Mourad Oudich

Propagation of acoustic waves and waveguiding in a two-dimensional locally resonant phononic crystal plate

We demonstrate the waveguiding of Lamb waves in a locally resonant phononic crystal (LRPC) and we present an analysis of the guiding of elastic waves in straight and bent waveguides. The finite element method combined with the supercell technique was used to analyze the band gap and the dispersion relation of LRPC waveguides. Unlike the traditional phononic crystals, we show the possibility of guiding only one confined mode inside a LRPC waveguide. We discuss the confinement and the transmission of the guided mode as a function of the width of the waveguide based on both the band structure and the displacement field.

Enlargement of a locally resonant sonic band gap by using double-sides stubbed phononic plates

We report on the theoretical analysis of the enlargement of locally resonant acoustic band gap in two-dimensional sonic crystals based on a double-side stubbed plate. A significant enlargement of the relative bandwidth by a factor of 2 compared to the classical one-side stubbed plates is obtained and discussed. Based on an efficient finite element method, we show that this band gap enlargement is due to the coupling between the same nature of the resonant eigenmodes (in-plane or out-of-plane) of the stubs located in each plate side, producing a strong interaction with the plate’s Lamb modes. Acoustic displacement fields are computed to illustrate such mechanism and to discuss the physics behind it.

Acousto-optic couplings in two-dimensional phoxonic crystal cavities

We investigate the acousto-optic coupling, based on both photo-elastic and opto-mechanical mechanisms, in periodic structures with simultaneous photonic and phononic band gaps. The investigations are focused on a cavity defect in which the strong confinement of acoustic and optic waves enhances the interaction. We calculate the modulation of each photonic mode frequency by each phononic mode confined in the cavity. We compare the strength for the photo-elastic and opto-mechanical effects in the different cases. Both mechanisms can be in phase or out of phase and produce additive or subtractive effects in the total acousto-optic coupling.

Surface acoustic wave devices based on AlN/sapphire structure for high temperature applications

AlN/sapphire layered structure has been investigated as a potential substrate for surface acoustic wave (SAW) devices operating at high temperatures up to 950 °C under air atmosphere. Frequency characterizations of the SAW delay lines based on this structure indicate a slight increase of 2 dB in the insertion losses after annealing for 30 min at 900 °C. Scanning electron and atomic force microscopy as well as x-ray diffraction measurements suggest that theses losses are due to the deterioration of the Pt/Ta electrodes and to a slight oxidation of the AlN film.

Dispersion curves of surface acoustic waves in a two-dimensional phononic crystal

Reliable numerical simulations of band structure for surface acoustic waves propagating in a two-dimensional phononic crystal are reported. Through an efficient finite element method and specific boundary conditions, a theoretical approach allowing a direct computation of surface acoustic wave’s band structure for phononic crystal is proposed. Three types of phononic structures are investigated; fluid/solid, solid/solid, and air connected stubbed substrate. Using sound cone limitation, calculated results show the propagation of surface acoustic waves in the nonradiative region of the substrate. In addition, the modal displacements of the original guided surface modes supported by the studied structures are computed showing their original characteristics.

Broadband plate-type acoustic metamaterial for low-frequency sound attenuation

We show experimentally that plate-type acoustic metamaterials can serve to totally prohibit low frequency structure-borne sound at selective resonance frequencies ranging from 650 to 3500 Hz. Our metamaterial structures are consisting of a periodic arrangement of composite stubs (tungsten/silicone rubber) deposited on a thin aluminium plate. We report that these metamaterials present a broadband gap of out-of-plane modes at frequencies where the relevant sound wavelength in air is about three orders of magnitude larger than the plate thickness. Confinement and waveguiding of structure-borne sound in this sub-wavelength resonant regime is also experimentally evidenced and discussed.

Opening of simultaneous photonic and phononic band gap in two-dimensional square lattice periodic structure

We discuss two points related to the simultaneous existence of phononic and photonic band gaps in a two-dimensional crystal constituted by a square array of holes drilled in a matrix. In a first part, using the case of a sapphire sample in the microwave range, we show that in addition to the phononic gap, an absolute photonic gap may be obtained making use of the high values as well as the anisotropy of the dielectric matrix elements in the microwave regime. In a second part, using the case of silicon in the telecom frequency range, we demonstrate that absolute photonic and phononic gaps may be obtained by making a combination of two crystals having slightly different filling factors. The calculations of the band structures and transmission coefficients were mainly computed using the finite difference time domain method.

Modeling light-sound interaction in nanoscale cavities and waveguides

Abstract The interaction of light and sound waves at the micro and nanoscale has attracted considerable interest in recent years. The main reason is that this interaction is responsible for a wide variety of intriguing physical phenomena, ranging from the laser-induced cooling of a micromechanical resonator down to its ground state to the management of the speed of guided light pulses by exciting sound waves. A common feature of all these phenomena is the feasibility to tightly confine photons and phonons of similar wavelengths in a very small volume. Amongst the different structures that enable such confinement, optomechanical or phoxonic crystals, which are periodic structures displaying forbidden frequency band gaps for light and sound waves, have revealed themselves as the most appropriate candidates to host nanoscale structures where the light-sound interaction can be boosted. In this review, we describe the theoretical tools that allow the modeling of the interaction between photons and acoustic phonons in nanoscale structures, namely cavities and waveguides, with special emphasis in phoxonic crystal structures. First, we start by summarizing the different optomechanical or phoxonic crystal structures proposed so far and discuss their main advantages and limitations. Then, we describe the different mechanisms that make light interact with sound, and show how to treat them from a theoretical point of view. We then illustrate the different photon-phonon interaction processes with numerical simulations in realistic phoxonic cavities and waveguides. Finally, we introduce some possible applications which can take enormous benefit from the enhanced interaction between light and sound at the nanoscale.

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.

Acoustic energy harvesting based on a planar acoustic metamaterial

We theoretically report on an innovative and practical acoustic energy harvester based on a defected acoustic metamaterial (AMM) with piezoelectric material. The idea is to create suitable resonant defects in an AMM to confine the strain energy originating from an acoustic incidence. This scavenged energy is converted into electrical energy by attaching a structured piezoelectric material into the defect area of the AMM. We show an acoustic energy harvester based on a meta-structure capable of producing electrical power from an acoustic pressure. Numerical simulations are provided to analyze and elucidate the principles and the performances of the proposed system. A maximum output voltage of 1.3 V and a power density of 0.54 μW/cm3 are obtained at a frequency of 2257.5 Hz. The proposed concept should have broad applications on energy harvesting as well as on low-frequency sound isolation, since this system acts as both acoustic insulator and energy harvester.