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International audience; Multi-dielectric thin films are usually studied as waveguiding structures with low absorption effectbecause of the low imaginary part of the refractive index. However, when properly designed, wedemonstrated that multi-dielectric stacks can sustain large optical fields. We briefly present hereour design method leading the fabrication of such multi-dielectric stacks, which can be optimizedfor arbitrary wavelengths, indices or polarizations. We then report on our experimental character-ization in near and far field, using a photon scanning tunneling microscope and scattering opticalsetup, respectively. This investigation may find applications for ultra-sensitive optical sensors orintegrated light sources to mention a few.

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International audience

Optical interference filters, most often consisting of a stack of dielectric layers deposited on a glass substrate, perform essential functions in any current optical system (antireflection, polarizers, selectors, separators, mirrors, equalizers ...). In general, the phenomena of thermal origin in these filters, which appear when they are subjected to laser flux, cannot be neglected. On the one hand, they can be responsible for many problems ranging from the degradation of optical performance of the filter to its irreversible damage. On the other hand, they are at the basis of various methods of characterizing the absorption of filters, such as photo-thermal deflection, common-path interferometry, or Lock-In Thermography.However, describing these phenomena with precision remains a difficult step. Existing models are often incomplete, based on approximations or are not adapted to the field of interference filters. The objective of this thesis is therefore first to propose theoretical tools to accurately model the photo-induced thermal phenomena in optical thin films, in arbitrary illumination regime (pulsed, clocked, continuous).These photo-induced processes originate from the absorption of multi-dielectric components, whose bulk density is the heat source responsible for a temperature rise in the component. Thus, the photo-induced temperature is the first thermal phenomenon studied in this thesis and a complete description based on layer and illumination parameters is proposed. It is mainly based on an analogy between optics and thermics, allowing to obtain an analytical model of the temperature. This procedure allows to better target the influence of the different input parameters, and to further address the phenomena of laser damage or self-organization of nanoparticles.The temperature increase leads to a modification of the thermal radiation of the filter. Very often, the modeling of this radiation in monochromatic illumination regime is reduced to a calculation of emissivity in connection with Kirchhoff's law, which stipulates its equality with the absorption. However, this method is insufficient when we want to go further into the details of the balance of thermal processes (evanescent waves, guided modes by coupling, transient regimes). Thus, in this thesis, we propose a direct modeling of photo-induced thermal radiation. This means that we rely on the various works of statistical physics that allow to link the thermal agitation of particles to volume densities of electric current. These currents are inserted into Maxwell's equations to obtain the electromagnetic field radiated in free space, called thermal radiation. The method of resolution is based on the work related to luminescent microcavities and the volume scattering of light in filters.The resolution method also allows the design filters that confine and exalt their own thermal radiation in a small spectral or angular bandwidth. More broadly, this opens the way to the control of thermal radiation by multilayer planar structures, which is an important research area in the Defense and Energy sectors. An analytical synthesis method is proposed to generate giant field exaltations in microcavities, with application to thermal radiation.Finally, this precise