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International audience; The optical properties of chiral plasmonic metasurfaces depend strongly on their architecture, in particular the orientation and spacing between the individual building blocks assembled into large arrays. However, methods to obtain chiral metamaterials with fully tunable chiroptical properties in the UV, visible and near infrared range are scarce. Here, we show that the chiroptical properties of silver nanowires assembled in helical nanostructures by Grazing Incidence Spraying and Layer-by-Layer assembly can be finely tuned over a broad wavelength range using simple design principles. The angle between the oriented nanowire layers controls the intensity of the circular dichroism, reaching ellipticity values higher than 13° and g-factor values up to 1.6, while the shape of the circular dichroism spectra depends strongly on the spacing between the layers which can be tuned at the nm-scale. The structure-dependent optical properties of the assembly are successfully modeled using a transfer matrix approach.

International audience; The optical properties of a monolayer of nanocomposite film (PMMA/gold nanocubes) were provided by fitting a proposed theoretical model to spectroscopic ellipsometry (SE) measurements. For such a thin film, these features cannot be successfully determined by means of experimental and conventional effective medium theory such as Maxwell-Garnett or Bruggeman. To make it possible, we developed a model of two classical Lorentz oscillators; one for a PMMA layer and the other for GNCs, revealing one homogeneous layer and rapid analysis without the need for large computational resources. Additionally, we tailored both the size and number of GNCs in the PMMA layer by tuning the synthesis parameters as seen in scanning electron microscopy (SEM) images. In parallel, SE measurements clearly highlighted the change in the optical properties of GNCs as a function of their density on the substrate and dimensions. Our findings demonstrate that SE is an alternative method to characterize layered GNCs on opaque substrates efficiently, which has potential implications for designing other morphologies in the future.

We study the local density of optical states (LDOS) for lossy dielectric substrates whose electric permittivity has a vanishing real part, approaching zero from the positive side of the real axis. A criterion for evaluating the threshold height above (below) which radiative (non-radiative) processes dominate for a dipole emitter is established. We focus on the case of a vertical dipole above the ϵ -near-zero (ENZ) substrate and show that, in the lossless case, complete LDOS cancellation originates from radiative modes in its near field. We evaluate the performance of commercially available ENZ materials and quantify the limits of such cancellation effects with the intrinsic losses of the substrate.

International audience; Chiral materials appear as excellent candidates to control and manipulate the polarization of light in optical devices. In nano-photonics, the self-assembly of colloidal plasmonic nanoparticles gives rise to strong resonances in the visible range and, when such organizations are chiral, strong chiroplasmonic effect can be observed. In the present work, we describe the optical properties of chiral artificial nano-photonic materials, Goldhelices, which are hierarchically organized by Grazing Incidence Spraying. These Goldhelices are made by plasmonic nanoparticles (gold) grafted onto helical templates made from silica nanohelices. A comparison of oriented vs non-oriented surfaces has been performed by Mueller matrix polarimetry, showing the importance of the organization of the Goldhelices regarding their interaction with light. Moreover, mono-vs multi-layers photonic films are created and the measured optical properties are discussed and compared to simulations.

Transparent nanoscale-film heterojunctions based on Cu2O and ZnO were fabricated by atomic layer deposition and reactive magnetron sputtering. The constitutive layers exhibit high crystalline quali...

International audience; We propose an advanced photothermal model based on a modified Takami model (MTM) to explain the mechanisms of shape changes of colloidal gold nanorods (NRs) induced by nanosecond laser pulses. This model takes into account the orientation of NRs, the radiative and convective losses, and the phase transitions of NRs. It was applied to the determination of the evolution of temperature and the shape and size transformations of NRs during the laser exposure. A series of measurements arising from the interaction between Au NRs and nanosecond laser pulses were analyzed by TEM measurements and the MTM model. We have demonstrated that the fragmentation and reshaping processes govern the nanoparticle (NP) shape. At high laser fluence, the complete fragmentation leads to a population of nearly spherical NPs, while at a moderate laser fluence, the partial fragmentation and reshaping processes generate a bimodal distribution. At low laser fluence, uncommon ϕ-shape NPs were produced as a result of the competition of cooling and reshaping processes. We also demonstrated that it is possible by the MTM model to determine the laser fluence required to suppress some specific NR shapes and to predict the NP size and shape distributions obtained after the laser exposure.

