Design of metallic nanoparticles gratings for filtering properties in the visible spectrum

Plasmonic resonances in metallic nanoparticles are exploited to create efficient optical filtering functions. A Finite Element Method is used to model metallic nanoparticles gratings. The accuracy of this method is shown by comparing numerical results with measurements on a two-dimensional grating of gold nanocylinders with elliptic cross section. Then a parametric analysis is performed in order to design efficient filters with polarization dependent properties together with high transparency over the visible range. The behavior of nanoparticle gratings is also modelled using the Maxwell-Garnett homogenization theory and analyzed by comparison with the diffraction by a single nanoparticle. The proposed structures are intended to be included in optical systems which could find innovative applications.Comment: submitted to Applied Optic

Waveguide mode in the box with an extraordinary flat dispersion curve

The extraordinary flattening of the dispersion curve of the so-called cavity resonator integrated guided-mode resonance filters (CRIGFs) is analyzed and explained as due to the intramode coupling imposed by the external Bragg resonators. CRIGFs are composed of a grating coupler (guided-mode resonance filter, GMRF) put between two distributed Bragg reflectors (DBRs). They form a cavity box in which the excited guided mode is confined. This confinement provides resonances with small spectral width (smaller than 1 nm for optical wavelengths) and extraordinary wide angular acceptance (several degrees). At a first glance, one may think that similar performances could be obtained while putting the GMRF and the DBR one above the other, forming a so-called "doubly periodic" grating, as in this configuration also the DBR confines the mode. Yet, the angular acceptance of CRIGFs is an order of magnitude greater than in classical gratings, even with complex pattern. The aim of the present paper is to identify the phenomenon responsible for the extraordinary large angular acceptance of CRIGFs. We numerically calculate, for the first time to the best of our knowledge, the dispersion curve of the mode excited in the CRIGF. The dispersion curve shows a flat part, where the resonance wavelength is quasi-independent of the angle of incidence, and the flattening grows with the width of the Bragg reflector. We develop an approximate coupled four-wave model, which predicts the extraordinary flattening as a consequence of an additional coupling of the waveguide modes of the GMRF provided by the Bragg grating, that does not exist in the "doubly periodic" gratings

Comparative study of total absorption of light by two-dimensional channel and hole array gratings

International audienceA detailed study of light absorption by silver gratings having two-dimensional periodicity is presented for structures constructed either of channels or of holes with subwavelength dimensions. Rigorous numerical modelling shows a systematic difference between the two structures: hole (cavity) gratings can strongly absorb light provided the cavity is sufficiently deep, when compared to the wavelength, whereas very thin channel gratings can induce total absorption. A detailed analysis is given in the limit when the period tends towards zero, and an explanation of the differences in behavior is presented using the properties of effective optical index of the metamaterial layer that substitutes the periodical structure in the limit when the period tend to zero

Plasmonic antiresonance through subwavelength hole arrays

International audienceIt has been shown both experimentally and numerically that the phenomenon of extraordinary transmission through subwavelength hole arrays is generally associated with a drop in transmission located very close to it. Paradoxically, this antiresonant drop occurs at the wavelength that, at first glance, should provoke a resonant excitation of a surface plasmon propagating along the metallic surface of the screen. The present paper gives a theoretical demonstration of this phenomenon, which dispels the paradox. Our theory is supported by numerical calculations

Electro-optic effect in BaTiO3 for spectral tuning of narrowband resonances

International audienceWe study the electro-optic (E-O) properties of a BaTiO3 thin layer placed in a stack of dielectric layers, including a subwavelength diffraction grating with a two-dimensional periodicity, aiming to tune spectrally the position of the resonant reflection peak that is used for narrowband optical filtering. BaTiO3 is chosen due to its strong E-O properties. When an external electric field is applied to the E-O layer, it leads to a spectral shift of the resonant peak. We study numerically different configurations with either weak or strong spectral tunability, presenting some arguments to explain these different behaviors. Taking into account only the linear part of the E-O effect (Pockels effect), the tuning of the peak that has 0.1 nm spectral width is approximately 33 nm for a 1.5×10Exp7 V/m applied field. The shift is multiplied by three (97 nm) when also taking into account the quadratic E-O effect

Semi-phenomenological effective permittivity approach to metallic periodic structures

International audienceA detailed review of the theory of effective permittivity for one- and two-dimensional periodic structures shows its limited validity for metal-dielectric structures in the visible and near infra-red if the feature dimensions are comparable with the metal skin depth. We propose a phenomenological correction to the static formulae using a realistic assumption for the electric field behavior inside the metal features. This approach allows us to obtain analytical expressions for the effective permittivity in the case when the electric field is not sufficiently homogeneous within the unit cell of the gratings. A comparison with the numerical results of the Fourier modal method demonstrates the validity of the analytical formulae. Additional study is made on the impedance approximation at the outer boundaries of the periodical structure in order to propose analytical formulae for the reflection coefficient that permits better correspondence with the numerical results. The link between the values of effective permittivity and permeability defined as the ratios between the averaged fields, and the metamaterial permittivity and permeability is discusse

Optimization of plasmon excitation at structured apertures

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Nonlinear conversion in Cavity-Resonator Integrated Grating Filters

In this manuscript, we review the recent progress made in nonlinear parametric conversion in grating-coupled Fabry-Pérot planar microcavities known as Cavity-Resonant Integrated Grating Filters (CRIGFs). Having previously established that enhanced second harmonic generation can be obtained in these devices, we discuss the design and experimental demonstrations of technical implementations allowing the achievement of critical coupling and improved conversion efficiencies