analysis of the thermal radiation allows, for the first time, to precisely quantify the part of the photo-induced thermal radiation that is transferred as guided modes to the multi-dielectric structure.; Les filtres optiques interférentiels, constitués le plus souvent d’un empilement de couches diélectriques déposées sur un substrat de verre, réalisent des fonctions essentielles à tout système optique actuel (antireflets, polariseurs, sélecteurs, dichroïques, séparateurs, miroirs, égaliseurs…). De façon générale, les phénomènes d’origine thermique dans ces filtres, qui apparaissent notamment lorsqu’ils sont soumis à des flux lasers, ne peuvent pas être négligés. D’une part, ils peuvent être responsables de nombreux problèmes allant de la dégradation des performances optiques du filtre à son endommagement irréversible. Et d’autre part, ils sont à la base des différents procédés de caractérisation de l’absorption des filtres, comme la déflexion photo-thermique, l’interférométrie photo-thermique ou encore la thermographie par détection synchrone.Cependant, décrire avec précision ces phénomènes demeure une étape difficile. Les modèles existants sont souvent incomplets, reposent sur des approximations ou ne sont pas adaptés au domaine des filtres interférentiels. L’objectif de cette thèse est donc en premier lieu de proposer des outils théoriques permettant de modéliser avec précision les phénomènes thermiques photo-induits dans les couches minces optiques, en régime arbitraire d’illumination (pulsé, cadencé, continu).Ces processus photo-induits trouvent leur origine dans l’absorption des composants multi-diélectriques, dont la densité volumique constitue la source de chaleur responsable d’une élévation de température dans le composant. Ainsi, la température photo-induite est le premier phénomène thermique étudié dans cette thèse et une description complète à partir des paramètres des couches et de l’éclairement est proposée. Elle s’appuie principalement sur une analogie entre l’optique et la thermique, permettant d’obtenir un modèle analytique de la température. Cette procédure permet de mieux cibler l’influence des différents paramètres d’entrée, et d’aborder plus avant les phénomènes d’endommagement laser ou d’auto-organisation de nanoparticules.L’élévation de température donne lieu à une modification du rayonnement thermique du filtre. Bien souvent, la modélisation de ce rayonnement en régime d’éclairement monochromatique est ramenée à un calcul d’émissivité en lien avec la loi de Kirchhoff, qui stipule son égalité avec l’absorption. Cependant, cette méthode s’avère insuffisante lorsque l’on veut aller plus loin dans le détail du bilan des processus thermiques (ondes évanescentes, modes guidés par couplage, régimes transitoires). Ainsi dans ce travail de thèse, on propose une modélisation directe du rayonnement thermique photo-induit. Cela signifie que l’on s’appuie sur les différents travaux de la physique statistique qui permettent de relier l’agitation thermique des particules à des densités volumiques de courant électrique. Ces courants sont insérés dans les équations de Maxwell pour obtenir le champ électromagnétique rayonné en espace libre, appelé rayonnement thermique. La méthode de résolution s’appuie sur les travaux en lien avec les microcavités luminescentes et la diffusion volumique de la lumière dans les filtres.La méthode de résolution permet également de concevoir des filtres qui confinent et exaltent leur propre rayonnement thermique dans une faible bande spectrale ou angulaire. De manière plus large, cela ouvre la voie au contrôle du rayonnement thermique par des cavités planaires multicouches, qui constitue un important domaine de recherche dans les secteurs de la Défense et de l’Energie. Une méthode analytique de synthèse est proposée pour générer des exaltations géantes de champ en microcavité, avec application au rayonnement thermique.Enfin, cette analyse précise du rayonnement thermique permet, pour la première fois, de quantifier avec précision la part du rayonnement thermique photo-induit qui est transférée sous forme de modes guidés à la structure multi-diélectrique.

