Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.Comment: 5 page

Contribution à l'étude des structures phononiques et photoniques unidimensionnelles périodiques et quasi-périodiques (super-réseaux solide-fluide et guides monomodes)

Ce travail de thèse présente une contribution à l'étude de la propagation et la localisation des ondes acoustiques et électromagnétiques dans les cristaux phononiques et photoniques unidimensionnels. Notre intérêt a porté principalement sur la propagation des ondes acoustiques dans les matériaux multicouches périodiques de type solide-fluide et les ondes électromagnétiques dans les guides d'ondes monomodes quasipériodiques. L'objectif étant de chercher de nouveaux matériaux avec des bandes interdites larges et étudier l'effet de la présence des inhomogénéités (défaut) dans ces structures telles que : la surface libre, l'interface avec un substrat homogène ou l'existence d'une cavité. Ces résultats sont obtenus à partir d'un calcul analytique détaillé des fonctions de Green qui nous a permis de déterminer les relations de dispersion, les densités d'états locale et totale et les différents coefficients de transmission et de réflexion ainsi que les temps de phase correspondants. Dans le cas des ondes acoustiques de polarisation sagittale dans les super-réseaux solide-fluide, nous avons mis en évidence l'existence et le comportement des modes de surface et d'interface ainsi qu'une règle générale sur l'existence de ces modes. Aussi, nous avons montré que ces systèmes peuvent présenter des gaps omnidirectionnels qui réfléchissent les ondes quelque soit l'angle d'incidence (miroirs acoustiques) ainsi qu'une transmission sélective à travers les modes de défaut et d'interface (filtres acoustiques). Dans le cas des ondes électromagnétiques dans les guides d'ondes monomodes quasi-périodiques (de type Fibonacci) à base de câbles coaxiaux, nous avons mis en évidence certaines propriétés liées à ces systèmes telles que l'auto-similarité avec un facteur d'échelle dans le cas d'une seule séquence de Fibonacci. Aussi, II a été montré que ces systèmes peuvent présenter dans certaines régions de fréquences, des vitesses subluminiques et superluminiques. Dans le cas des séquences périodiques de Fibonacci, nous avons montré des propriétés liées aux bandes permises telles que la fragmentation des spectres de fréquences selon une loi en puissance ainsi que deux types de modes de surface. Les modes de volume et de surface dans ces structures peuvent présenter des comportements liés aux systèmes multifractals. Tous ces résultats sont obtenus à partir de la mesure de l'amplitude et la phase de la transmission à travers ces cristaux photoniques placés soit horizontalement entre deux guides soit verticalement sur un guide. Les résultats expérimentaux sont en bon accord avec les résultats théoriques.LILLE1-BU (590092102) / SudocSudocFranceF

Trapped-mode-induced Fano resonance and acoustical transparency in a one-dimensional solid-fluid phononic crystal

International audienceWe investigate theoretically and numerically the possibility of existence of Fano and acoustic-induced transparency (AIT) resonances in a simple though realistic one-dimensional acoustic structuremade of solid-fluid layers inserted between two fluids. These resonances are obtained by combining appropriately the zeros of transmission (antiresonance) induced by the solid layers and the local resonances induced by the solid or combined solid-fluid layers with surface free boundary conditions. In particular, we show the possibility of trapped modes, also called bound states in continuum, which have recently found a high renewal interest. These modes appear as resonances with zero width in the transmission spectra as well as in the density of states (DOS). We consider three different structures: (i) a single solid layer inserted between two fluids. This simple structure shows the possibility of existence of trapped modes, which are discrete modes of the solid layer that lie in the continuum modes of the surrounding fluids. We give explicit analytical expressions of the dispersion relation of these eigenmodes of the solid layer which are found independent of the nature of the surrounding fluids. By slightly detuning the angle of incidence from that associated to the trapped mode, we get a well-defined Fano resonance characterized by an asymmetric Fano profile in the transmission spectra. (ii) The second structure consists of a solid-fluid-solid triple layer embedded between two fluids. This structure is found more appropriate to show both Fano and acoustic-induced transparency resonances. We provide detailed analytical expressions for the transmission and reflection coefficients that enable us to deduce a closed-form expression of the dispersion relation giving the trapped modes. Two situations can be distinguished in the triple-layer system: in the case of a symmetric structure (i.e., the same solid layers) we show, by detuning the incidence angle theta, the possibility of existence of Fano resonances that can be fitted following a Fano-type expression. The variation of the Fano parameter that describes the asymmetry of such resonances as well as their width versus theta is studied in detail. In the case of an asymmetric structure (i.e., different solid layers), we show the existence of an incidence angle that enables to squeeze a resonance between two transmission zeros induced by the two solid layers. This resonance behaves like an AIT resonance, its position and width depend on the nature of the fluid and solid layers as well as on the difference between the thicknesses of the solid layers. (iii) In the case of a periodic structure (phononic crystal), we show that trapped modes and Fano resonances give rise, respectively, to dispersionless flat bands with zero group velocity and nearly flat bands with negative or positive group velocities. The analytical results presented here are obtained by means of the Green's functionmethod which enables to deduce in closed form: dispersion curves, transmission and reflection coefficients, DOS, as well as the displacement fields. The proposed solid-fluid layered structures should have important applications for designing acoustic mirrors and acoustic filters as well as supersonic and subsonic materials

