of the field of metamaterials at the turn of the millennium. Indeed, the proven possibility to manipulate the magnetic permeability of materials at microwave frequencies induced a considerable excitement in the scientific community as it opened the way to the design of unprecedented tools and devices for the control of light propagation. Extensions to higher frequencies of IR and visible light were soon proposed and tested by downsizing the artificial structures. The fabrication of negative index materials, optical cloaks or hyper-lenses seemed within reach. Two decades later, and after considerable research efforts, the applications of the AOM are still scarce and the concept seems to face a number of fundamental physical limits. We review in this chapter the state-of-the-art of the bottom-up approach whereby nanochemistry and colloidal physics are used to engineer hybrid metastructures exhibiting AOM in visible light or near IR.

International audience; Within the frame of a simple, long-wavelength, quasi-static description, we present a theoretical characterization of the optical response of metal nanoparticles doped with active gain elements in a core-shell (metallic core within an active dielectric shell) and nano-shell (active dielectric core within a metallic shell) configurations. The common feature of these structures is that, adding gain to the system produces an increase of the quality of the plasmon resonance, which becomes sharper and sharper until a singular point, after which, the system switches from absorptive to emissive (nanolaser). We use this aforementioned simple model to develop a general method allowing to calculate both the expected singular plasmon frequency and the gain level needed to realize it, and to discuss the spectral deformation occurring before and after this singular point. Finally we propose a way to calculate if the singular behavior is reachable using realistic amounts of gain.

International audience; Hyperbolic metamaterials use the concept of controlling the propagative modes through the engineering of the dispersion relation, and are considered highly promising to reach different meta-properties. Here we propose a novel bottom-up fabrication technique for uniaxial anisotropic metamaterials presenting a strongly anisotropic dispersion relation in the visible wavelength range, using self-assembled nanostructured block copolymers hybridized with gold nanoparticles. The materials consist in periodic lamellar stacks of period 28 nm, of alternating layers of pure polymer (dielectric) and layers of composite of polymer loaded with a high density of 7 nm gold nanoparticles. The spectral variation of their anisotropic effective dielectric permittivity is determined by variable-angle spectroscopic ellipsometry using appropriate effective medium models, as a function of the density of plasmonic nanoparticles. For large gold loading and close to the plasmon resonance of the nanoparticles, the lamellar stack presents ordinary and extraordinary components of the dielectric function of opposite signs. We therefore demonstrate for the first time the possibility of using a self-assembly methodology for the fabrication of bulk hyperbolic metamaterial.

The plasmonic response of a metal nanoparticle in the presence of surrounding gain elements is studied, using a space and time-dependent model, which integrates a quantum formalism to describe the gain and a classical treatment for the metal. Our model fully takes into account the influence of the system geometry (nanosphere) and offers for the first time, the possibility to describe the temporal evolution of the fields and the coupling among the multipolar modes of the particle. We calculate the lasing threshold value for all multipoles of the spaser, and demonstrate that the dipolar one is lowest. The onset of the lasing instability, in the linear regime, is then studied both with and without external field forcing. We also study the behaviour of the system below the lasing threshold, with the external field, demonstrating the existence of an amplification regime where the nanoparticle’s plasmon is strongly enhanced as the threshold is approached. Finally, a qualitative discussion is provided on later, non-linear stages of the dynamics and the approach to the steady-state of the spaser; in particular, it is shown that, for the considered geometry, the spasing is necessarily multi-modal and multipolar modes are always activated.

International audience; Disordered gold nanocomposite films were obtained by introducing spherical gold nanoparticles into a polymer matrix. These films were structurally characterized using microscopy techniques, and then analyzed for their effective optical indices using spectroscopic ellipsometry. It is found that even for a low volume fraction of gold nanoparticles ....

We report a powerful and general concept for the design of three-dimensional metamaterials suitable for optically probed nanoscale sensing, combining the environmental sensitivity of localized surface plasmon resonances (LSPRs) and a low absorption of the probe light, even when the LSPRs are supported by lossy plasmonic elements. This concept is based on the tuning of the plasmon-induced Brewster conditions (angle of incidence and photon energy) of the material by suitable tailoring of its structure. This tuning is made possible by the interplay between LSPRs and optical interferences, i.e., the so-called plasmonic optical interferences. The plasmon-induced Brewster photon energy can be set outside the spectral range of the LSPR, thus in a spectral region of low optical absorption. Such off-resonant' plasmon-induced Brewster conditions are examplified in a self-assembled lamellar three-dimensional metamaterial consisting of stacked polymer layers doped with gold nanoparticles alternating with undoped polymer layers. At the off-resonant' plasmon-induced Brewster conditions, an abrupt jump of the ellipsometric phase angle δ occurs in the photon energy space. In the spectral vicinity of this jump, δ shows strong sensitivity to the spectral features of the LSPR, potentially allowing the detection of infinitesimal LSPR energy shifts (