Capitalizing on a previous theoretical paper, we propose a novel approach, to our knowledge, that is different from the usual scattering measurements, one that is free of any mechanical movement or scanning. Scattering is measured along a single direction. Wide-band illumination with a properly chosen wavelength spectrum makes the signal proportional to the sample roughness, or to the higher-order roughness moments. Spectral shaping is carried out with gratings and a spatial light modulator. We validate the technique by cross-checking with a classical angle-resolved scattering set-up. Though the bandwidth is reduced, this white light technique may be of key interest for on-line measurements, large components that cannot be displaced, or other parts that do not allow mechanical movement around them.

International audience; In less than 20 years, extraordinary progress has been made on the design and manufacturing of planar optical components, especially in terms of minimizing light scattering (a few 10^-6 of the incident flux). This progress is closely linked to polishing techniques, which allow to obtain, on amorphous substrates, roughness lower than a fraction of nanometer in the optical frequency window. Moreover, with modern filter manufacturing technologies, the roughness of the substrate is reproduced almost identically by each of the thin layers constituting the stack.However, at this level of qualification, new problems and concerns arise, in particular the presence of localized defects in the component. These defects are of sub-micron size and appear during the manufacturing of the filters. Their density is low (less than one defect for a 100 micrometers diameter disk), so for conventional components, their contribution can be neglected. However, for Space optics and gravitational waves detection, the impact of these isolated defects can become dominant in the light scattering process and its accurate quantification remains a challenge. To address this problematic, the CONCEPT Group of the Institut Fresnel developed a SPatially and Angulary Resolved Scatterometry Equipment (SPARSE). The principle of the instrument is based on the coupling of imaging abilities with a scatterometer. It is designed to record up to 440 thousand BRDF with a single measurement on a one inch diameter component. A beam shaping arm is added to tailor the illumination to the shape of the component and to offer the capability to “turn off” defects in order to increase the detectivity. SPARSE allows the measurement of scattering level as low as 10^-8 sr^-1$ and the data processing is designed to discriminate and quantify the weight of localized defects, contamination, scratches and roughness in the scattering budIn less than 20 years, extraordinary progress has been made on the design and manufacturing of planar optical components, especially in terms of minimizing light scattering (a few 10^-6 of the incident flux). This progress is closely linked to polishing techniques, which allow to obtain, on amorphous substrates, roughness lower than a fraction of nanometer in the optical frequency window. Moreover, with modern filter manufacturing technologies, the roughness of the substrate is reproduced almost identically by each of the thin layers constituting the stack.However, at this level of qualification, new problems and concerns arise, in particular the presence of localized defects in the component. These defects are of sub-micron size and appear during the manufacturing of the filters. Their density is low (less than one defect for a 100 micrometers diameter disk), so for conventional components, their contribution can be neglected. However, for Space optics and gravitational waves detection, the impact of these isolated defects can become dominant in the light scattering process and its accurate quantification remains a challenge. To address this problematic, the CONCEPT Group of the Institut Fresnel developed a SPatially and Angulary Resolved Scatterometry Equipment (SPARSE). The principle of the instrument is based on the coupling of imaging abilities with a scatterometer. It is designed to record up to 440 thousand BRDF with a single measurement on a one inch diameter component. A beam shaping arm is added to tailor the illumination to the shape of the component and to offer the capability to “turn off” defects in order to increase the detectivity. SPARSE allows the measurement of scattering level as low as 10^-8 sr^-1 and the data processing is designed to discriminate and quantify the weight of localized defects, contamination, scratches and roughness in the scattering budget. We propose in this paper a detailed description of the set-up with its metrological qualification and some examples of measurements performed on representative Space Optics.get. We propose in this paper a detailed description of the set-up with its metrological qualification and some examples of measurements performed on representative Space Optics.

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International audience; We present the main improvements to a scatterometer developed at the Fresnel Institute allowing the spatially resolved recording (up to 1 million elementary pixels) of the light transmitted or scattered by a plane sample.

Photo-induced thermal radiation of multilayer optics is analytically modelized and implemented. The results provide temporal, wavelength, and angular radiation patterns. Specific designs are presented to shape thermal emission.

Photo-induced temperature in multilayer optics is analytically modelized to investigate the impact of source parameters on the temperature elevation. Repetition rate and thermal fringes are discussed, together with temporal and 3D spatial resolution.

First-order scattering from rough layers vanishes in the anti-scattering configuration. An exact electromagnetic theory is used to predict the higher-order scattering. It is shown how this scattering is influenced by interfaces height distribution.

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International audience; We present an analytical method for the optimization of luminescent micro-cavities to create a substrate that is extremely sensitive to contamination. Giant optical enhancement can thus be controlled arbitrarily and simultaneously at various frequencies within the substrate’s evanescent field with the aim of obtaining ultra-sensitive optical sensors. This process provides an alternative to sensors based on illumination in free space.

International audience; We apply reverse engineering techniques in the time domain THz regime to recover the multilayer structure of a sunflower leaf. Results are successful and allow to analyze the time sunflower behavior under water stress

International audience; The photo-induced temperature in multilayer systems is calculated using an original analytical model based on optical/thermal analogies. Various illumination regimes are considered, ranging from ps pulses to a continuous regime, while taking into account a variable repetition rate. The temporal and spatial (3D) resolutions are quantified, and the distributions of temperature and stationary optical field are compared. The temperature spectra are given as a function of the illumination wavelength. Thermal and electromagnetic damage thresholds are compared. Lastly, the thermal fringes resulting from structured optical illumination are studied.

Following on from previous studies on motionless scatterometers based on the use of white light, we propose a new, to the best of our knowledge, experiment of white-light scattering that should overtake the previous ones in most situations. The setup is very simple as it requires only a broadband illumination source and a spectrometer to analyze light scattering at a unique direction. After introducing the principle of the instrument, roughness spectra are extracted for different samples, and the consistency of results is validated at the intersection of bandwidths. The technique will be of great use for samples that cannot be moved.

International audience; Despite limiting the performance of multilayer optical thin-films, light scattering properties are not as yet controllable by current design methods. These methods usually consider only specular properties: transmittance and reflectance. Among other techniques, design of thin-film components assisted by deep neural networks have seen growing interest over the last few years. This paper presents an implementation of a deep neural network model for light scattering design and proposes an optimization process for complex multilayer thin-film components to comply with expectations on both specular and scattering spectral responses.

