Somatosensory cortical facilitation during step preparation restored by an improved body representation in obese patients.

Background: The anticipatory postural adjustments (APA) associated with step initiation are impaired in obese patients (e.g. longer duration, greater lateral center of pressure excursion).

Critical coupling and extreme confinement in nanogap antennas.

Nanogap antennas are compelling structures for squeezing light into ultrasmall volumes. However, when gaps are shrunk to the nanometer scale, the mode losses dramatically increase. In this Letter, we report the conditions of critical coupling between the arrays of nanogap resonant metal-insulator-metal (MIM) antennas and free space.

Ultrathin mono-resonant nano photovoltaic device for broadband solar conversion.

Nano-resonators can be used in photovoltaics to drastically improve the ability of the device to absorb light and generate photo-carriers, therefore enabling a reduction of the absorber volume. Conventionally, the harvest of the spectrally broad solar spectrum is achieved via the tedious engineering of multiple optical resonances. In this paper, we propose a breakthrough approach, which consists in reducing the solar spectral range with a spectral conversion layer to match only one resonance that can then be easily designed.

Pixel-sized infrared filters for a multispectral focal plane array.

We present a theoretical study of guided-mode-resonance filters made of two sub-wavelength metallic gratings and a dielectric waveguide, with lateral geometries compatible with the size of infrared focal plane array pixels. Contrary to most of the studies described in the literature, we consider here a focused beam on finite-sized filters, and we investigate the optical properties of a mosaic of 30 μm-long filters. We demonstrate the spectral filtering ability and low crosstalk of such components.

Field extension inside guided-mode-resonance filters under a focused beam.

We present a theoretical study of the mid-infrared guided mode resonance spectral filters made of two subwavelength metallic gratings and a dielectric waveguide under a focused beam with a finite spot size. This Letter shows that, at the resonant wavelength, the lateral extension of the electromagnetic field in the waveguide is close to the width of the beam. We compare the performance of filters using gratings with a one-slit pattern and gratings with a two-slit pattern, and we show that the latter gratings (biatom gratings) provide a higher transmission and a better limitation of field extension, due to an improved angular acceptance.

Integration of nanostructured planar diffractive lenses dedicated to near infrared detection for CMOS image sensors.

This paper deals with the integration of metallic and dielectric nanostructured planar lenses into a pixel from a silicon based CMOS image sensor, for a monochromatic application at 1.064 μm. The first is a Plasmonic Lens, based on the phase delay through nanoslits, which has been found to be hardly compatible with current CMOS technology and exhibits a notable metallic absorption.

Two-mode model for metal-dielectric guided-mode resonance filters.

Symmetric metal-dielectric guided-mode resonators (GMR) can operate as infrared band-pass filters, thanks to high-transmission resonant peaks and good rejection ratio. Starting from matrix formalism, we show that the behavior of the system can be described by a two-mode model. This model reduces to a scalar formula and the GMR is described as the combination of two independent Fabry-Perot resonators.

Extraordinary transmission in optical Helmholtz resonators.

Optical Helmholtz resonators (OHRs) have been adapted from acoustics designs for light absorbing structures, exhibiting extreme light confinement. Here, extraordinary transmission of light is theoretically demonstrated through symmetric OHRs, comprising a cavity with two λ/500 narrow slits on either side. This device has appealing features to act as a spectral bandpass filter in the context of multispectral imaging, in particular its high angular tolerance because of the localized nature of the resonance.

Electromagnetic modelization of spherical focusing on a one-dimensional grating thanks to a conical B-spline modal method.

Focusing light onto nanostructures thanks to spherical lenses is a first step in enhancing the field and is widely used in applications. Nonetheless, the electromagnetic response of such nanostructures, which have subwavelength patterns, to a focused beam cannot be described by the simple ray tracing formalism. Here, we present a method for computing the response to a focused beam, based on the B-spline modal method adapted to nanostructures in conical mounting.

Fast modal method for crossed grating computation, combining finite formulation of Maxwell equations with polynomial approximated constitutive relations.

We present a modal method for the fast analysis of 2D-layered gratings. It combines exact discrete formulations of Maxwell equations in 2D space with polynomial approximations of the constitutive equations, and provides a sparse formulation of the eigenvalue equations. In specific cases, the use of sparse matrices allows us to calculate the electromagnetic response while solving only a small fraction of the eigenmodes.

Analytical description of subwavelength plasmonic MIM resonators and of their combination.

