Mahmoud Addouche

Superlensing effect for surface acoustic waves in a pillar-based phononic crystal with negative refractive index

We demonstrate super resolution imaging for surface acoustic waves using a phononic structure displaying negative refractive index. This phononic structure is made of a monolithic square lattice of cylindrical pillars standing on a semi-infinite medium. The pillars act as acoustic resonator and induce a surface propagating wave with unusual dispersion. We found, under specific geometrical parameters, one propagating mode that exhibits negative refraction effect with negative effective index close to −1. Furthermore, a flat lens with finite number of pillars is designed to allow the focusing of an acoustic point source into an image with a resolution of λ3, overcoming the Rayleigh diffraction limit.

Ultra-wide acoustic band gaps in pillar-based phononic crystal strips

An original approach for designing a one dimensional phononic crystal strip with an ultra-wide band gap is presented. The strip consists of periodic pillars erected on a tailored beam, enabling the generation of a band gap that is due to both Bragg scattering and local resonances. The optimized combination of both effects results in the lowering and the widening of the main band gap, ultimately leading to a gap-to-midgap ratio of 138%. The design method used to improve the band gap width is based on the flattening of phononic bands and relies on the study of the modal energy distribution within the unit cell. The computed transmission through a finite number of periods corroborates the dispersion diagram. The strong attenuation, in excess of 150 dB for only five periods, highlights the interest of such ultra-wide band gap phononic crystal strips.

Acoustically induced transparency using Fano resonant periodic arrays

A three-dimensional acoustic device, which supports Fano resonance and induced transparency in its response to an incident sound wave, is designed and fabricated. These effects are generated from the destructive interference of closely coupled one broad- and one narrow-band acoustic modes. The proposed design ensures excitation and interference of two spectrally close modes by locating a small pipe inside a wider and longer one. Indeed, numerical simulations and experiments demonstrate that this simple-to-fabricate structure can be used to generate Fano resonance as well as acoustically induced transparency with promising applications in sensing, cloaking, and imaging.

Subwavelength waveguiding of surface phonons in pillars-based phononic crystal

In this study, we theoretically analyze the guiding of surface phonons through locally resonant defects in pillars-based phononic crystal. Using finite element method, we simulate the propagation of surface phonons through a periodic array of cylindrical pillars deposited on a semi-infinite substrate. This structure displays several band gaps, some of which are due to local resonances of the pillar. By introducing pillar defects inside the phononic structure, we show the possibility to perform a waveguiding of surface phonons based on two mechanisms that spatially confine the elastic energy in very small waveguide apertures. A careful choice of the height of the defect pillars, allows to shift the frequency position of the defect modes inside or outside the locally resonant band gaps and create two subwavelenght waveguiding mechanisms. The first is a classical mechanism that corresponds to the presence of the defect modes inside the locally resonant band gap. The seconde is due to the hybridation between the phonon resonances of defect modes and the surface phonons of the semi-infinite homogenous medium. We discuss the nature and the difference between both waveguiding phenomena.

Experimental evidence of ultrasonic opacity using the coupling of resonant cavities in a phononic membrane

We report the practical realization of phononic membrane with sub-wavelength apertures, inducing a broadband ultrasonic opacity. The ultrasonic experiments confirm the existence of deep and wide attenuation in the transmission spectrum, through periodic aperture arrays in silicon substrate immersed in water. This attenuation reaches 30 dB on a relative bandwidth of 31% with a center frequency of 0.9 MHz. The arrays act as Fabry-Perot acoustic resonators, and through the coupling effect between them, we obtain a series of asymmetric shape peaks in the transmission spectra. This leads to an enhanced transmission at the resonance frequencies as well as to improve the attenuation significantly at the antiresonance frequencies.

Extraordinary nonlinear transmission modulation in a doubly-resonant acousto-optical structure

Acousto-optical modulators usually rely on coherent diffraction of light by a moving acoustic wave, leading to bulky devices with a long interaction length. We propose a subwavelength acousto-optical structure that instead relies on a double resonance to achieve strong modulation at near-infrared wavelengths. A periodic array of metal ridges on a piezoelectric substrate defines cavities that create a resonant dip in the optical transmission spectrum. The ridges simultaneously support large flexural vibrations when resonantly excited by a radio-frequency signal, effectively deforming the cavities and leading to strongly nonlinear acousto-optical modulation. The nano-optical structure could find applications in highly compact photonic devices.