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The development of optical interference filters for multispectral imagers has revealed new problems specific to these thin film components. The numerous physical interfaces between their inner layers generate light scattering which can alter the use of the filter. It is therefore essential to understand and control this phenomenon. Three tools have been developed for that purpose: modeling, metrology, and synthesis.An electromagnetic model has been implemented by the CONCEPT group of the Fresnel Institute. It considers the structural features of the multilayer and the roughness of each interface.The metrology relies on the development of SALSA (Spectral and Angular Light Scattering characterization Apparatus), a scatterometer which operated on wavelengths between 400 and 1000 nm. As part of this thesis, the spectral range has been extended to 1700 nm with the same performances than those in the visible spectrum. SALSA measures down to 1E−8 sr^−1 for scattered light and up to 13 optical densities in specular, which is around 6 orders of magnitude better than the international state of the art. Experimental results have been compared to the model and reach an excellent agreement.The implementation of the electromagnetic model paves the way for the synthesis of interference filters based on scattering properties. Until now, no method controls both specular and scattering spectral responses. We propose a model based on deep neural networks to solve this inverse problem. The first results show the ability to control and reduce the scattering of an interference filter of one decade on a specific spectral band.; Le développement des filtres interférentiels pour les imageurs multispectraux a mis en évidence de nouvelles problématiques propres à ces composants couches minces. Les interfaces séparant leurs couches constitutives génèrent de la diffusion lumineuse qui perturbe leur utilisation, il est primordial de comprendre et maîtriser ce phénomène. Trois outils ont été développées : la modélisation, la métrologie et la synthèse.Un modèle électromagnétique de la diffusion a été mis au point par l’équipe CONCEPT de l’Institut Fresnel. Il prend en compte les caractéristiques structurelles de l’empilement et la rugosité de chaque interface.La métrologie repose sur le développement de SALSA (Spectral and Angular Light Scattering characterization Apparatus), un diffusomètre qui opérait initialement sur des longueurs d’onde entre 400 et 1000 nm. Dans le cadre de cette thèse, son fonctionnement a été étendu jusqu’à 1700 nm, tout en conservant les performances acquises sur le spectre visible. SALSA mesure jusqu’à 1E−8 sr^−1 en diffusion et 13 densités optiques en spéculaire, soit environ 6 ordres de grandeur de mieux que l’état de l’art international. La comparaison des résultats expérimentaux avec le modèle est excellente.Enfin, le modèle électromagnétique ouvre la voie à la synthèse de composants sous contrôle des propriétés de diffusion. Aucune méthode ne permet de contrôler à la fois les réponses spéculaires et les réponses diffuses. Nous proposons une optimisation par réseaux de neurones artificiels profonds pour résoudre ce problème inverse. Les premiers résultats montrent la possibilité de réduire la diffusion d’un composant complexe d’une décade sur une gamme spectrale choisie.

Optical coatings have been extensively characterized these last decades, due to more and more severe requirements. These characterizations include optical properties, uniformity, hardness, adhesion and stress, damage threshold, absorption, scattering and others… Despite this state of the art, photo-induced thermal radiation [1]–[3] in optical coatings was rarely investigated in detail until now, while emissivity plays a key role in numerous sectors related to energy and defence, space optics and MIR imaging... This is the scope of this paper to provide an exact theory of thermal radiation in optical multilayers submitted to an arbitrary illumination (continuous, pulsed, modulated). The spectral range of thermal radiation (TR) is temperature (Tp) dependent. Hence it varies with incident power, and with the imaginary indices of the thin film materials. Index dispersion also plays a key role for short duration beams. As always with thin films, the TR waves follow Maxwell equations and their angular and spectral patterns are shaped by the coating design, which may open an opportunity to control these patterns [4], [5]. Relying on the fluctuation dissipation theorem [1], we introduce in Maxwell equations the adequate currents responsible for thermal radiation. These currents are temperature related, for which reason the depth and time distribution of temperature is also calculated for arbitrary illumination regimes. The Tp calculation relies on the analogy [6], [7] between thermal (diffusion equation) and optics (propagation equation) in metallic media. The resulting currents are bulk currents which vary within the depth of the stack, which creates a similarity with luminescent micro-cavities [8]. Eventually we calculate the waves emitted from these currents and this gives the angular, wavelength and temporal patterns of thermal radiation which merges in free space. The TR patterns are analysed for a series of coatings and a few ideas are presented to reduce, confine, or enhance the thermal waves.