International audience; A new methodology, to the best of our knowledge, is developed to determine the shape distribution profile of gold nanoparticles (NPs) from optical spectroscopic measurements. Indeed, an artificial neural network (ANN) approach was introduced to classify Au NP shape distributions from their normalized absorption spectra. This ANN quantitatively analyzes the absorption spectra and provides the posterior probability to have a bimodal or unimodal shape distribution. Several colloidal suspensions were considered to investigate the robustness of the ANN approach. The comparison between ANN classification and TEM analysis was also given and discussed. We demonstrate that ANN classification is a suitable tool to inspect rapidly Au colloidal suspensions after their synthesis.

International audience; Plasmonic Cu nanoparticles (NP) were successfully deposited on ZnO substrates by atomic layer deposition (ALD) owing to the Volmer–Weber island growth mode. An evolution from Cu NP to continuous Cu films was observed with an increasing number of ALD cycles. Real and imaginary parts of the NP dielectric functions, determined by spectroscopic ellipsometry using an effective medium approach, evidence a localized surface plasmon resonance that can be tuned between the visible and near-infrared ranges by controlling the interparticle spacing and size of the NP. The resulting Cu NP/ZnO device shows an enhanced photoresponse under white light illumination with good responsivity values, fast response times, and stability under dark/light cycles. The significant photocurrent detected for this device is related to the hot-electron generation at the NP surface and injection into the conduction band of ZnO. The possibility of tuning the plasmon resonance together with the photoresponsivity of the device is promising in many applications related to photodetection, photonics, and photovoltaics.

We report a comparative study between Forouhi-Bloomer, Tauc-Lorentz and Tanguy dispersion laws for determining the reliable dielectric function of crystallized ZnO. ZnO layers were prepared by sol-gel method and deposited on crystalline silicon (c-Si) by spin coating. Spectroscopic ellipsometry (SE) was performed on ZnO/c-Si and each dispersion law was considered in the physical model for fitting SE experimental data. A best agreement was found between measurements and model. This applies in particular to the Tanguy dispersion. The physical parameters such as excitonic energy, optical gap, damping factor, real and imaginary parts of dielectric function were determined and analyzed. The temperature-dependent photoluminescence spectroscopy (PL) measurements were also used to approve the adequate dispersion law for ZnO material. We found by SE and PL measurements that Tanguy law dispersion can be considered as the most appropriate one for a correct description of ZnO optical dielectric function and for the interpretation of the absorption tail band and for the excitonic band of crystallized ZnO. The band-gap energy, excitonic energy and damping factor parameter are determined and analyzed. Their values (3.37 eV, 48 meV and 39 meV, respectively) extracted from ellipsometry are in good agreement with those obtained by PL measurements.

International audience; We present a method to characterize optical properties of liquids by spectroscopic ellipsometry. The experiments use a specific liquid cell that avoids disturbance of waves at air-liquid interface and allows the determination of the real and the imaginary part of the refractive index, with a sensitivity of the latter below 10−4. The method is illustrated by results obtained with a spectroscopic phase modulation ellipsometer on several liquids such as deionised water, microscope oil and protein solution. Comparisons of the method with standard techniques are given.

6th International Conference on Spectroscopic Ellipsometry (ICSE), Kyoto, JAPAN, MAY 26-31, 2013; International audience; We describe a spectroscopic ellipsometer in the visible domain (400-800 nm) based on a rotating compensator technology using two detectors. The classical analyzer is replaced by a fixed Rochon birefringent beamsplitter which splits the incidence light wave into two perpendicularly polarized waves, one oriented at +45 degrees and the other one at-45 degrees according to the plane of incidence. Both emergent optical signals are analyzed by two identical CCD detectors which are synchronized by an optical encoder fixed on the shaft of the step-by-step motor of the compensator. The final spectrum is the result of the two averaged. and. spectra acquired by both detectors. We show that. and. spectra are acquired without systematic errors on a spectral range fixed from 400 to 800 nm. The acquisition time can be adjusted down to 25 ms. The setup was validated by monitoring the first steps of bismuth telluride film electrocrystallization. The results exhibit that induced experimental growth parameters, such as film thickness and volumic fraction of deposited material can be extracted with a better trueness.

