Jean-Marc Frigerio

Estimate of the degree of inhomogeneity of the refractive index of dielectric films from spectroscopic ellipsometry

Dielectric thin films often present microstructures that give rise to a variation of the refractive index with the distance from the substrate. We propose a method of analysis of ellipsometric data for homogeneous and slightly inhomogeneous films that are deposited on transparent substrates. Assuming a linear refractive-index gradient, we are able to determine not only the average index and the thickness but also the degree of inhomogeneity of the films by spectroscopic ellipsometry at variable angles of incidence. We apply this method to titanium dioxide films deposited on glass, which present different degrees of inhomogeneity depending on the preparation conditions.

Controlled modification of single colloidal CdSe/ZnS nanocrystal fluorescence through interactions with a gold surface.

Single colloidal CdSe/ZnS nanocrystals are deposited at various distances from a gold film in order to improve their performance as single photon sources. Photon antibunching is demonstrated and the experimental curves are accurately fitted by theoretical equations. Emission lifetime and intensity are measured and found in excellent agreement with theoretical values. The various effects of a neighbouring gold film are discussed : interferences of the excitation beam, interferences of the fluorescence light, opening of plasmon and lossy-surface-wave modes, modification of the radiation pattern leading to a modified objective collection efficiency. At 80 nm from the gold film, when using an objective with 0.75 numerical aperture, about a 2.4-fold increase of the detected intensity is evidenced.

Manipulating emission of CdTeSe nanocrystals embedded in three-dimensional photonic crystals

We report experimental and theoretical results on the photoluminescence of CdTeSe nanocrystals, embedded in a silica opaline structure by infiltration of a highly diluted solution. Strong modification of emission diagrams of embedded nanocrystals have been observed in good agreement with theoretical models. At macroscopic scale, we measured the difference of nanocrystals emission lifetime embedded either in an opal for which the emission is in the gap, or in an opal of smaller balls diameter for which the emission is outside the gap. The photonic bandgap effect leads to a lifetime increase of the order of 10%. These lifetime variations are shown to be in good agreement with the calculated local density of states modification due to the pseudogap.

Raman investigation of orthorhombic MII Ga2 (S, Se)4 compounds

Raman investigation of isostructural polycrystalline MII Ga 2 (S, Se)4 compounds with MII = Pb, Sr, Eu, Yb and Ca was performed. We observed that the effect of MII atomic mass on the phonon frequencies is negligible and that the vibrations are essentially influenced by the sizes of the MII cations. As the effects of cation substitution are small we concluded that the orthorhombic thiogallates MII Ga 2 S4 exhibit nearly the same phonon energies: about 280 cm−1 (~35 meV) and 360 cm−1 (~45 meV) for the most intense vibration modes. The substitution of S atoms by heavier Se atoms causes the shift of the Raman spectrum to lower frequencies. The orthorhombic MII Ga 2 Se4 compounds present phonon energy at about 185 cm−1 (~23 meV) for the most intense vibration mode. Results confirm that a molecular model is more adequate to describe the vibrations of these compounds than the factor-group analysis.

Effective medium equivalent to a symmetric multilayer at oblique incidence

It is shown that a multilayer with repeated symmetric unit structure is equivalent to a homogeneous and optically biaxial medium for obliquely incident light, the three principal axes being normal to the plane of incidence, normal to the film plane, and normal to the other two. Unit thickness dependence and incident angle dependence of the dielectric tensor and the effective thickness are calculated using exact formulas. It is found that, at small unit structure thickness, the equivalent medium is almost uniaxial, the optical axis being normal to the film, and the effective thickness is very close to the mechanical one. Absorbing films are also envisaged and the domain of validity (in terms of unit structure thickness) of the uniaxial equivalent medium is defined by numerical examples.

Hysteresis of the metal-insulator transition of ; evidence of the influence of microscopic texturation

Vanadium dioxide films have been deposited on silicon substrates by reactive RF cathodic sputtering from and targets. The optical measurements show a good contrast at the semiconductor-metal transition, but exhibit two kinds of hysteresis cycle: a narrow and symmetrical one, and a wider and asymmetrical one. We have mainly studied the microstructure of these samples by means of grazing-incidence x-ray diffraction at the LURE synchrotron radiation facility. We analysed in detail the intensities of the diffraction peak spectra and of the portions of diffraction rings. We found a clear relation between a narrow and symmetrical hysteresis cycle, and a (011) texture of the films. We attribute this result to an improvement in the cooperativity of the transition phenomena.

Determination of refractive-index profiles by a combination of visible and infrared ellipsometry measurements.

Model inhomogeneous silicon oxynitride films were produced by ion-beam sputtering and characterized by ellipsometry in the visible and infrared ranges. These films exhibit strong intentional gradients of the refractive index that cannot be considered linear. A discrete description of the index profile with a few layers or a continuous description with a polynomial are examined; regressions by the use of measurements in the visible only or in the total spectral range (visible and infrared) are performed. Acquisition of data in an extended range is found to be a guarantee of the reliability of the calculated index profiles.

Determination of the Absorption and Scattering Coefficients of Pigments: Application to the Identification of the Components of Pigment Mixtures

In most scattering media, the scattering centers have random shapes and sizes, preventing any theoretical approach for the determination of their absorption and scattering coefficients. Three different experimental protocols are here selected and briefly described. The obtained values are compared and then validated from a comparison with the most rigorous method. The Kubelka–Munk method, using a paint layer applied to both white and black backgrounds, is chosen to build a database of these coefficients for reference pigments. Using this database, the identification of the components of a pigment mixture using spectral measurements is realized and validated.

Optical coherence tomography: non-destructive imaging and spectral information of pigments

Optical Coherence Tomography (OCT) is a non-invasive and non-destructive technique which is very interesting for the study of works of art. Based on a Michelson interferometer, our device has been adapted for the recording of information in the visible range. This system allows three-dimensional imaging of painting with a micrometer resolution. 3D imaging of pictorial layers is presented where the pigments embedded in the binder are clearly visible. Furthermore, an adapted signal processing gives access to the spectral information issued from pigments. Tests on pigments in powder are presented and show that their spectral responses are different.

Skin color modeling using the radiative transfer equation solved by the auxiliary function method

The auxiliary function method is an efficient technique for solving the radiative tranfer equation without adding any assumption and was applied until now only for theoretical stratified media. The first application (to our knowledge) of the method to a real case, the human skin, is presented. This makes it possible to validate the method by comparing model results with experimental reflectance spectra of real skin. An excellent agreement is obtained for a multilayer model of the skin made of 22 sublayers and taking into account the anisotropic phase function of the scatterers. Thus there is the opportunity to develop interest in such models by quantitatively evaluating the influence of the parameters commonly used in the literature that modify skin color, such as the concentration of the scatterers and the thickness of each sublayer.