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International audience; New techniques for agriculture science are widely explored since several decades in order to improve production yield. Measurements of optical properties at different scales of the crop are investigated and exploited to assess different parameters of interest such as state of stress. For instance, nowadays, there exists acquisition systems embedded in drones, mobile machines and satellites that are able to collect huge amount of hyperspectral imaging data. Identification of optical signature extracted from these techniques can help agronomist with adapting irrigation or distinguishing different plant varieties. These techniques allow to improve greatly the agricultural management, however they do not provide information about the internal structure of the plant leaf and their interaction with electromagnetic fields. Knowing precisely the plant leaf structure can bring critical information that can lead to the development of new techniques for phenotyping and precocious stress detection. To do this it is necessary to probe the plant at the leaf scale using THz instead of optical frequencies because the scattering sensitive phenomenon for plants is more drastic at optical frequencies. To find out how the light interact with the leaf, in a deterministic way, we can model the vegetal tissue as a stack of different physical layers characterized by the thickness and the optical index. In this study, funded by ANR project OptiPAG, we use a well-known reverse engineering technique to retrieve leaf architecture from the reflection data. In time domain, a short Terahertz pulse illuminates a multilayer sample that reflects a part of the signal carrying information about the sample structure. Using a numerical fit in the frequency domain allows to identify each layer and deduce the respective optical index over the input frequency range. We use a few classical (inorganic) etalon samples and analyze the echoes to reveal their thicknesses under the assumption of negligible absorption. Then, we use reverse engineering technique to fit the data in the THz range by taking into account the absorption, making an excellent agreement with the previous results with more accuracy. The measured thickness of the samples correspond very well with the manufacturing specifications. And finally we use this technique with vegetal tissues (sunflower leaves), that poses a much more complex situation. Results emphasize a 8-layer stack including trichomes, cuticules, epidermis and mesophyll layers and for each layer we extract the thickness and the complex index. To our knowledge this is the first time that the leaf multilayer structure is extracted with accuracy using a non-contact techniques.

Optical sources within micro-cavities have been extensively studied for years [1]–[3]. It is today well known that over-coating these cavities with multilayer optics allows to modify their emission patterns in free space in the surrounding media (substrate and superstrate). While sources within single overcoats radiate a quasi-lambertian pattern, more elaborated structures allow to confine the angular patterns in narrow angular or spectral regions. Applications concern lightening and bio-photonics, micro and single photon sources, antennas… However, most overcoats in micro-cavities are classical multilayer designs currently used in the field of thin-film optics, such as mirrors and narrow-band filters, so that one may wonder whether these cavities can be optimized with much more (huge) efficiency. Furthermore, until now the presence of guided waves was not considered in this optimization process. The objective of this paper is to introduce specific techniques to design huge enhancement of emission patterns from micro-cavities. We first introduce zero-admittance layers [4]–[6] in the cavity and show that they also strongly increase (by several decades) in free space the emission pattern. Then we are interested in designing a cavity for which the modal light would largely dominate the free-space light; here we use modal light to designate the light trapped in the cavity and which propagates in the form of guided modes without radiation losses. In order to reach this goal, we show how to design specific poles of the reflection factor in the modal range, that are known to be the modal constants of the layered structure. All results allow to reach a total modal energy much higher than the integrated free-space light, or conversely. Applications may concern micro-lasers, guided waves and lightning, and address the extension of design techniques to the situation of multilayer waveguides.

Optical thin-film components have known major developments over the last three decades and find now applications in many scientific fields. But it remains difficult to measure and characterize complex components. Light scattering properties are usually neglected in the characterization even though it is well-known that it can be critical for some very demanding applications where the miniaturization of optical instruments make them more sensitive to crosstalk. We present in this paper an upgraded version of a spectrally and angularly resolved scatterometer operating on a continuous spectral range from 400 to 1700 nm and reaching unprecedented levels of detection and accuracy.

International audience; Alternative energy strategies based on plant biomass-derived bioenergy and biofuels rely on understanding and optimization of plant structure, chemistry, and performance. Starch, a constitutive element of all green plants, is important to food, biofuels, and industrial applications. Models of carbohydrate storage granules are highly heterogeneous in representing morphology and structure, though a deeper understanding of the role of structure in functional behavior is emerging. A better understanding of the in situ nanoscale properties of native granules is needed to help improve the starch quality in food crops as well as optimize lignocellulosic biomass production in perennial nonfood crops. Here, we present a new technique called soft mechanical nano-ablation (sMNA) for accessing the interior of the granules without compromising the inner nanostructure. We then explore the nanomechanics of granules within the ray parenchyma cells of Populus xylem, a desirable woody biofuel feedstock. The employed soft outer layer nanoablation and atomic force microscopy reveal that the inner structure comprises 156 nm blocklets arranged in a semicrystalline organization. The nanomechanical properties of the inner and outer structures of a single starch granule are measured and found to exhibit large variations, changing by a factor of 3 in Young's modulus and a factor of 2 in viscoplastic index. These findings demonstrate how the introduced approach facilitates studies of structure−function relationships among starch granules and more complex secondary cell wall features as they relate to plant performance.