The optical properties of spherical gold and silver nanoparticles distributed in size are investigated through a new effective medium theory. It is about the modified Maxwell-Garnett-Mie theory (MMGM) that takes into account the intrinsic confinement, dynamic effects and the nanoparticle radius distribution. In the case of monodispersed nanoparticles with a radius in the 1 nm-25 nm range, MMGM is equivalent to the Mie theory. Compared to the conventional effective medium theory, we demonstrate that MGMM gives better description of the measured extinction spectra of colloidal gold and silver solutions. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; A new effective medium theory is introduced to describe the optical properties of a two-dimensional array of metallic nanoislands. This model which takes into account both the nanoisland orientation and their shape distribution is successfully used to interpret the ellipsometric measurements performed on gold nanoislands sputtered on a silicon substrate. By coupling ellipsometry to atomic force microscopy measurements, we show that the growth mechanism involves a Volmer-Weber growth mode. The optical anisotropy of uniaxial films was attributed to in-plane preferential self-orientation of gold nanoislands. Finally, we demonstrate that the optical birefringence and dichroism of nanoisland layers can be tuned during the film growth and are due to the splitting of the plasmon resonance into two modes: the transversal and the longitudinal modes of gold nanoislands.

We report on the realization of functional infrared light concentrators based on a thick layer of air-polymer metamaterial with controlled pore size gradients. The design features an optimum gradient index profile leading to light focusing in the Fresnel zone of the structures for two selected operating wavelength domains near 5.6 and 10.4 μm. The metamaterial which consists in a thick polymer containing air holes with diameters ranging from λ/20 to λ/8 is made using a 3D lithography technique based on the two-photon polymerization of a homemade photopolymer. Infrared imaging of the structures reveals a tight focusing for both structures with a maximum local intensity increase by a factor of 2.5 for a concentrator volume of 1.5 λ3, slightly limited by the residual absorption of the selected polymer. Such porous and flat metamaterial structures offer interesting perspectives to increase infrared detector performance at the pixel level for imaging or sensing applications.

International audience; We report on structural and optical properties of ZnO thin films deposited on different Si-based substrates presenting different porosities. ZnO layers were prepared by sol gel method and deposited on crystalline silicon (ZnO/Si), mesoporous silicon (ZnO/PS+) and nanoporous silicon (ZnO/PS-) by spin coating. Several techniques such as scanning electron microscope (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence spectroscopy (PL) and spectroscopic ellipsometry (SE) were used to study the influence of the pore size of porous silicon (PS) on physical properties of ZnO films. SEM images revealed the formation of ZnO granular nanoparticles on Si, PS- and PS+ substrates. We show by the XRD analysis that hexagonal crystallized (002) ZnO is mainly obtained for ZnO/PS- system causing by a strong absorption of the capillary effect and high adhesion to PS- surface. An intense PL related to ZnO and PS- was demonstrated for ZnO/PS- in UV and visible ranges. Optical properties of ZnO were determined and analyzed by SE using Tanguy dispersion model. For each sample, a specific optical model was carried out. SE confirms a good physical properties of ZnO/PS- comparing to ZnO/Si and ZnO/PS+. For example, the good crystallinity is characterized by low damping factor value (Gamma). This value was found by SE to be low (29 meV) for the ZnO/PS-, while the damping factors of ZnO/Si and ZnO/PS+ are 47 meV and 70 meV, respectively. The amplitude of dielectric function of ZnO/PS- around 3.4 eV reveals an increase of grain size and crystallinity of ZnO layer.

