Abdelkrim Choujaa

Guiding and bending of acoustic waves in highly confined phononic crystal waveguides

We demonstrate experimentally the guiding and the bending of acoustic waves in highly confined waveguides formed by removing rods from a periodic two-dimensional lattice of steel cylinders immersed in water. Full transmission is observed for a one-period-wide straight waveguide within the full band gap of the perfect phononic crystal. However, when the waveguide width is doubled, destructive interference causes the transmission to vanish in the center of the passband. Waveguiding over a wide frequency range is obtained for a one-period-wide waveguide with two sharp 90° bends. Finite-difference time-domain computations are found to be in good agreement with the measurements.

Complete band gaps and deaf bands of triangular and honeycomb water-steel phononic crystals

Phononic crystals with triangular and honeycomb lattices are investigated experimentally and theoretically. They are composed of arrays of steel cylinders immersed in water. The measured transmission spectra reveal the existence of complete band gaps but also of deaf bands. Band gaps and deaf bands are identified by comparing band structure computations, obtained by a periodic-boundary finite element method, with transmission simulations, obtained using the finite difference time domain method. The appearance of flat bands and the polarization of the associated eigenmodes is also discussed. Triangular and honeycomb phononic crystals with equal cylinder diameter and smallest spacing are compared. As previously obtained with air-solid phononic crystals, it is found that the first complete band gap opens for the honeycomb lattice but not for the triangular lattice, thanks to symmetry reduction.

Acoustic channel drop tunneling in a phononic crystal

We study both theoretically and experimentally the possibility of resonant tunneling of acoustic waves between two parallel guides created in a phononic crystal composed of steel cylinders in water. In the absolute band gap of the phononic crystal, ranging from 250to325kHz, a full transmission band exists for propagation inside a straight waveguide. We show that the transfer of a particular wavelength can occur between two parallel waveguides coupled together through an appropriate coupling structure. The latter is composed of isolated cavities interacting with stubs located at the sides of the waveguides.

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.

Octave omnidirectional band gap in a three-dimensional phononic crystal

We report on the experimental observation of a one-octave-large omnidirectional elastic band gap for longitudinal waves in a three-dimensional phononic crystal consisting of face-centered-cubic arrays of close-packed steel beads in a solid epoxy matrix. The 60% relative width of the band gap is larger than expected based on the contrast in material properties, and is confirmed by band structure diagram and transmission spectra computations. The coupling between shear and longitudinal polarizations is pointed out as a mechanism for enlarging the band gap by comparing with the steel beads in water matrix case.

Experimental study of guiding and filtering of acoustic waves in a two dimensional ultrasonic crystal

Abstract We present a combined experimental and theoretical study of the guiding, bending and filtering of acoustic waves in an ultrasonic crystal. The crystal consists of a two-dimensional periodical array of steel rods immersed in water, for wich a complete acoustic band gap extending from 240 to 325 kHz is found experimentally. Waveguides for acoustic waves are further created by removing a line defect, on which stubs can be added by removing rods from the side-walls of the waveguide. Full transmission is observed for a one-period-wide straight waveguide within the full-band-gap of the perfect phononic crystal, i.e. for a waveguide aperture smaller than one acoustic wavelength. Waveguiding over a wide frequency range is also obtained for a one-period-wide waveguide with two sharp 90° bends. Finite-difference time-domain computations are found to be in good agreement with the measurements in all experimental configurations.

Experimental study of band gaps and defect modes in a two-dimensional ultrasonic crystal

We report on the experimental observation of the existence and the interaction of localized defect modes in a full acoustic band gap in a two-dimensional lattice of steel cylinders immersed in water. The confinement of defect modes and the splitting of their resonance frequencies are observed and are explained by their evanescent coupling. A new type of waveguiding in a phononic crystal based on the evanescent coupling of defect modes is proposed and demonstrated experimentally. The finite-difference time-domain (FDTD) method is used to interpret the experimental data and it is found that theoretical predictions properly account for the observed spectra.

Guiding and filtering acoustic waves in a two-dimensional phononic crystal

An experimental and theoretical study of the guiding, bending and filtering of acoustic waves in an ultrasonic crystal is reported. The crystal consists of a 2D periodical array of steel rods immersed in water, for which a complete acoustic band gap is found experimentally. Waveguides for acoustic waves are created by removing one row of rods and the possibility of including sharp bends, owing to the strong spatial confinement within the full band gap, is demonstrated. Stubs can be further added by removing rods from the side-walls of the waveguide. Depending on the stub geometry, definite wavelengths are reflected from the stubs creating a 1D bandgap within the waveguide transmission spectrum, the bandwidth of which can be controlled by arranging a proper sequence of stubs.

The OmniSaw device concept (OmniSAW: Omnidirectional band gap for surface acoustic wave)

We report the theoretical evidence for the occurrence of omnidirectional elastic band gap in one-dimensional phononic crystal structures. The structure is constituted by a periodic layered deposited on a specific substrate that exhibit total reflection of waves for all incident angles and polarizations in a given frequency range the omnidirectional band gap. We present the influence of the nature and filling fraction of the layered structures as well as the substrate nature on the omnidirectional band gap. By introducing a defect piezoelectric layer, for instance AlN or ZnO, in the finite size layered structure, under certain condition a selective resonance modes occur within the omnidirectional bang gap. In this case, the elastic energy is localized in the defect layer. The frequencies of the defect modes are sensitive to the nature of the material and to the layer thicknesses. In order to excite the localized mode in the defect layer. We introduce interdigital transducers on the top of the surface. This implies the introduction of a new spatial periodicity and the reduction of the parallel wave vector in the first Brillouin zone. The omnidirectional character of the bad gap in this case is crucial to confine the elastic energy in the defect layer.

Ultrasonic and hypersonic phononic crystals

We report on the experimental and theoretical investigation two kinds of acoustic waves in two dimensional phononic crystal: bulk acoustic waves and surface acoustic waves. For bulk acoustic waves, the work focuses on the experimental observation of full acoustic band gaps in a two-dimensional lattice of steel cylinders immersed in water as well as deaf bands that cause strong attenuation in the transmission for honeycomb and triangular lattices. For surface acoustic waves, complete acoustic band gaps found experimentally in a two-dimensional square-lattice piezoelectric phononic crystal etched in lithium niobate will be presented. Propagation in the phononic crystal is studied by direct generation and detection of surface waves using interdigital transducers. The complete band gap extends from 203 to 226 MHz, in good agreement with theoretical predictions. Near the upper edge of the complete band gap, it is observed that radiation to the bulk of the substrate dominates. This observation is explained by introducing the concept of sound line.