Due to the diversity and complexity of optical functions they address, optical coatings are widely used in space optics but the level of both their specifications and performances is now so high that the characterization of their optical properties requires the development of extreme metrology. In this context we present in this paper the last state-of-the-art instrument for thin-films characterization. This apparatus is unique by the performances and the levels of detectivity achieved. It operates spectrally and angularly resolved characterization of Angle Resolved Scattering (ARS) over a wide and continuous spectral range from 400 nm to 1.7 μm. The spectral resolution is lower than 1 nm, while the angular resolution is lower than 1° with a precision better than 1%. The lowest levels of scattering measured are close to 10-8 sr-1, near to the limit of air particles scattering. The large angular range available offers the opportunity to evaluate scattering functions both in the forward and backward half-spaces. In specific configurations, this same setup records specular properties, achieving again unreached levels of detection and precision over the same spectral range and with the same resolution. All measurements have shown great agreement with theoretical designs and numerical predictions, for both scattering and transmittance records. This instrument is an upgraded version of an apparatus developed in the Scattering group for more than five years formerly operating in the visible wavelength spectrum and now extended to near-infrared with the same level of performances.

We measured very low light power backscattered by optical surfaces using low coherence interferometry. The backscattered light can indeed be a critical noise source for interferometric gravitational wave detectors such as LISA, the laser interferometer space antenna developed by ESA, with the support of NASA and European national space agencies. We present the preliminary measurement results of light backscattered from a silver mirror and a bare N-BK7 window 1 mm thick using a superluminescent diode centered at 1060 nm as broadband light source. The use of low coherence interferometry allows light backscattered by both sides of the N-BK7 window to be recorded independently with a noise floor of around -110 dB.

International audience; In next generation space instrumentation for Earth and Universe Observation, new instrument concepts include often non planar gratings. Their realization is complex and costly. We propose a new technology for designing and realizing convex blazed gratings for high throughput spectrographs. For this purpose, our requirements are driven by a Digital-Micromirror-Device-based (DMD) MOS instrument to be mounted on the Telescopio Nazionale Galileo (TNG) and called BATMAN. The two-arm instrument is providing in parallel imaging and spectroscopic capabilities. The objects/field selector is a 2048 x 1080 micromirrors DMD, placed at the focal plane of the telescope; it is used as a programmable multi-slit mask at the entrance of the spectrograph. The compact Offner-type spectrograph design contains a low density convex grating to disperse light. For optimization of the spectrograph efficiency, this convex grating must be blazed. A blazed reflective grating has been designed with a period of 3300 nm and a blaze angle of 5.04°, and fabricated into convex substrates with 225 mm radius of curvature and a footprint diameter of 63.5 mm. The blaze is optimized for the center wavelength of 580 nm within the spectral range of 400 – 800 nm. Such gratings have been fabricated and coated with a silver-based layer, with a final 7° blaze angle over the whole surface. Efficiency close to 90% on the 1st diffraction order at 700nm has been obtained, measured on BATMAN spectroscopic arm. Detailed mapping of the blazed grating showed a very good period uniformity with up to 0.5% deviation. Grating depth and blaze angle have higher deviation, up to 7%. An optimized device with the exact required blaze angle would reach the same efficiency and be centered on the mid of 400-800nm wavelength band: its realization is on-going. The grating brings a significant contribution in the total amount of straylight at instrument level. Their straylight level remains a critical issue, and its reduction by specific and controlled implementation of improvements in manufacturing process is a challenge to tackle. Preliminary straylight measurement has been done and shows a lowest straylight level below 10-2 sr-1 between the diffraction orders. This new type of non-planar reflective gratings will be the key component for future high throughput spectrographs in space missions.

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We numerically demonstrate the capability of a single metasurface to simultaneously separate and focus spectral features in accordance with the specifications of a pushbroom hyperspectral imager. This is achieved through the dispersion engineering of a library of two-level TiO2 nano-elements. Sommerfeld integrals are used to confirm our numerical simulations provided by our solver based on Fourier modal method. As a proof of concept, a metasurface with a 175 µm diameter is designed to be compatible with hyperspectral imaging over a spectral range of ± 50 nm around 650 nm with a spectral resolution of 8.5 nm and a field of view of 8° around the normal incidence (angular resolution of 0.2°).