International audience; By irradiating a cylindrical silver target rotated at a high-speed within the range 300–2400 rpm (lateral speed 0.16–1.25 m/s) in pure water, we prepare ligand-free Ag nanoparticles (NPs) with a size of 4 ± 2 nm which are likely to be primary particles. Usually, the generation of NPs showing such a small size requires either a laser post-treatment and/or chemical additives. As the rotation rate of the target is increased, calculated 3D flow patterns revealed different hydrodynamic regimes which clearly influence the ablation rate and repeatability of the process as well as the colloidal properties. In addition to revealing the importance of fluid dynamics in pulsed-laser ablations in liquids, this study provides a way for producing in one step pure NPs with sizes below 5 nm which are suitable for applications in catalysis.

The optical properties of single walled carbon nanotube (SWCNT) films, produced by vacuum filtration, are correlated with their diameter and their in plane preferred orientation by coupling transmission and scanning electron microscopy to conventional ellipsometry. We focused on the optical anisotropy of this material and we demonstrated that it was originated from a breakdown in the selection rules.

A comparative study was made of sorted semi-conducting single walled carbon nanotube (SWCNT) films and unsorted SWCNT films for gas sensing applications. The transmission line method is used to monitor separately the SWCNTs film resistance and the contact resistance between electrodes and the SWCNTs, thus revealing that the sensing mechanism mainly relies on a modification of the tube conductivity during gas exposure. The fabricated sensors demonstrate a detection limit of 20 ppb NO2 and 600 ppb NH3 mainly attributed to experimental setup limitations. Moreover, semi-conducting nanotubes happened to be 2.5 times more sensitive to NH3 than unsorted ones, thus proving that selectivity can be improved by sorting the SWCNTs. The temperature dependence of the sensor sensitivity was studied, and a good agreement was found between experimental results and the Langmuir adsorption model.

The growth of silver nanoparticles in mesostructured hybrid silica films under laser illumination is investigated by optical absorption spectroscopy and transmission electron microscopy. At low laser doses in the UV range, a blue-shift of the plasmon resonance is observed for increasing particle size. This size dependence is interpreted on the basis of the Mie theory assuming a two-layer core−shell model resulting from the localization of the nanoparticles in the block copolymer part of the film. At higher laser doses, the observed red-shift of the resonance wavelength and decrease of the absorbance level at resonance are attributed to the formation of a high refractive index shell around the nanoparticles that thickens slowly with the dose. The growth kinetics of silver nanoparticles is also studied for different illumination wavelengths in the UV and visible ranges. Surprisingly, the nanoparticles also grow under visible illumination. This is partly attributed to the release of electrons by the degradin...

International audience; We report on a comparative study between dielectric functions of Si nanoparticles (Si-NPs) obtained from Bruggeman effective medium approximation (BEMA), Maxwell-Garnett (MG), and a modified Maxwell-Garnett (MMG) models. Unlike BEMA and MG, a size-distribution dependent dielectric function of Si-NPs is considered in the introduced MMG model. We show that the standard deviation sigma of a size distribution can be evaluated by analyzing the imaginary part of the dielectric functions of Si-NPs extracted from BEMA and MMG. In order to demonstrate this, several samples composed of Si-NPs embedded in silicon-rich silicon nitride are investigated by spectroscopic ellipsometry over the photon energy range varying between 2 and 4 eV. Assuming a lognormal size distribution of the Si nanoparticles, it is evidenced that the parameter sigma ranges between 1.15 and 1.35. The values of size dispersion deduced by this methodology are in good agreement with TEM observations.

International audience; The optical properties of metallic spherical nanoparticles embedded in host liquid matrix are studied. Extended Maxwell-Garnett-Mie formulation which accounts for size dispersion, the intrinsic confinement, and extrinsic size effect, is proposed for the calculation of the effective dielectric function and absorption coefficient of size dispersion of colloidal solution of Au and Ag nanoparticles in water. We demonstrate that the size distribution induces an inhomogeneous broadening and an increase of the amplitude of the plasmon band. A large redshift of the plasmon band is also observed for silver nanoparticles. Compared to the conventional Maxwell Garnett theory, we demonstrated that this model gives better description of the measured absorption spectra of colloidal gold solutions.

