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.

A plane-wave-expansion method suited to the analysis of surface-acoustic-wave propagation in two-dimensional piezoelectric phononic crystals is described. The surface modes of a square-lattice Y-cut lithium niobate phononic crystal with circular void inclusions with a filling fraction of 63% are identified. It is found that a large full band gap with a fractional bandwidth of 34% exists for surface acoustic waves of any polarization and incidence, coincidentally with the full band gap for bulk waves propagating in the plane of the surface. The excitation of surface acoustic waves by interdigital transducers is discussed.

The plane-wave-expansion (PWE) approach dedicated to the simulation of periodic devices has been extended to 1-3 connectivity piezoelectric composite structures. The case of simple but actual piezoelectric composite structures is addressed, taking piezoelectricity, acoustic losses, and electrical excitation conditions rigorously into account. The material distribution is represented by using a bidimensional Fourier series and the electromechanical response is simulated using a Bloch-Floquet expansion together with the Fahmy-Adler formulation of the Christoffel problem. Application of the model to 1-3 connectivity piezoelectric composites is reported and compared to previously published analyses of this problem.

It was shown that elastic waves propagating out-of-plane in a two-dimensional phononic crystal can experience full-band-gaps for nonzero values of the wave-vector component parallel to the rods. By further inserting a rod defect, it is demonstrated that modes propagating along the rod defect can be localized within the band-gaps of the phononic crystal. Such waveguide modes are exhibited for a tungsten/epoxy composite containing an aluminum nitride rod as the rod defect. It is expected that guided modes of such a structure can be excited and detected electrically owing to the piezoelectric effect.

The elastic modes guided along the axis of an optical fiber are obtained for an arbitrary finite cross section using waveguide finite element analysis. The band structure of acoustic phonons is obtained from this fullvector computation. The analysis is applied to the case of a photonic crystal fiber possessing a honeycomb lattice. It is shown that this fiber exhibits band gaps for elastic modes propagating along the longitudinal fiber axis. For frequencies within a band gap, the external boundary of the fiber becomes a defect of the phononic crystal that supports the propagation of guided elastic modes. Such boundary modes are very sensitive to the boundary conditions. The further introduction of a defect within the two-dimensional phononic crystal leads to the formation of highly confined elastic waveguide modes that copropagate in the same core volume as the guided optical mode. We consider the application of these properties to the suppression of stimulated Brillouin scattering and to enhanced collinear acousto-optical interactions. In particular, we obtain the optimum elastic modal shape that maximizes the acousto-optical scattering coefficient for given optical modes.

We have used a plane-wave-expansion model to study the out-of-plane propagation of elastic waves in a two-dimensional phononic band-gap material. The case of quartz rods embedded in an epoxy matrix has been computed. Band gaps for nonzero values of the wave-vector component parallel to the rods are shown to exist and are investigated. For wavelengths smaller than the period of the structure, modes are found that are localized in the epoxy intersites, and propagate perpendicularly to the plane of the structure.

The need for optimized acoustic transducers for the development of high-quality imaging probes requires efficient simulation tools providing reliable descriptions of the behavior of real devices. The purpose of this work is the implementation of a finite-element model for the simulation of periodic transducer arrays. By using the assumption of harmonic excitation, the harmonic admittance of the studied structure can be derived. It is then shown how the mutual admittance is deduced from this feature, allowing one to estimate the amount of cross-talk effects for a given periodic transducer. Computation results are reported for standard linear acoustic probes, 2-2 ͑one-dimensional periodic͒ and 1-3 ͑two-dimensional periodic͒ piezocomposite materials. In the case of 2-2 connectivity composites, a comparison between nonperiodic and periodic computations of the mutual admittance is conducted, from which the minimum number of periods for which periodic computations can be trustfully considered can be estimated.

International audienceThe development of new surface acoustic wave devices exhibiting complicated electrode patterns or layered excitation transducers has been favored by an intense innovative activity in this area. For instance, devices exhibiting interdigital transducers covered by piezoelectric or dielectric layers have been fabricated and tested, but the design of such structures requires simulation tools capable to accurately take into account the actual shape of the wave guide elements. A modeling approach able to address complicated surface acoustic wave periodic structures (defined in the saggital plane) exhibiting any geometry then has been developed and implemented. It is based on the combination of a finite element analysis and a boundary element method. A first validation of the computation is reported by comparison with standard surface wave devices. Surface transverse wave resonators covered by amorphous silica have been built and consequently used for theory/experiment assessment. Also the case of recessed electrodes has been considered. The proposed model offers large opportunities for modeling any two-dimensional periodic elastic wave guide