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International audience; We use balanced optical low-coherence interferometry to measure the ultralow light power back-reflected and/or back-scattered by optical interfaces. Indeed, backscattered light from optical surfaces can be a critical source of noise in interferometric gravitational-wave detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, or the Laser Interferometer Space Antenna (LISA), and therefore, it is required to characterize, with high sensitivity, the scattering properties of elementary components involved in their optical design. The use of a broadband light source makes an easy identification of the scattering interface possible, while the implementation of balanced interferometric detection allows very low light levels to be detected, thanks to the simultaneous use of coherent gain and source intensity noise suppression. Experimental results are reported, first on bare glass optical windows (N-BK7 and S-LAH66) and then on a silver-coated mirror, that demonstrate the ability of this setup to quantify specular- or diffuse-reflectance coefficients as low as 10−11.

International audience; Three experts in the field of thin-film optics present a detailed and self-contained theoretical study of planar multilayers and how they can be effectively exploited in both traditional and modern applications. Starting with a discussion of the relevant electromagnetic optics, the fundamental optical properties of multilayers are introduced using an electromagnetic approach based on a direct solving of Maxwell's equations by Fourier transforms. This powerful approach is illustrated through the comprehensive description of two of the most important phenomena in multilayers, i.e. giant field enhancement in dielectric stacks and light scattering from thin-film optical filters. The same approach is extended to the description of the operation of planar microcavities and the balance of energy between radiated and trapped light. This book will be valuable to researchers, engineers and graduate students with interests in nanophotonics, optical telecommunications, observational astronomy and gravitational wave detection.

International audience; We analyze the multilayer structure of sunflower leaves from Terahertz data measured in the time-domain at a ps scale. Thin film reverse engineering techniques are applied to the Fourier amplitude of the reflected and transmitted signals in the frequency range f < 1.5 Terahertz (THz). Validation is first performed with success on etalon samples. The optimal structure of the leaf is found to be a 8-layer stack, in good agreement with microscopy investigations. Results may open the door to a complementary classification of leaves.

International audience

International audience; New instruments to characterize vegetation must meet cost constraints while providing accurate information. In this paper, we study the potential of a laser speckle system as a low-cost solution for non-destructive phenotyping. The objective is to assess an original approach combining laser speckle with chemometrics to describe scattering and absorption properties of sunflower leaves, related to their chemical composition or internal structure. A laser diode system at two wavelengths 660 nm and 785 nm combined with polarization has been set up to differentiate four sunflower genotypes. REP-ASCA was used as a method to analyze parameters extracted from speckle patterns by reducing sources of measurement error. First findings have shown that measurement errors are mostly due to unwilling residual specular reflections. Moreover, results outlined that the genotype significantly impacts measurements. The variables involved in genotype dissociation are mainly related to scattering properties within the leaf. Moreover, an example of genotype classification using REP-ASCA outcomes is given and classify genotypes with an average error of about 20%. These encouraging results indicate that a laser speckle system is a promising tool to compare sunflower genotypes. Furthermore, an autonomous low-cost sensor based on this approach could be used directly in the field.

International audience; The growing need for classical as well as quantum optical sensing places increasingly stringent requirements upon the desired characteristics of the engendered fields. Specifically, achieving superior field enhancement plays a critical role in applications ranging from chem-bio sensing, Raman and infrared spectroscopies to ion trapping and qubit control in emerging quantum-information science. Due to their low optical losses and ability to exhibit resonant field enhancements, all dielectric multilayers are emerging as an optical material system not only useful to classical photonics and sensing but also of potential to be integrated with quantum materials and quantum sensing. The recently introduced concept of zero-admittance layers [1] within dielectric multilayer materials, enables the creation and control of resonant fields orders of magnitude larger than the exciting field. Here, invoking the zero-admittance concept, we design, fabricate, and characterize an all-dielectric nonabsorbing stack and demonstrate the engendered huge field enhancement. Describing the fields in terms of Bloch surface waves, we connect the surface field to the semiperiodicity in the dielectric domains of the stack. As a specific application of the resonant field, we propose and demonstrate refractive-index sensing for the detection of trace amounts of an analyte. The results include a quantification of the sensitivity of the device with respect to the profile of the exciting field. The experimental results are shown to be in good agreement with theoretical calculations.