Intent of the research was to develop metamaterials with large attenuation capacities in a targeted field of frequencies, reducing the size of the resonators from the centimeter-level used in audible frequencies to microresonators able to function at ultrasonic frequencies, focusing on two approaches. The core-shell approach aimed to construct objects composed of a hard core showing a high density contrast with its surrounding elastic soft shell, the result embedded in a hard matrix. The second ( resonant-emulsion ) approach aimed at fabricating a material in which the incoming wave exhibits a Mie resonance with the inclusions due to the sound velocity contrast between the inclusions and surrounding matrix. For efficiency we focused on the resonant-emulsion approach, fabricating a soft material composed of oil emulsion droplets suspended in a liquid water-based gel, allowing us to vary the average size and droplet volume fraction in the material. Applying a wave frequency to this material indeed showed variable attenuation at different wavelengths. Varying the droplet diameters allowed polydispersity in the material, reducing the individual scattering peaks and applying the attenuation coefficient to a broader range of frequencies. By doping the material with ferrofluid we hoped to specifically tune the local Mie resonance frequencies to set ultradamping at particular frequencies (for adaptive filters and acoustic mirrors or other applications), and research bifurcated into synthesis of fluorinated ferrofluid, and acoustic measurements of the obtained material. Once the ferrofluid was synthesized, application of the magnetic field elongated the droplets, shifting resonance peaks to higher frequencies, and this attenuation strongly changes from one geometry to another (whether droplets were elongated perpendicular or parallel to the arriving waves). Research now looks to synthesize and investigate materials for more intense scattering such as silicon polymers.

We present a dynamical model for a classically treated metal spherical inclusion in a gain assisted medium described using quantum formalism. The model integrates geometry via a proper set of boundary conditions and it is shown to be consistent with the statical formula for polarizability, which can be obtained in the low amplitude linear approximation. This dynamical approach is able to describe transient regimes, to study instabilities and to account the effects of a pulsed pump.

The optical response of a surface covered with an array of gold nanopillars is studied both experimentally, using spectroscopic ellipsometry, and numerically. When treating the gold pillars as an effective layer, we find that it is possible to extract anisotropic effective optical indices that fit the experimental data. However, a detailed numerical study of the behavior of the metamaterial under illumination shows the presence of localized, specific coupling effects with the underlying layer, suggesting that homogeneization, and effective indices should be considered with caution and have only limited applicability.

In this work, we study the electromagnetic properties of a metamaterial made by a cubic array of TiO2 microspheres embedded in a host medium at terahertz frequencies. By employing the method reported in [1] by Silveirinha, we are able to consider in the computation the effect of magnetoelectric coupling between TiO2 microspheres, as well as spatial dispersion. After calculation of the dominant mode inside the structure, we show that the effect of spatial dispersion on the effective parameters for this kind of systems is relatively weak. Accuracy of the results and effectiveness of homogenized parameters have been demonstrated by comparison against full-wave simulations for a 5-layer slab of such a metamaterial.

International audience; We present two routes of millifluidic-based synthesis of solid core-solid shell particles with submillimeter size. These particles, which behave as mechanical resonators, are key components of targeted three-dimensional locally resonant acoustic metamaterials working in the ultrasonic frequency range. The first route involves the encapsulation of a solid core into a liquid shell, which is then in-line solidified through gelation. The second route involves two successive coflows followed by two solidifications triggered by UV irradiation and complexation, respectively. Original solutions to several practical issues are presented. The advantages and drawbacks of both routes are analyzed in the light of the targeted specifications of the resonators.

6 pages; International audience; We present high refractive index optical metamaterials assembled via a microfluidic evaporation technique. This technique enables fabrication of truly three-dimensional bulk samples from a suspension of nanoparticles with a number of layers well in excess of 600, surpassing rival techniques by at least an order of magnitude. In addition to their large dimensions, the assembled metamaterials show a high degree of homogeneity and warrant an easy and rapid optical characterization using spectroscopic ellipsometry. We believe that the suggested inexpensive method considerably reduces the complexity in assembling optical metamaterials and opens new avenues in engineering bulk optical devices by choice of nanoparticle composition and geometry.