We show that a periodic array of metal-insulator-metal resonators can be described as a high refractive index metamaterial. This approach permits to obtain analytically the optical properties of the structure and thus to establish conception rules on the quality factor or on total absorption. Furthermore, we extend this formalism to the combination of two independent resonators.

Metal-dielectric bi-atomic structure for angular-tolerant spectral filtering.

We theoretically study metal-dielectric structures made of bi-atomic metallic gratings coupled to a guided-mode dielectric resonator. The bi-atomic pattern grating allows tailoring of the Fourier spectrum of the inverse grating permittivity in order to adapt the frequency gap and obtain a flat dispersion band over a wide angular range. A significant enhancement (two-fold) of the angular tolerance as compared to a simply periodic structure is obtained.

Mason's rule and signal flow graphs applied to subwavelength resonant structures.

Widely used for the design of resonant electronic devices, Mason's scalar rule is adapted here to the study of resonant subwavelength optical structures. It turns out to be an efficient formalism, especially when dealing with multiple wave interference mechanisms. Indeed it allows to comprehend the underlying physical mechanisms of the structure in a straightforward way and fast analytical formulae can be retrieved.

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.

Complex optical index of single wall carbon nanotube films from the near-infrared to the terahertz spectral range.

We retrieve the complex optical index of single-walled carbon nanotube (CNT) films in the 0.6-800 μm spectral range. Results are obtained from a complete set of optical measurements, reflection and transmission, of free-standing CNT films using time domain spectroscopy in the terahertz (THz) and Fourier transform infrared (IR) spectroscopy in the visible-IR.

Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas.

In this Letter, we demonstrate experimentally that a patchwork of four metal-insulator-metal patches leads to an unpolarized wideband omnidirectional infrared absorption. Our structure absorbs 70% of the incident light on a 2.5 μm bandwidth at 8.

Harvesting light at the nanoscale by GaAs-gold nanowire arrays.

A nanoscale metal-semiconductor-metal photodetector with a 40 nm-thick GaAs absorbing layer has been studied numerically and experimentally. A gold nanowire array is the top mirror of a Fabry-Perot cavity and forms interdigitated Schottky contacts. Nearly perfect absorption is achieved in TE polarization.

Light funneling mechanism explained by magnetoelectric interference.

We investigate the mechanisms involved in the funneling of optical energy into subwavelength grooves etched on a metallic surface. The key phenomenon is unveiled thanks to the decomposition of the electromagnetic field into its propagative and evanescent parts. We unambiguously show that the funneling is not due to plasmonic waves flowing toward the grooves, but rather to the magnetoelectric interference of the incident wave with the evanescent field, this field being mainly due to the resonant wave escaping from the groove.

Perfect extinction in subwavelength dual metallic transmitting gratings.

We investigate the strong electromagnetic coupling that settles in dual metallic grating structures. This coupling is evidenced to lead to a perfect optical extinction in the transmission spectrum. The behavior of this perfect extinction that strongly depends on the longitudinal space and the lateral displacement between the two gratings can be explained by a simple model that describes the interference between a propagating mode and a couple of evanescent modes.

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.

Reduced scattering-matrix algorithm for high-density plasmonic structures.

We describe a method to compute S-matrix interface terms using a selection of eigenmodes. When solving the modal equation, the computation of left and right eigenvectors leads to rectangular eigenmodes matrices. Expressions of S-matrix interface terms are then expressed so as to allow for a significant reduction of the computation cost.

Fast modal method for subwavelength gratings based on B-spline formulation.

We present a B-spline modal method for analyzing a stack of complex structured layers. Thanks to a B-spline approximation of the field, we solve the Maxwell equations. Diffraction calculation is based on the scattering matrices algorithm.

Tailoring radiative and non-radiative losses of thin nanostructured plasmonic waveguides.

Thin nanostructured metal films allow to control radiative and non-radiative losses of surface plasmon polariton modes without changing their group velocities. This effect is studied in plasmonic waveguides made of thin gold films drilled with very narrow slits and deposited on a GaAs substrate. The analysis is supported by high-resolution angle-resolved transmission measurements and rigorous electromagnetic calculations.

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.

Waveguiding in nanoscale metallic apertures.

We study the optical properties of subwavelength metallic waveguides made of nanoscale apertures in a metal. We develop analytical expressions for the fundamental optical modes in apertures. The results are in excellent agreement with finite element calculations.