Anne-Laure Fehrembach

Angular tolerant resonant grating filters under oblique incidence

Resonant grating filters have been proposed as a promising alternative to multilayer stacks for narrowband free-space filtering. The efficiency of such filters under normal incidence has been demonstrated. Unfortunately, under oblique incidence, the limited angular tolerance of the resonance forbids any filtering applications with use of standard collimated incident beams. Using a multimode planar waveguide and a bi-atom grating, we show how to increase the angular tolerance up to the divergence of standard beams (0.2 deg) without modifying the spectral bandwidth (0.1 nm), under any oblique angle of incidence.

Phenomenological theory of filtering by resonant dielectric gratings

Using a phenomenological theory of diffraction gratings made by perturbing a planar waveguide allows us to deduce important properties of the sharp filtering phenomena generated by this kind of structure when the incident light excites a guided wave. It is shown that the resonance phenomenon occurring in these conditions acts on one of the two eigenvalues of the Hermitian reflection matrix only. As a consequence, we deduce a mathematical expression of the reflectivity and demonstrate that high-efficiency filtering of unpolarized light requires the simultaneous excitation of two uncoupled guided waves. Numerical examples are given.

Highly directive light sources using two-dimensional photonic crystal slabs

We have designed a microcavity with periodic microstructure that extracts nearly all the power emitted by a luminescent source and confines 80% of the energy radiated in the superstrate in a cone of half width 0.2° about the normal of the device.

Unpolarized narrow-band filtering with resonant gratings

Periodically corrugated planar waveguides present sharp peaks in their reflectivity spectrum due to the excitation, by the incident beam, of an eigenmode of the structure. This property can be used to design free-space narrow-band filters. However, the extreme sensitivity of the resonances with respect to the angle and polarization under oblique incidence, has prevented, up to now, the development of efficient filters with this approach. We succeed in getting round these limitations by exiting four eigenmodes at the same time, and by modifying deeply the eigenmodes properties with an appropriate periodic corrugation.

Enhanced transmission of light through a circularly structured aperture

Using the differential theory of light diffraction by finite cylindrical objects, we study light transmission through a small circular aperture in a metallic screen with concentric corrugation around the nanohole. Poynting vector maps in the region below the screen show that the field enhancement compared with an unstructured aperture is obtained with corrugation lying on the entrance face of the screen. Corrugation on the exit face leads to a more directional radiation close to the normal to the screen. The spectral dependence of the transmission shows a sharp maximum linked with surface plasmon excitation.

Optimization of plasmon excitation at structured apertures.

Surface plasmon excitation that is due to a single or a structured circular aperture in a flat metallic screen is investigated theoretically and numerically with a view to enhancing the electric field close to the metallic surface. A systematic study of the homogeneous solution of the electromagnetic scattering problem is made with cylindrical coordinates, expanding Maxwell equations on a Fourier-Bessel basis. A perturbation analysis devoted to simple physical analyses of different types of cylindrical nanostructure is developed for the optimization of plasmon excitation by a normally incident linearly polarized monochromatic plane wave. The conclusions drawn from this analysis agree well with the results of rigorous electromagnetic calculations obtained with the differential theory of diffraction in cylindrical coordinates.

High performance bi-dimensional resonant grating filter at 850nm under high oblique incidence of ∼60°

We present an experimental demonstration of a polarization quasi-independent narrow-band (less than 0.8nm) filter operating under high oblique incidence. The structure is a resonant grating with a hexagonal lattice which has been carefully designed to ensure polarization quasi-independence of the narrow-band resonance peak over a wide angular range around 60° of incidence. A good agreement between experimental results and theoretical calculations is shown.

Filtering of unpolarized light by gratings

The filtering effect we are dealing with in the present paper is obtained by using a diffraction grating made by perturbing a planar waveguide. A phenomenological theory allows us to deduce important properties of the sharp filtering phenomena generated by this kind of structure when the incident light excites guided waves. It is shown that the resonance phenomenon occurring when a single guided mode is excited acts on a single eigenvalue of the Hermitian reflection matrix. As a consequence, we demonstrate that a high efficiency filtering of unpolarized light requires the simultaneous excitation of two uncoupled guided waves. This criterion is reached when these guided waves propagate in orthogonal directions. Numerical examples are given.

2 × 1D crossed strongly modulated gratings for polarization independent tunable narrowband transmission filters

We design a narrowband polarization independent transmission guided mode resonance filter whose center wavelength is tunable with respect to the angle of incidence. The device is composed of two identical structures assembled back to back. Each half structure is a dielectric multilayer stack in which a grating is engraved. This so-called 2×1D crossed gratings component has already been demonstrated for reflection filtering [Opt. Lett.36, 1662 (2011)OPLEDP0146-959210.1364/OL.36.001662; Opt. Lett.39, 6038 (2014)OPLEDP0146-959210.1364/OL.39.006038]. The functioning in transmission requires the use of a high index material for the grating bumps. For the design, we resort to a clustering global optimization algorithm, used for the first time to our knowledge for grating structures. We demonstrated two filters with a quality factor of about 4000, tunable over more than 15 nm when the angle of incidence varies over a range of 4°, and with a transmittivity at resonance greater than 95% whatever the incident polarization.

Narrow-band filtering with whispering modes in gratings made of fibers

We present a numerical study of whispering modes in gratings made of fibers. Due to the strong localization of the modes inside each fiber, it is possible to obtain narrow-band filters with very broad angular tolerance.