Our initial idea was to build metamaterials with specific properties such as large attenuation capacities in a targeted field of frequencies. If much work has been up to now performed using centimetre-sized handmade mechanical resonators adapted to audible frequencies [1-3], fewer attempts have been made on smaller micro-resonators, in the 100 m range, i.e. for ultrasonic frequencies. Following this path, we recently proposed two different microfluidic approaches to get such micro-resonators: 1. The core-shell approach 2. The resonant emulsion approach.

One issue in using metallic nanostructures for metamaterial applications at optical frequencies is their high level of losses. A most promising strategy to circumvent this obstacle is loss compensation, where the structures are coupled to active compounds enabled to transfer energy and therefore amplify the desired response. We here present the first unified theory of the response of plasmonic nanoparticles assisted by optical gain media, in the case of a nanoparticle coated with a shell of optically active dipoles (fluorescent molecules or dyes). The mechanism of the losses compensation is based on nonradiative energy transfer (ET) or quenching between the layer of gain elements and nanoparticle [1, 2]. We establish a complete description of the optical response of the system based on Green’s functions, which allows us to investigate high molecular coverage of nanoparticle with either regular or random distribution of dye molecules, taking into account not only the interactions between NP (treated in a multipolar approach) and dye dipoles, but also between dyes molecules, either directly or via the nanoparticle [3–5]. We then obtain the optical response of the core-shell aggregate in terms of its equivalent polarizability composed of the direct response from the nanoparticle and the contribution rising from the energy transfer mechanism. Our numerical calculations reveals that cooperative plasmon-mediated coupling between optically active dyes and metal nanostructure leads to the compensation of plasmon losses and some instability that is resolved either by surface plasmon amplification of stimulated emission (spasing states) or by enhanced absorption in the system.

7 pages; International audience; We experimentally demonstrate that gain materials properly encapsulated into the shell surrounding metal nanoparticles (NPs) are responsible for the modification of the overall plasmon response of engineered nanostructures. A comparison between designed systems based on functionalized core-shell NPs having different encapsulated dye molecules is presented. Experimental observations of Rayleigh scattering enhancement, accompanied by an increase of transmission as a function of gain, reveal striking optical loss compensation effects. Fluorescence lifetime measurements demonstrate a quenching of dye photoluminescence in functionalized core-shell NP samples with respect to pure dye solutions, confirming the strong resonant coupling occurring between the gain medium and gold NPs. Experimental evidence of a selective modification of the gain functionalized core-shell Au NP extinction curve is found, in good agreement with the results of a simplified theoretical model. The model verifies the causality principle through Kramers-Kronig dispersion relations for the investigated gain functionalized plasmonic nanostructure.

We characterize the modes with real and complex wavenumbers for both longitudinal and transverse polarization states (with respect to the mode traveling direction) in three dimensional (3D) periodic arrays of titanium dioxide (TiO2) microspheres in the frequency range between 250 GHz and 350 GHz. Modal results are computed by using a single magnetic dipole approximation (SDA) and an SDA model with correction (SDA-WC) that assumes the array to be embedded in a host with an effective permittivity computed through Maxwell Garnett formulas. Moreover, for the transverse polarization case, modal wavenumbers are computed also by fitting the full-wave simulation magnetic field (one point per unit cell) in a finite thickness structure, and their agreement and disagreement are discussed. The longitudinal polarization is not affected by the artificial correction introduced in the SDA-WC; in the transverse polarization case, instead, the correction is needed to obtain results in better agreement with the full-wave data fit. In the observed frequency range, there are one longitudinal mode and two transverse modes, one forward and one backward, where the forward one is "dominant" (i.e., it contributes mostly to the field in the array). Therefore, in the case of transverse polarization, we describe the composite material in terms of homogenized refractive index and relative permeability, comparing results from (i) modal analysis (with and without correction), (ii) Maxwell Garnett formulas, and (iii) Nicolson-Ross-Weir retrieval method from scattering parameters of finite thickness structures. The agreement among the different methods justifies the performed homogenization procedure in the case of transverse polarization. We show that artificial magnetism is generated from a nonmagnetic composite material. © 2012 Optical Society of America.