Acoustic demultiplexer based on Fano and induced transparency resonances in slender tubes

We give an analytical demonstration of the possibility to realize a simple phononic demultiplexer based on Fano and acoustic induced transparency resonances. The demultiplexer consists of a Y-shaped waveguide with an input line and two output lines. Each output line contains two stubs grafted either at a given position or at two positions far from the input line. We derive in closed form the expressions for a selective transfer of a single propagating mode through one line keeping the other line unaffected

Surface acoustic waves interaction with confined acoustic phonons in a coupled nanoridges dimer set atop of a multilayer design

AbstractWe present a comprehensive study using finite element numerical analysis of the acoustic localized phonons supported by a gold nanoridges dimer-based multilayer design. The latter consists in a SiO2-substrate over which a gold film covered with a thin polymer is deposited. We investigate first the mechanical eigen-modes analysis of a single monomer ridge, where we find flexural and compressional type modes in the sub-GHz frequency range. This is realized by either setting the ridge in a periodic structure, which enables to get the dispersion curve of the modes, or by considering an isolated system bounded by perfect matched layers, where we use the equivalent of the local density of states to track the modes. A good agreement is obtained between the two methods. Similarly, we find in case of the coupled dimer ridges hybridized modes, namely the flexural and compressional modes of a monomer split-up into in- and -out-of-phase type modes. We demonstrate efficient coupling between the monomer/dimer localized phonons with surface acoustic waves (SAWs) as the simulated transmission spectra show dips at the frequencies of the monomer/dimer eigenmodes. For symmetry reasons, some of the dimer modes are expected to be optomechanically active. The proposed SAW-based device is meant to help design acousto-optic modulators or ultrasensitive sensors

Optical transmission properties of an anisotropic defect cavity in one-dimensional photonic crystal

International audienceWe investigate theoretically the possibility to control the optical transmission in the visible and infrared regions by a defective one dimensional photonic crystal formed by a combination of a finite isotropic superlattice and an anisotropic defect layer. The Green's function approach has been used to derive the reflection and the transmission coefficients, as well as the densities of states of the optical modes. We evaluate the delay times of the localized modes and we compare their behavior with the total densities of states. We show that the birefringence of an anisotropic defect layer has a significant impact on the behavior of the optical modes in the electromagnetic forbidden bands of the structure. The amplitudes of the defect modes in the transmission and the delay time spectrum, depend strongly on the position of the cavity layer within the photonic crystal. The anisotropic defect layer induces transmission zeros in one of the two components of the transmission as a consequence of a destructive interference of the two polarized waves within this layer, giving rise to negative delay times for some wavelengths in the visible and infrared light ranges. This property is a typical characteristic of the anisotropic photonic layer and is without analogue in their counterpart isotropic defect layers. This structure offers several possibilities for controlling the frequencies, transmitted intensities and the delay times of the optical modes in the visible and infrared regions. It can be a good candidate for realizing high-precision optical filters. (C) 2017 Elsevier B.V. All rights reserved

Terahertz multi-plasmon induced reflection and transmission and sensor devices in a graphene-based coupled nanoribbons resonators

International audienceWe demonstrate numerically multi-plasmon-induced reflection (PIR), multi-plasmon-induced transparency (PIT) and propose a highly sensitive refractive index nanosensor in a novel ultra-compact graphene-based nano-device, operating in the terahertz frequency range. The base structure of the system is composed out of a bus waveguide made out of graphene coupled to resonators inserted along or aside the waveguide where each resonator is constituted by a set of two coupled graphene nano-ribbons (CGNR). The latters behave as a molecule-like entity, whose surface plasmon eigen-modes originate from the splitting of the single ribbons modes. We show that the two CGNRs can also be regarded as an effective rectangular cavity-like system, so that the insertion of the latter along or aside the graphene bus waveguide (GBW) enables the design of tunable selective or rejective filters, respectively. By inserting two identical effective cavities in the structure, into a Δ-like state (in analogy with three level atomic systems), we show the possibility of realizing multiple plasmon induced reflection, and highlight its fast light-features. On the other hand, if the two identical cavities are set symmetrically on each side of the GBW with slightly detuned Fermi energy levels in a V-like configuration, multi-plasmon induced transparency can be obtained. In addition, a highly sensitive refractive index nanosensor is showcased. The latter consists in a GBW, side-coupled to an effective cavity-oscillator embedded with the analyte to be detected. The sensitivity of the sensor is numerically derived. Owing to its simple structure, the proposed sensor may offer the advantage of being easily fabricated. The showcased graphene-based devices proposed in this work should help the design of tunable and highly integrated optical devices such as nano-filters, optical switches and highly sensitive nanosensors