Among direct-bandgap semiconducting nanomaterials, single-walled carbon nanotubes (SWCNT) exhibit strong quasi-one-dimensional excitonic optical properties, which confer them a great potential for their integration in future photonics devices as an alternative solution to conventional inorganic semiconductors. In this paper, we will highlight SWCNT optical properties for passive as well as active applications in future optical networking. For passive applications, we directly compare the efficiency and power consumption of saturable absorbers (SAs) based on SWCNT with SA based on conventional multiple quantum wells. For active applications, exceptional photoluminescence properties of SWCNT, such as excellent light-emission stabilities with temperature and excitation power, hold these nanometer-scale materials as prime candidates for future active photonics devices with superior performances.

Équipe 104 : Nanomatériaux; International audience; Silicon nanocrystals (Si-NC) with different sizes (2-6 nm) are synthesized by evaporation. The system is composed of a single Si-NC layer that is well controlled in size. The numerical modeling of such system, without a large size distribution, is suitable to perform easily the optical calculations. The nanocrystal size and confinement effects on the optical properties are determined by photoluminescence (PL) measurements, absorption in the UV visible range, and spectroscopic ellipsometry (SE). The optical constants and the bandgap energies are then extracted and analyzed. The dependence of the optical responses with the decrease of the size of the Si-NC occurs not only with a drastic reduction of the amplitudes of dielectric function but also by a significant expansion of the optical gap. This study supports the idea of a presence of a critical size of Si-NC for which the confinement effect becomes weak. The evolution of those bandgap energies are discussed in comparison with values reported in literature.

International audience; We studied the nanosecond laser-induced shape modifications of Ag colloids exposed to a converging laser beam during the ablation of a Ag target in water. To this end, we performed a series of laser ablation experiments in which the laser energy was varied while all other parameters were kept constant. In addition to transmission electron microscopy (TEM), the shape distribution of the Ag nanoparticles was determined by modelling the extinction spectra of the final colloidal solutions using theoretical calculations based on shape distributed effective medium theory (SDEMT). From these calculations, two physical parameters named sphericity and dispersity were introduced and used to gauge the evolution of the shape distribution of the particles. As the laser energy on the target was increased from 5 to 20 mJ/pulse, an apparently abrupt modification of the shape distribution of the particles was evidenced by both TEM and SDEMT calculations. This change is explained in terms of competitive fragmentation, growth and reshaping processes. On the basis the heating-melting-vaporization model, we demonstrate how the competition between these processes, occurring at different locations of the converging beam, determines the shape distribution of the final product. We highlight the relevance of the fluence gradient along the beam path and the laser interaction volume on the laser-induced modifications of the suspended particles during the ablation process.

This paper quantifies the migration of silver contained within mesostructured hybrid silica films and mesoporous titania films under exposure to modulated light. This quantization allows to demonstrate that all of the initial silver salt can be concentrated and reduced in domains accumulating the higher photonic energy. Entirely reduced in the form of nanoparticles of few nanometers size embedded in the silica matrix, silver is then quite stable even under subsequent homogeneous exposures. It is also shown that thanks to the relatively slow nanoparticle growth, successive multiple exposures can be used to create complex 3D microstructures within silica films using a simple dual beam interferometer. In mesoporous titania films, the UV photo-growth of silver nanoparticles remains limited to the vicinity of the film interface because of the matrix absorption and cannot provide deep 3D patterns of silver nanoparticles. However, 2D refractive index patterns can be obtained under UV exposure, erased with visible light and updated thanks to a reversible photochromic behavior. In such films, opposite migration flows of silver species are proven under UV intensity gradient and homogeneous visible exposure.

The composition, the structure and the optical properties of mesostructured hybrid silica films elaborated by sol-gel routine are studied versus the temperature of the post treatment by comparing ellipsometric measurements, atomic force microscopy and electronic microscopy characterizations, X-ray diffraction and thermal analysis. This paper shows that the refractive index variation is a combination between the structure contraction, the solvent evaporation, the silica wall condensation and the pyrolysis of the copolymer. We also investigate the optical properties of thermally grown silver nanoparticles in the mesotructured films and we demonstrate that these properties depend on the optical properties of the host matrix and on the silver concentration profile in the film.