5 pages; International audience; We study theoretically the polarizability of a single metallic nanoparticle immersed into an externally pumped, active gain medium able to couple to the plasmon resonance. Within the frame of a simple long-wavelength, macroscopic description, and under steady-state conditions, we show that localized plasmons can be strongly amplified, until becoming singular for a specific amount of surrounding gain; however, we find that such gain-assisted singular plasmons exhibit spectrally spread imaginary responses and are therefore intrinsically different from singular plasmons in idealized, lossless metals. More generally, we carry a systematic study of how the plasmonic response transforms under changes in the amount of gain, and show that the coupled particle and activemedium act as a self-tuned Fano resonant system. The resulting plasmons exhibit strongly distorted line shapes with unusual but interesting features. One particularly attractive situation is that of "conjugate plasmons," which, at resonance, display a strong real response in association with minimal losses. These findings could have some applications in plasmonics, nanoantennas, nanosensing, and optical metamaterials.

International audience

We present the first unified theory, to the best of our knowledge, of the response of a plasmonic nanosphere (NS) assisted by optical gain media, in the case of a NS coated with a layer of optically active dipolar dyes. We obtain the optical coherent response of the core–shell aggregate in terms of its equivalent polarizability composed of the direct response from the NS and the contribution arising from the cooperative coupling between dyes and surface plasmons of the NS. We identify a mechanism of superradiance-like plasmonic aggregate cooperative emission similar to the conventional Dicke effect with reduced intraband relaxation bandwidth due to the loss compensation in the system. The analysis of the aggregate resonances based on the system eigenvalues provides physical insight into the total loss compensation mechanism and resonance frequency shifts.

We employ the generalized Lorentz-Lorenz method to investigate how both magnetoelectric coupling and spatial dispersion influence the artificial magnetic capabilities at terahertz frequencies of the representative case of a metamaterial consisting of a three-dimensional (3D) lattice of TiO2 microspheres. The complex wavenumber dispersion relations pertaining to modes supported by the array, traveling along one of the principal axes of the array with electric or magnetic field polarized transversely and longitudinally (with respect to the mode traveling direction), are studied and thoroughly characterized. One mode with transverse polarization is dominant at any given frequency for the analyzed dimensions, proving that the 3D lattice can be treated as a homogeneous medium with defined electromagnetic material parameters. We show, however, that bianisotropy is a direct consequence of magnetoelectric coupling, and the dyadic expressions of both effective and equivalent material parameters are derived. In particular, we analyze the effect of spatial dispersion on the effective parameters relative to a composite material made by a 3D lattice of TiO2 microspheres with filling fraction around 30% and near the first Mie magnetic dipolar resonance. Finally, we homogenize the metamaterial in terms of equivalent index and impedance, and by comparison with full-wave simulations, we explain the presence of the unphysical antiresonance permittivity behavior observed in previous work. © 2014 Optical Society of America.

Chemical and self-assembly approaches are increasingly used for the fabrication of nanostructured materials with controlled optical properties.We have considered the simplest case of composite films comprised of a non-structured polymer with spherical gold nanoparticle inclusions, with varied thickness and filling fractions. Optical properties have been studied by means of spectroscopic ellipsometry in the ultraviolet-visible-near infrared range. For thick, three-dimensional films (30-200 nm) with filling fractions below 6%, the behavior of the optical index as a function of the wavelength is well described using a modified Maxwell-Garnett mixing law modified with anisotropic polarisabilities, which accounts for electromagnetic couplings between neighboring particles. Very dense samples with filling fractions up to 30% have also been fabricated, and exhibit near-zero or negative electric permittivities in a finite frequency range. For very thin films (of thickness close to the nanoparticle size), we have shown that the complex optical index is not an intrinsic descriptor of the system and that ellipsometric analysis cannot determine a sample thickness unambiguously. However, the analysis gives access to the particles extinction cross-section : we find that when the particles lie very close to the silicon substrate, experimental values are enhanced compared to theoretical predictions, even taking into account image dipole effects within the substrate. This work has shed light on the relation between the optical properties and the nanostructure of composite films, and opens the way for the fabrication of nanocomposite materials with novel optical properties via chemical routes., Les méthodes de synthèses chimiques et d'auto-assemblage sont aujourd'hui envisagées pour fabriquer des matériaux nanostructurés aux propriétés optiques contrôlées. Nous avons considéré le cas le plus simple de composites combinant un polymère non-structuré et des nanoparticules d'or sphériques, dont nous fabriquons des films d'épaisseur et de fraction volumique en or variables et étudions les propriétés optiques, par ellipsométrie spectroscopique, dans les domaines ultraviolet-visible-proche infrarouge. Pour des films tridimensionnels épais (30-200 nm) de fraction volumique en nanoparticules inférieure à 6%, les variations de l'indice optique avec la longueur d'onde sont bien décrites par une loi de mélange de Maxwell-Garnett modifiée par l'introduction de polarisabilités anisotropes rendant compte du couplage électromagnétique entre particules proches. Des d'échantillons très denses en nanoparticules ont été fabriqués (jusqu'à des fractions volumiques de 30%), et montrent, dans une bande spectrale finie, des permittivités électriques proches de zéro ou négatives. Pour des échantillons très minces (épaisseur de l'ordre de la taille des nanoparticules), nous montrons que l'indice optique complexe n'est plus un descripteur intrinsèque du système et que l'analyse ellipsométrique ne permet pas de déterminer l'épaisseur de manière non-ambigüe. On a accès en revanche à la section efficace d'extinction des particules : lorsque les particules sont déposées très près du substrat de silicium, les valeurs expérimentales sont exaltées par rapport aux valeurs théoriques, même en tenant compte d'effets de dipôles image dans le substrat. Les conclusions de ce travail ont permis d'éclaircir le lien entre les propriétés optiques mesurées et la nanostructure de films composites, et laissent entrevoir la possibilité d'élaborer par voie chimique des matériaux nanocomposites aux propriétés optiques inédites.

