Nathalie Bardou

Highly directional radiation generated by a tungsten thermal source

We report the design of a tungsten thermal source with extraordinarily high directivity in the near infrared, comparable to the directivity of a CO2 laser. This high directivity is the signature of the long-range correlation of the electromagnetic field in the source plane. This phenomenon is due to the resonant thermal excitation of surface-plasmon polaritons.

Total funneling of light in high aspect ratio plasmonic nanoresonators

We demonstrate the total extinction of the reflectivity for a transverse magnetic polarized wave on a gold surface etched on 6% of its area by both narrow (150 nm) and deep (2 μm) grooves. These high aspect ratio metallic grooves were fabricated using a mold cast technique based on an electrolytic growth of gold. They exhibit two resonance peaks corresponding to the first and second cavity modes inside the grooves. We also evidence the incidence-invariance of their spectral response, which undoubtedly shows the localized nature of the resonances. These experimental results confirm the prediction of total funneling of light in very narrow grooves.

Free-standing subwavelength metallic gratings for snapshot multispectral imaging

A mosaic of ten spectral filters has been fabricated in a single 20 mm2 membrane drilled by nanoslits and coated by a gold layer. The nanostructured core-shell gratings exhibit 70% average maximum transmission efficiency in 15% aperture area, which represents a fivefold enhancement compared to the geometrical transmission. This mosaic of bandpass filters regularly spaced in the 3–5 μm wavelength range is used to demonstrate real-time spectral imaging in a multichannel camera.

Guided mode resonance in subwavelength metallodielectric free-standing grating for bandpass filtering

We present the experimental study of a free-standing metallic guided-mode resonant structure, for bandpass filtering applications in the mid-IR wavelength range. Structure consists of a subwavelength gold grating with narrow slits deposited on a silicon nitride membrane. High optical transmission is measured with up to 78% transmission at resonance. Angularly resolved spectra are presented revealing Fano-type resonance.

Optical extinction in a single layer of nanorods.

We demonstrate that almost 100% of incident photons can interact with a monolayer of scatterers in a symmetrical environment. Nearly perfect optical extinction through free-standing transparent nanorod arrays has been measured. The sharp spectral opacity window, in the form of a characteristic Fano resonance, arises from the coherent multiple scattering in the array. In addition, we show that nanorods made of absorbing material exhibit a 25-fold absorption enhancement per unit volume compared to unstructured thin film. These results open new perspectives for light management in high-Q, low volume dielectric nanostructures, with potential applications in optical systems, spectroscopy, and optomechanics.

Ultrathin GaAs Solar Cells With a Silver Back Mirror

We report on the fabrication and characterization of ultrathin GaAs solar cells with a silver back mirror and absorber thicknesses of only t = 120 nm and t = 220 nm. The silver back mirror is combined with localized ohmic contacts. Without antireflection coating, Fabry-Perot resonances lead to strong enhancement over single-pass absorption (up to 4), and external quantum efficiency reaches 0.8 at resonance wavelengths. An analytical model is used to determine the resonance wavelengths and the absorption maxima. A short-circuit improvement of 27% results from the enhanced absorption induced by the Fabry-Perot resonances. By implementing an additional antireflection coating, short-circuit currents reach 16.3 mA/cm2 fort = 120 nm and 20.7 mA/cm2 for t = 220 nm, corresponding to efficiencies of 8.7 % and 12.9 %, respectively.

Angle-resolved transmission measurements through anisotropic two-dimensional plasmonic crystals

We report on high-accuracy angle-resolved optical transmission measurements through anisotropic 2D plasmonic crystals made of gold films with large-area rectangular arrays of nanoscale square holes, deposited on GaAs substrates. The measurements reveal the dispersion relations of air-gold and gold-GaAs surface plasmon polaritons. The crystal anisotropy induces a separation between plasmonic modes propagating in different directions. Their symmetry and dispersion properties are discussed.

High energy resolution five-crystal spectrometer for high quality fluorescence and absorption measurements on an x-ray absorption spectroscopy beamline

Fluorescence detection is classically achieved with a solid state detector (SSD) on x-ray absorption spectroscopy (XAS) beamlines. This kind of detection however presents some limitations related to the limited energy resolution and saturation. Crystal analyzer spectrometers (CAS) based on a Johann-type geometry have been developed to overcome these limitations. We have tested and installed such a system on the BM30B/CRG-FAME XAS beamline at the ESRF dedicated to the structural investigation of very dilute systems in environmental, material and biological sciences. The spectrometer has been designed to be a mobile device for easy integration in multi-purpose hard x-ray synchrotron beamlines or even with a laboratory x-ray source. The CAS allows to collect x-ray photons from a large solid angle with five spherically bent crystals. It will cover a large energy range allowing to probe fluorescence lines characteristic of all the elements from Ca (Z = 20) to U (Z = 92). It provides an energy resolution of 1-2 eV. XAS spectroscopy is the main application of this device even if other spectroscopic techniques (RIXS, XES, XRS, etc.) can be also achieved with it. The performances of the CAS are illustrated by two experiments that are difficult or impossible to perform with SSD and the complementarity of the CAS vs SSD detectors is discussed.

Free-standing guided-mode resonance band-pass filters: from 1D to 2D structures

We study experimentally and theoretically band-pass filters based on guided-mode resonances in free-standing metal-dielectric structures with subwavelength gratings. A variety of filters are obtained: polarizing filters with 1D gratings, and unpolarized or selective filters with 2D gratings, which are shown to behave as two crossed-1D structures. In either case, a high transmission (up to ≈ 79 %) is demonstrated, which represents an eight-fold enhancement compared to the geometrical transmission of the grating. We also show that the angular sensitivity strongly depends on the rotation axis of the sample. This behavior is explained with a detailed description of the guided-mode transmission mechanism.

Spherically bent analyzers for resonant inelastic X-ray scattering with intrinsic resolution below 200 meV.

Resonant inelastic X-ray scattering with very high energy resolution is a promising technique for investigating the electronic structure of strongly correlated materials. The demands for this technique are analyzers which deliver an energy resolution of the order of 200 meV full width at half-maximum or below, at energies corresponding to the K-edges of transition metals (Cu, Ni, Co etc.). To date, high resolution under these conditions has been achieved only with diced Ge analyzers working at the Cu K-edge. Here, by perfecting each aspect of the fabrication, it is shown that spherically bent Si analyzers can provide the required energy resolution. Such analyzers have been successfully produced and have greatly improved the energy resolution in standard spherically bent analyzers.