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International audience; 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.

defect We report on the theoretical and experimental evidence for the occurrence of an omnidirectional elastic band gaps in one-dimensional phononic crystal structures. The structure is constituted by a periodically layered mirror deposited on a specific substrate that exhibits total reflection of waves for all incident angles and polarizations in a given frequency range. We study in this paper the influence of the nature and the filling fraction of the layered structures as well as of the substrate nature on the omnidirectional band gap. By introducing a defect piezoelectric layer in the finite size layered structure, selective resonance modes can occur within the omnidirectional bang gap under certain conditions. In this case, the elastic energy is confined to the defect layer. The frequencies of the defect modes are sensitive to the nature of the material and to the layer thicknesses. We excite the localized mode in the defect layer via interdigital transducers deposited on the substrate surface. We show a high frequency one port resonator at 4.3 GHz with high phase velocity of 43 km/s. The omnidirectional character of the band gap is crucial to confine the elastic energy in the layer.

The propagation of acoustic waves in a square-lattice phononic crystal slab consisting of a single layer of spherical steel beads in a solid epoxy matrix is studied experimentally. Waves are excited by an ultrasonic transducer and fully characterized on the slab surface by laser interferometry. A complete band gap is found to extend around 300 kHz, in good agreement with theoretical predictions. The transmission attenuation caused by absorption and band gap effects is obtained as a function of frequency and propagation distance. Well confined acoustic wave propagation inside a line-defect waveguide is further observed experimentally.

International audience; 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.

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 that cause strong attenuation in the transmission. 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. Using the same cylinders and the same distance between them, it is found that the center frequency of the first full band gap of the honeycomb lattice is lower than that of the triangular lattice, and that the width of this full band gap is also wider. Comparing our results with air/solid phononic crystals, we find that the fluid/solid system has larger band gaps than the air/solid system in the low frequency range

We study theoretically the possibility of transferring acoustic waves of a given wavelength between two parallel guides created in a phononic crystal. The structure considered in this work is composed of a periodic array of steel cylinders in water that displays an absolute bandgap. Within this gap, a full transmission band exists for propagation inside a straight wave guide. We show that by choosing an appropriate coupling structure between two parallel wave guides, one can transfer a particular acoustic wavelength from one guide to the other. The coupling element is composed of two hollow cylinders interacting with four other hollow cylinders located at each side of the wave guides. We discuss how the demultiplexing phenomenon is affected by varying the inner radius of the hollow cylinders. Keywords-component; acoustic waves; phononic crystal; demultiplexer

International audience; The transmission properties ofphononic crystal waveguides with grafted stub resonators were measured. The phononic crystal consists of a two-dimensional periodical array ofsteel rods immersed in water. Waveguides for acoustic waves are created by removing a line defect and stubs are formed by removing rods from the sidewalls of the waveguide. Depending on the stub geometry, definite wavelengths are reflected from the stub creating a one-dimensional bandgap within the waveguide transmission spectrum, the bandwidth ofwhich can be controlled by arranging a proper sequence ofstubs. These effects are interpreted as the interaction ofpropagating waveguide modes with localized stub resonances.

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.

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.

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 different 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 method is used to interpret the experimental data and it is found that theoretical predictions properly account for the observed spectra.

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.

The ultrasonic band gap properties of phononic crystals composed of arrays of steel cylinders immersed in water and arranged according to square, triangular or honeycomb lattices are compared theoretically and experimentally. In all three cases, complete band gaps are obtained, and the conditions of existence are identified by observing the complete band gap width as a function of the size of the inclusions. However, the measured transmission spectra reveal in the hexagonal symmetry cases (triangular and honeycomb lattices) the existence of deaf bands that cause strong attenuation in the transmission, with no band gap being involved. Band gaps and deaf bands are identified by comparing band structure computations for the infinite phononic crystal, obtained by a periodic‐boundary finite element method (FEM), with transmission simulations for the finite phononic crystal, obtained using the finite difference time domain (FDTD) method. The possibility of managing phononic wave guides within a defect line chan...

International audience; 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.