This PhD work was dedicated to the experimental, theoretical and numerical studyof the optical properties of self-assembled metamaterials made of gold nanoparticles, operatingat visible frequencies. We were interested in three different structure types and analyzed theiroptical properties experimentally by spectroscopic ellipsometry.We studied the effect of a high index substrate proximity on the plasmonic response of goldnanoparticles, both experimentally and numerically, on nanoparticle monolayers deposited ontopolyelectrolytes multilayers. We discuss an enhancement effect of the nanoparticle absorptioncross-section for small distances to the substrate. Dense polymer/gold nanoparticles compositefilms were shown to have tunable optical properties depending on their gold filling fractionand to behave, at high filling fraction, like a dielectric or a metal depending on the frequencyrange. We developed a phenomenological model, adapted to a large number of cases, whichsuccessfully describes the films optical properties. Finally, lamellar films made of block copolymersand gold nanoparticles were studied. We have shown how to access, for certain structuralconditions, the extraction and the modelisation of the effective permittivity anisotropy, affectedby the resonance of the nanoparticles., Ce travail de thèse a été consacré à l’étude expérimentale, théorique et numériquede métamatériaux auto-assemblés constitués de nanoparticules d’or, opérant dans la gammedu visible. Nous nous sommes intéressés à trois types de structures différentes et avons analyséleurs propriétés optiques par ellipsométrie spectroscopique.Nous avons étudié expérimentalement et numériquement l’effet de la proximité d’un substratde haut indice sur la réponse plasmonique d’une nanoparticule, grâce à un système expérimentalconstitué de monocouches de nanoparticules d’or sur une multicouche de polyélectrolytes.Nous avons discuté d’un effet d’exaltation de la section efficace d’absorption des nanoparticulesaux plus faibles distances au substrat. Nous avons montré que des films composites denses depolymère et de nanoparticules d’or ont des réponses contrôlées par la fraction en or et qu’ilsse comportent, à haute fraction en or, soit comme des diélectriques soit comme des métauxselon la gamme de fréquence. Nous avons développé un modèle phénoménologique, adapté àde nombreuses situations, qui décrit bien les propriétés optiques des films. Enfin, nous avonsétudié des nanocomposites lamellaires à base de copolymères à blocs et de nanoparticules d’or.Nous avons montré comment avoir accès, pour certains systèmes, à l’extraction et la modélisationde leur anisotropie de permittivité, qui est affectée par la résonance plasmonique desnanoparticules.