Guiding and confinement of interface acoustic waves in solid-fluid pillar-based phononic crystals

Pillar-based phononic crystals exhibit some unique wave phenomena due to the interaction between surface acoustic modes of the substrate and local resonances supported by pillars. In this paper, we extend the investigations by taking into account the presence of a liquid medium. We particularly demonstrate that local resonances dramatically decrease the phase velocity of Scholte-Stoneley wave, which leads to a slow wave at the solid/fluid interface. Moreover, we show that increasing the height of pillars introduces a new set of branches of interface modes and drastically affects the acoustic energy localization. Indeed, while some modes display a highly confined pressure between pillars, others exponentially decay in the fluid or only propagate in the solid without disturbing the fluid pressure. These theoretical results, performed by finite element method, highlight a new acoustic wave confinement suitable in various applications such as acoustophoresis, lab on chip and microfluidics.

Extensive tailorability of sound absorption using acoustic metamaterials

We present an experimental demonstration of sound absorption tailorability, using acoustic metamaterials made of resonant cavities that take advantage of the inherent visco-thermal characteristics of air. As confirmed by numerical calculation, we particularly show that using quarter-wave-like resonators made of deep subwavelength slits allows a high confinement of the acoustic energy of an incident wave. This leads to enhance the dissipation in the cavities and, consequently, generates strong sound absorption, even over a wide frequency band. This paves the way for tremendous opportunities in acoustic comfort because of their potentially low density, low volume, broadband, and tailorable capabilities.We present an experimental demonstration of sound absorption tailorability, using acoustic metamaterials made of resonant cavities that take advantage of the inherent visco-thermal characteristics of air. As confirmed by numerical calculation, we particularly show that using quarter-wave-like resonators made of deep subwavelength slits allows a high confinement of the acoustic energy of an incident wave. This leads to enhance the dissipation in the cavities and, consequently, generates strong sound absorption, even over a wide frequency band. This paves the way for tremendous opportunities in acoustic comfort because of their potentially low density, low volume, broadband, and tailorable capabilities.

Subwavelength sound screening by coupling space-coiled Fabry-Perot resonators

We explore broadband and omnidirectional low frequency sound screening based on locally resonant acoustic metamaterials. We show that the coupling of different resonant modes supported by Fabry-Perot cavities can efficiently generate asymmetric lineshapes in the transmission spectrum, leading to a broadband sound opacity. The Fabry-Perot cavities are space-coiled in order to shift the resonant modes under the diffraction edge, which guaranty the opacity band for all incident angles. Indeed, the deep subwavelength feature of the cavities leads to avoid diffraction that have been proved to be the main limitation of omnidirectional capabilities of locally resonant perforated plates. We experimentally reach an attenuation of few tens of dB at low frequency, with a metamaterial thickness fifteen times smaller than the wavelength (lambda / 15). The proposed design can be considered as a new building block for acoustic metasurfaces having a high level of manipulation of acoustic waves.

Solid-fluid interaction in a pillar-based phononic crystal

In this paper, we investigate the wave dispersion of two dimensional pillar-based phononic crystal surrounded in liquid medium. An unit cell structure with reduced pillar height (hp/a)=0.5 and reduced radius (rp/a)=0.3 is simulated using Finite Element Method. The geometrical parameter is chosen to demonstrate a local resonance mechanism that allow the confinement of elastic energy at the interface between the solid and the fluid. In order to identify the energy distribution, we represent the eigenmode at high symmetry (point X) in the first Brillouin zone. The decreasing trend of frequency is also boosted with the increase of pillar height. From the total displacement, the energy is mostly located inside the pillar and only a small value of displacement is present in the substrate. The results from this study could be useful for microfluidic and lab on chip application. We believe that the integration of pillar based phononic crystal with microfluidic could become a powerful tool in the sensor and actuator application for chemical and biological application.