International audience; Ellipsometry is an optical technique that is widely used for determining optical and geometrical properties of optical thin films. These properties are in general extracted from the ellipsometric measurement by solving an inverse problem. Classical methods like the Levenberg–Marquardt algorithm are generally too long, depending on direct calculation and are very sensitive to local minima. In this way, the neural network has proved to be an efficient tool for solving these kinds of problems in a very short time. Indeed, it is rapid and less sensitive to local minima than the classical inversion method. We suggest a complete neural ellipsometric characterization method for determining the index dispersion law and the thickness of a simple SiO2 or photoresist thin layer on Si, SiO2, and BK7 substrates. The influence of the training couples on the artificial neural network performance is also discussed.

International audience; Elaboration of mesostructured silica films with a triblock copolymer polyethylene oxide–polypropylene oxide–polyethylene oxide, (PEO–PPO–PEO) and controlled growth of silver nanoparticles in the mesostructure are described. The films are characterized using UV–visible optical absorption spectroscopy, TEM, AFM, SEM, X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). Organized arrays of spherical silver nanoparticles with diameter between 5 and 8nm have been obtained by NaBH4 reduction. The size and the repartition of silver nanoparticles are controlled by the film mesostructure. The localization of silver nanoparticles exclusively in the upper-side part of the silica-block copolymer film is evidenced by RBS experiment. On the other hand, by using a thermal method, 40nm long silver sticks can be obtained, by diffusion and coalescence of spherical particles in the silica-block copolymer layer. In this case, migration of silver particles toward the glass substratefilm interface is shown by the RBS experiment

Optimal performances of integrated optical devices are obtained by the use of an accurate and reliable characterization method. The parameters of interest, i.e., optical indices and thickness of the waveguide structure, are calculated from effective indices by means of an inversion procedure. We demonstrate how an artificial neural network can achieve such a process. The artificial neural network used is a multilayer perceptron. The first result concerns a simulated anisotropic waveguide. The accuracy in the determination of optical indices and waveguide thickness is 5 x 10(-5) and 4 nm, respectively. Then an experimental application on a silica-titania thin film is performed. In addition, effective indices are measured by m-lines spectroscopy. Finally, a comparison with a classical optimization algorithm demonstrates the robustness of the neural method.

In this work, methods are introduced to the determination of optical properties of thin silver films and nanostructures. We present an optical resonant system consisting of a mirror, a transparent layer and a thin silver film. The layer sequences and the nanostructure of the thin films are investigated by optical methods consist of reflectance measurements. The structures are analyzed by atomic force microscopy and scanning electron microscopy. The optical properties are determined by modeling the reflectance data. We have found that the growth mechanisms of the silver layer are correlated to its optical properties. It also found that temperature treatments produce isolated particles with a narrow plasmon resonance.

Single walled carbon nanotube (SWCNT) networks present outstanding potential for the development of SWCNT-based gas sensors. Due to the complexity of the transport properties of this material, the physical mechanisms at stake during exposure to gas are still under debate. Previously suggested mechanisms are charge transfer between gas molecules and SWCNT and Schottky barrier modulation. By comparing electrical measurements with an analytical model based on Schottky barrier modulation, we demonstrate that one mechanism or the other is predominant depending on the percolation of metallic carbon nanotubes. Below the metallic SWCNT percolation threshold, sensing is dominated by the modulation of the Schottky barrier, while above this threshold, it is only attributed to a charge transfer between SWCNT and gas molecules. Both mechanisms are discussed in terms of sensitivity and resolution leading to routes for the optimization of a gas sensor architecture based on highly enriched semiconducting carbon nanotube films.

A method dedicated to authentication of products submitted to a data inscription during their manufacturing process is suggested. It includes a management of the origin of the machine providing the reference product signature. The strength of the method is based on some links established by turns at the registration stage between all the elements to protect. Specific variations coming from product data photo-inscription physically induce a fundamental link between material and immaterial product components. This link is exploited via a specific signature extraction.