Unexpected Efficiency of a Luminescent Samarium(III) Complex for Combined Visible and Near-Infrared Biphotonic Microscopy.

An original samarium(III) complex based on a triazacyclononane platform functionalized with a charge-transfer antenna chromophore exhibited optimized brightness and was successfully used as an emissive species for two-photon microscopy experiments in both the visible and near-infrared spectral ranges.

Stimulated Raman scattering microscopy by spectral focusing and fiber-generated soliton as Stokes pulse.

We demonstrate stimulated Raman microscopy with broadband pump and Stokes pulses, using spectral focusing to attain spectral resolution and to rapidly acquire spectra within a spectral window determined by the bandwidth of the pulses. As the Stokes pulse, we use the redshifted soliton generated in a photonic crystal fiber, which allows for simple shifting of the accessible spectral window.

Membrane characterization by microscopic and scattering methods: multiscale structure.

Several microscopic and scattering techniques at different observation scales (from atomic to macroscopic) were used to characterize both surface and bulk properties of four new flat-sheet polyethersulfone (PES) membranes (10, 30, 100 and 300 kDa) and new 100 kDa hollow fibers (PVDF). Scanning Electron Microscopy (SEM) with "in lens" detection was used to obtain information on the pore sizes of the skin layers at the atomic scale. White Light Interferometry (WLI) and Atomic Force Microscopy (AFM) using different scales (for WLI: windows: 900 × 900 µm2 and 360 × 360 µm2; number of points: 1024; for AFM: windows: 50 × 50 µm2 and 5 × 5 µm2; number of points: 512) showed that the membrane roughness increases markedly with the observation scale and that there is a continuity between the different scan sizes for the determination of the RMS roughness.

Whispering gallery modes and other cavity modes for perfect backscattering and blazing.

We demonstrate the possibility to obtain perfect blazing both in Littrow and off-Littrow mountings using diffractive systems consisting of a plane metallic substrate and dielectric structures that can support cavity modes. The resonances are located at a relatively large distance between the metal and the dielectric structure, a condition that prevents the resonance increase of absorption. The high efficiency can be obtained in transverse electric or transverse magnetic polarization and at high incident angles.

Linearly damped modes at gap edges of photonic crystals.

It is shown that for one-dimensional dielectric photonic crystals, the Bloch modes, a vital tool in the analysis of these structures, cannot provide a complete representation of the electromagnetic field at the edges of bandgaps. On these points, the couple of Bloch modes representing the propagation on both sides of the crystal reduces to a single one, with a stationary field, and a complete representation of the field inside the crystal illuminated by a plane wave must include a linearly damped mode (LDM), the amplitude of which behaves linearly in space. The theory of transfer matrices and the use of basic properties of the field allow a precise description of the LDM from a few parameters.

Nature and anisotropy of cortical forces orienting Drosophila tissue morphogenesis.

The morphogenesis of developing embryos and organs relies on the ability of cells to remodel their contacts with neighbouring cells. Using quantitative modelling and laser nano-dissection, we probed the mechanics of a morphogenetic process, the elongation of Drosophila melanogaster embryos, which results from polarized cell neighbour exchanges. We show that anisotropy of cortical tension at apical cell junctions is sufficient to drive tissue elongation.

Multiscale analysis of the laser-induced damage threshold in optical coatings.

We have investigated the influence of laser beam size on laser-induced damage threshold (LIDT) in the case of single- and multiple-shot irradiation. The study was performed on hafnia thin films deposited with various technologies (evaporation, sputtering, with or without ion assistance). LIDT measurements were carried out at 1064 nm and 12 ns with a spot size ranging from a few tens to a few hundreds of micrometers, in 1-on-1 and R-on-1 modes.

Spectral analysis of three-dimensional photonic jets.

The spatial and spectral properties of three-dimensional photonic jets are studied in a framework employing rigorous Lorentz-Mie theory. The contributions to the field from each spectral component are studied quantitatively and highlight the distinctive features of photonic jets. In particular, the role of evanescent field in photonic jets generated by small spheres is investigated.

Multiple holographic optical tweezers parallel calibration with optical potential well characterization.

We report the extension to a multi-axes exploration of the potential well reconstruction method against drag force to simultaneously characterize the potential wells of several trapping sites generated with holographic optical tweezers. The final result is a robust, fast and automatic procedure we use to characterize holographic tweezers. We mainly focus on the reliability of the method and its application to address the dependence of the diffraction efficiency with the trap position in a given holographic traps pattern.

Direct imaging of photonic nanojets.

We report the direct experimental observation of photonic nanojets created by single latex microspheres illuminated by a plane wave at a wavelength of 520 nm. Measurements are performed with a fast scanning confocal microscope in detection mode, where the detection pinhole defines a diffraction-limited observation volume that is scanned in three dimensions over the microsphere vicinity. From the collected stack of images, we reconstruct the full 3 dimensional photonic nanojet beam.

Total absorption of unpolarized light by crossed gratings.