This PhD work was dedicated to the theoretical and numerical study of metamaterials in the terahertz range (or submillimetric) made of dielectric spheres embedded in a host medium. The dipolar electric and magnetic Mie resonances of a set of dielectric particles can provide metamaterials properties such as artificial magnetism or negative index of refraction. In this work, the materials to use, as well as the sizes and the filling fractions of the spheres to achieve magnetism in the terahertz range were precised. As a second step, we interested in the size distribution effects on the effective permeability. We showed that this effect can be detrimental to the emergence of metamaterials properties, but nevertheless, in a controlled amount, polydispersity can generate a wide negative permeability zone with a magnitude close to zero. Finally, we studied the electromagnetic behaviour of bidisperse array of TiO2 spheres with numerical simulations. A Bloch mode analysis of this kind of structure seems to proove the existence of a negative index band for high filling fractions.; Ce travail de thèse a été consacré à l'étude théorique et numérique de métamatériaux pour la gamme submillimétrique (domaine térahertz), formés de sphères diélectriques présentant des résonances dipolaires de Mie électrique et magnétique, dispersées dans un milieu-hôte. Tout d'abord, les matériaux à utiliser ainsi que les tailles et fraction volumique des sphères permettant l'obtention d'effets de perméabilité ont été précisées. Ensuite, nous avons mené une étude de la polydispersité en taille et donné les distributions en taille limite à ne pas dépasser lors d'une synthèse chimique. Nous avons également montré qu'une polydispersité contrôlée pouvait engendrer un élargissement des zones de perméabilité négative ou une zone de perméabilité nulle. Enfin, nous avons étudié le comportement électromagnétique d'assemblages bidisperses de sphères de TiO2 par simulations numériques, et avons procédé à une analyse des modes de Bloch, semblant indiquer l'existence d'une bande d'indice négatif pour des fractions volumiques élevées.

Chemical and self-assembly approaches are increasingly used for the fabrication of nanostructured materials with controlled optical properties.We have considered the simplest case of composite films comprised of a non-structured polymer with spherical gold nanoparticle inclusions, with varied thickness and filling fractions. Optical properties have been studied by means of spectroscopic ellipsometry in the ultraviolet-visible-near infrared range. For thick, three-dimensional films (30-200 nm) with filling fractions below 6%, the behavior of the optical index as a function of the wavelength is well described using a modified Maxwell-Garnett mixing law modified with anisotropic polarisabilities, which accounts for electromagnetic couplings between neighboring particles. Very dense samples with filling fractions up to 30% have also been fabricated, and exhibit near-zero or negative electric permittivities in a finite frequency range. For very thin films (of thickness close to the nanoparticle size), we have shown that the complex optical index is not an intrinsic descriptor of the system and that ellipsometric analysis cannot determine a sample thickness unambiguously. However, the analysis gives access to the particles extinction cross-section : we find that when the particles lie very close to the silicon substrate, experimental values are enhanced compared to theoretical predictions, even taking into account image dipole effects within the substrate. This work has shed light on the relation between the optical properties and the nanostructure of composite films, and opens the way for the fabrication of nanocomposite materials with novel optical properties via chemical routes.; Les méthodes de synthèses chimiques et d'auto-assemblage sont aujourd'hui envisagées pour fabriquer des matériaux nanostructurés aux propriétés optiques contrôlées. Nous avons considéré le cas le plus simple de composites combinant un polymère non-structuré et des nanoparticules d'or sphériques, dont nous fabriquons des films d'épaisseur et de fraction volumique en or variables et étudions les propriétés optiques, par ellipsométrie spectroscopique, dans les domaines ultraviolet-visible-proche infrarouge. Pour des films tridimensionnels épais (30-200 nm) de fraction volumique en nanoparticules inférieure à 6%, les variations de l'indice optique avec la longueur d'onde sont bien décrites par une loi de mélange de Maxwell-Garnett modifiée par l'introduction de polarisabilités anisotropes rendant compte du couplage électromagnétique entre particules proches. Des d'échantillons très denses en nanoparticules ont été fabriqués (jusqu'à des fractions volumiques de 30%), et montrent, dans une bande spectrale finie, des permittivités électriques proches de zéro ou négatives. Pour des échantillons très minces (épaisseur de l'ordre de la taille des nanoparticules), nous montrons que l'indice optique complexe n'est plus un descripteur intrinsèque du système et que l'analyse ellipsométrique ne permet pas de déterminer l'épaisseur de manière non-ambigüe. On a accès en revanche à la section efficace d'extinction des particules : lorsque les particules sont déposées très près du substrat de silicium, les valeurs expérimentales sont exaltées par rapport aux valeurs théoriques, même en tenant compte d'effets de dipôles image dans le substrat. Les conclusions de ce travail ont permis d'éclaircir le lien entre les propriétés optiques mesurées et la nanostructure de films composites, et laissent entrevoir la possibilité d'élaborer par voie chimique des matériaux nanocomposites aux propriétés optiques inédites.