International audience; Black paints are commonly used to provide broadband light absorbers in high-precision optics. We show how multidielectric coatings improve the performances of these absorbers. The coated rough paints still exhibit a quasi-lambertian diffuse reflection, but this scattering pattern can be reduced by several orders of magnitude, which strongly enhances absorption. Predictions are based on an exact electromagnetic theory of light scattering from arbitrary rough multilayers. Results are also compared to useful approximate theories.

Optical components realized for space applications have to be mastered in term of parasitic light. This paper present the last improvements performed at the Institute Fresnel to predict and measure scattering losses of optical components with a special care to complex optical coatings. Agreement between numerical models and metrology is now excellent. Some examples will be presented.

We analyze Terahertz (THz) echoes by reflection on a sunflower leaf in order to evaluate the internal leaf structure (geometry, complex indices and thicknesses). The analysis is based on the thin film multilayer formalism in time and frequency domains. A high agreement is emphasized between experiment and theory, and we evaluate how realistic the multilayer solution can be in regard to our knowledge related to the sunflower leaf. A test campaign is performed in Charles Coulomb laboratory, which is equipped with the THz spectrometer.

International audience; We present a new version of a light scattering measurement setup, using a high-power supercontinuum laser source, two volume hologram filters, and two low-noise scientific grade cameras. This configuration enables spectral and angle resolved characterization of the light scattered by complex thin-film filters from 400 to 1650 nm. Measurements carried out on specific filters illustrate the performances of the setup.

International audience

International audience

[Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]INSPIRE [ADD1_IRSTEA]Équiper l'agriculture; International audience; This study aims to investigate the combination of speckle pattern analysis, polarization parameters, and chemo-metric tools to predict the optical absorption and scattering properties of materials. For this purpose, an optical setup based on light polarization and speckle measurements was developed, and turbid samples were measured at 405 and 660 nm. First, a backscattered polarized speckle acquisition was performed on a set of 41 samples with various scattering (µ s) and absorbing (µ a) coefficients. Then, several parameters were computed from the polarized speckle images, and prediction models were built using stepwise multiple linear regression. For scattering media, µ s was predicted with R 2 = 0.9 using two parameters. In the case of scattering and absorbing media, prediction results using two parameters were R 2 = 0.62 for µ s and R 2 = 0.8 for µ a. The overall results obtained in this research showed that the combination of speckle pattern analysis, polarization parameters, and chemometric tools to predict the optical bulk properties of materials show interesting promise.

International audience; An international round-robin experiment has been conducted to test procedures and methods for the measurement of angle-resolved light scattering. ASTM E2387-05 has been used as the main guide, while the experience gained should also contribute to the new ISO standard of angle-resolved scattering currently under development (ISO/WD 19986:2016). Seven laboratories from Europe and the United States measured the angle-resolved scattering from Al∕SiO 2-coated substrates, transparent substrates, volume diffusors, quasi-volume diffusors, white calibration standards, and grating samples at laser wavelengths in the UV, VIS, and NIR spectra. Results were sent to Fraunhofer IOF, which coordinated the experiments and analyzed the data, while ESA-ESTEC, as the project donor, defined conditions and parameters. Depending mainly on the sample type, overall good to reasonable agreements were observed, with largest deviations at scattering angles very close to the specular beam. Volume diffusor characterization unexpectedly turned out to be challenging. Not all participants provided measurement uncertainty ranges according to the Guide to the Expression of Uncertainty in Measurement; often, a single general scatterometer-related measurement uncertainty value was stated. Although relative instrument measurement uncertainties close to 1% are sometimes claimed, the comparison results did not support these claims for specular scattering samples as mirrors, substrates, or gratings.

International audience

All-dielectric multilayer structures are emerging as optical components of great utility for experiments in which strong but localized fields are needed. These structures allow for an incoming field to resonantly interact with the layers in such a way as to create a giant field at the surface of the structure, where sensing, imaging, and other applications can take place. We present how we synthesize such structures to achieve optical responses with orders of magnitude field enhancements. Furthermore, we introduce and optimization method to achieve enhancements up to 104. We will then present the advantages and drawbacks, and discuss the impact of the illumination bandwidths (angular divergence and spectral range). Subsequently, we will estimate the fabrication errors (in thicknesses and refractive index) over the optical response of the all-dielectric component and especially the field enhancement at the free interface. We will finally give a brief comparison with the surface plasmon resonances to evidence the assets and drawbacks for each case in the context of sensing applications.