We present both experimental and numerical data showing the absorption of unpolarized, normally incident light by a gold crossed grating having a shallow sinusoidal profile. We show furthermore that the total absorption of unpolarized light can be achieved for an angle of incidence of 30 degrees with a crossed grating having its period adjusted appropriately from the normal incidence case to preserve the plasmonic resonance responsible for the enhanced absorptance. We contrast the process for achieving high absorptance in the principal plane of incidence aligned with the grooves of one of the gratings, with that for the principal plane at 45 degrees to each grating.

Laser damage resistance of hafnia thin films deposited by electron beam deposition, reactive low voltage ion plating, and dual ion beam sputtering.

A comparative study is made of the laser damage resistance of hafnia coatings deposited on fused silica substrates with different technologies: electron beam deposition (from Hf or HfO(2) starting material), reactive low voltage ion plating, and dual ion beam sputtering. The laser damage thresholds of these coatings are determined at 1064 and 355 nm using a nanosecond pulsed YAG laser and a one-on-one test procedure. The results are associated with a complete characterization of the samples: refractive index n measured by spectrophotometry, extinction coefficient k measured by photothermal deflection, and roughness measured by atomic force microscopy.

Determination of the refractive indices of layers in a multilayer stack by a guided-wave technique.

The m-lines technique is used to measure the refractive indices and thicknesses of layers embedded in a multilayer stack. The multilayer considered is deposited by ion plating. Its formula is silica-H-L-H-L-H-air, where H and L denote Ta(2)O(5) and SiO(2)lambda/4 layers, respectively, with lambda = 514.

Light emission from sources located within metallodielectric planar microcavities.

A simple, rigorous electromagnetic formula is derived for predicting the electromagnetic power provided by sources located in transparent or dissipative planar microcavities. With this simple approach, we compare numerically and experimentally the electromagnetic power that escapes the microcavity when the source is located in a metallodielectric or in an all-dielectric resonant planar structure. Although a strong light-extraction coefficient might be expected for metallodielectric microcavities, we show that these attractive structures suffer from metal absorption even when thin metallic layers are used.

Minimum description length synthetic aperture radar image segmentation.

We present a new minimum description length (MDL) approach based on a deformable partition--a polygonal grid--for automatic segmentation of a speckled image composed of several homogeneous regions. The image segmentation thus consists in the estimation of the polygonal grid, or, more precisely, its number of regions, its number of nodes and the location of its nodes. These estimations are performed by minimizing a unique MDL criterion which takes into account the probabilistic properties of speckle fluctuations and a measure of the stochastic complexity of the polygonal grid.

Toward the simulation of the strain of female pelvic organs.

Surgical simulators have undergone a significant development, especially since the rise of mini-invasive surgery. The main simulators of digestive surgery have been developed for solid organs such as the liver and spleen. Studies relating to soft tissues like the pelvic organs are rare.

Non-Bloch plasmonic stop-band in real-metal gratings.

Recent studies of plasmon surface wave (PSW) propagation in short-period perfectly conducting gratings have shown formation of stop-band that are not linked to the interaction between two (counter) propagating surface waves. We study the properties of this stop-band in real metals. While for both perfectly conducting and real metals the propagation constant of PSW grows with the groove height, the stop-band in real metals appears for groove heights significantly smaller than in perfect metals.

Nonparametric statistical snake based on the minimum stochastic complexity.

We propose a nonparametric statistical snake technique that is based on the minimization of the stochastic complexity (minimum description length principle). The probability distributions of the gray levels in the different regions of the image are described with step functions with parameters that are estimated. The segmentation is thus obtained by minimizing a criterion that does not include any parameter to be tuned by the user.

Light-scattering characterization of transparent substrates.

Angle-resolved light scattering has been used for decades to quantify the surface roughness of optical components. However, because this technique is affected by the contribution of both interfaces of the sample, it cannot be applied to transparent substrates. We show how to overcome this issue and apply these principles to the characterization of superpolished samples.

Photothermal deflection in multilayer coatings: modeling and experiment.

A model of the photothermal deflection signal in multilayer coatings is presented that takes into account optical interference effects and heat flow within the stack. Measurements are then taken of high-reflectivity HfO2/SiO2 ultraviolet mirrors made by plasma ion assisted deposition and compared to calculations. Good agreement is found between the experimental results and the model.

Determination of refractive indices of opaque rough surfaces.

The refractive indices of optical materials are usually determined from spectrophotometric and ellipsometric measurements of specular beams. When the roughness of the interfaces increases, the energy in the specularly reflected and transmitted beams decreases and scattering becomes predominant. For strong roughness (compared to the incident wavelength) a surface does not exhibit specular reflection or transmission, making difficult the determination of the refractive index.

Optimized metrology for laser-damage measurement: application to multiparameter study.

An automatic test apparatus for refined testing of laser damage is presented that permits an in situ analysis of the tested area before, during, and after pulsed irradiation. Spatial and temporal beam profiling are performed in real time and give access to the localized fluence for each shot. Furthermore, an optimization of the initiation of damage detection is undertaken by use of image processing and yields a resolution better than 1 microm.

In situ and air index measurements: influence of the deposition parameters on the shift of TiO2/SiO2 Fabry-Perot filters.

We measure the refractive index of thin films of TiO2 and SiO2 for given deposition parameters. Two complementary methods are used. The first is a postdeposition technique which uses the measurements of reflectance and transmittance in air.