Bertrand Dubus

Waveguiding in two-dimensional piezoelectric phononic crystal plates

We investigate the possibility of designing phononic crystal-based devices for telecommunication applications using materials commonly employed in microfabrication. We focus our attention on a phononic crystal made of a square array of cylindrical holes drilled in an active piezoelectric PZT5A matrix. Two different structures are considered, namely, a freestanding phononic crystal plate and a plate deposited on a silicon substrate. The geometrical characteristics of the phononic crystal plates (lattice parameter and thickness) were chosen to ensure the existence of an absolute band gap around 1.5GHz; a common frequency in radio frequency telecommunications. Computations of the dispersion curves of these active structures were conducted with the help of the finite element method. We demonstrate the existence of absolute band gaps in the band structure of the phononic crystal plates and, then, the possibility of guided modes inside a linear defect created by removing one row of air holes in the phononic cry...

Cone-like bubble formation in ultrasonic cavitation field.

A new phenomenon in ultrasonic cavitation field is reported. Cavitation bubbles are observed to self-arrange in a cone-like macrostructure in the vicinity of transducer radiating surface. The cone-like macrostructure is stable while its branch-like pattern microstructure changes rapidly. The structure is constituted by moving bubbles which undergo attractive and repulsive Bjerknes forces caused by high acoustic pressure gradients and strongly nonlinear oscillations of cavitation bubbles. The cone-like bubble structure is a chemically active formation. Its remarkably high activity is confirmed by chemiluminescence experiments.

Switchable and tunable strontium titanate electrostrictive bulk acoustic wave resonator integrated with a Bragg mirror

The realization and the radio-frequency characterization of a tunable strontium titanate (STO) electrostrictive solidly mounted acoustic resonator (SMR) bulk acoustic wave are reported. For a 430nm thick strontium titanate layer, the resonance frequency at 2.2GHz can be switched on with a bias voltage of 6V and tuned ±0.85% with a bias voltage between 6 and 30V. No hysteresis is observed. The SMR tunability is found to be affected by (i) the variation of coupling factor versus bias which is the dominant effect and (ii) the variation of SrTiO3 stiffness at constant electric displacement.

Experimental demonstration of the negative refraction of a transverse elastic wave in a two-dimensional solid phononic crystal

The negative refraction of transverse elastic waves is demonstrated experimentally in a two-dimensional phononic crystal (PC) made of a square lattice of cylindrical air cavities in an aluminum matrix. Dispersion curves of elastic waves in this PC exhibit a unique branch with phase and group velocities of opposite signs in a broad frequency range. Measurement of refraction angles through prismatic PC included in an aluminum block demonstrates negative refraction of elastic transverse wave.

Analysis of mechanical limitations of high power piezoelectric transducers using finite element modelling

Abstract The design of high power electro-acoustic transducers is a difficult problem in many fields of acoustics. The mechanical, electrical, thermal or acoustical phenomena which limit the available power need to be controlled. To determine the mechanical limitations of a transducer, knowledge of both the mechanical limit of the materials and the stress field in the structure is essential. Although there are many experimental data available to designers, the classical transducer models are limited to simple geometries. The present study uses the finite element code ATILA to compute the stress field, which is then compared to the mechanical limit of the material using classical criteria. The method has been applied to analysis of the mechanical behaviour of a length expander transducer used in macrosonics, the problem of interest being application of prestress, as well as the dynamic behaviour at high drive levels. In both cases, very good agreement is found when the computed results are compared to strain gauge measurements.

On the physical origin of conical bubble structure under an ultrasonic horn.

The cavitation field generated by an ultrasonic horn at low frequency and high power is known to self-organize into a conical bubble structure. The physical mechanism at the origin of this bubble structure is investigated using numerical simulations and acoustic pressure measurements. The thin bubbly layer lying at horn surface is shown to act as a nonlinear thickness resonator that amplifies acoustic pressure and distorts acoustic waveform. This mechanism explains the self-stabilization of the conical bubble structure as well as the generation of shock wave and the focusing at very short distance.

UHF/VHF resonators using lamb waves co-integrated with bulk acoustic wave resonators

Bulk Acoustic Wave (BAW) resonators exhibit attractive properties in terms of power handling capacity and on- chip integration to realize filters in the GHz range. A current problem in the design of BAW resonators is the existence of spurious resonances due to Lamb waves propagation. This work demonstrates that Lamb wave resonances can be used to realize resonators with high quality factor in the Ultra-High Frequency/Very High Frequency (UHF/VHF) ranges. Design of lateral resonators are performed using finite element modelling. Influence of electrode geometry and type of excitation (one- or two-phase) upon coupling of fundamental or higher order resonances is discussed. Lamb wave and Film Bulk Acoustic Resonators (FBAR) are simultaneously manufactured using surface micro-machining process compatible with Above IC integration. S0 Lamb wave resonances from 20 to 277 MHz are measured showing good agreement with simulation. High quality factors (Qs and Qp) of 2000 and k 2 ~0.8% (k coupling coefficient) are obtained at 92 MHz for 3 rd order resonator. For co- integrated BAW resonators, thickness extensional resonance is measured at 2.5 GHz.

Numerical analysis of negative refraction of transverse waves in an elastic material

A numerical analysis of negative refraction process is reported using a phononic crystal with an elastic solid matrix. The phononic crystal considered in this study is made of a periodic arrangement of holes in aluminum. Dispersion curves are discussed and conditions for which negative refraction can appear are identified. These conditions are obtained for the transverse waves, whereas the longitudinal waves are evanescent. A calculation is performed with a prism shaped phononic crystal, and it clearly exhibits a negative refraction angle. Several analyses are provided with a view to characterize the wave going out of the phononic crystal. Finally, improvements, with respect to the impedance matching and index tuning, are discussed.

6H-5 Design, Elaboration and Characterization of Coupled Resonator Filters for WCDMA Applications

This work presents the design, manufacturing and characterization of bulk acoustic wave coupled resonator filter (BAW CRF) for W-CDMA applications. Aluminum nitride resonators with molybdenum electrodes are acoustically coupled through a W/SiO2 multi-layered stack and insulated by a fully dielectric SiN/SiOC Bragg mirror. This configuration provides galvanic isolation between input and output making CRF suitable for single-to-balanced conversion. Measurements of both single-ended and single-to-balanced filters display bandwidth and out-of-band attenuation close to W-CDMA specifications. Minimum insertion loss of 2.9 dB is measured whereas strong spurious resonances are observed. Experimental results on spurious modes suppression using specific border region are discussed

Finite-element modeling of lead magnesium niobate electrostrictive materials: Dynamic analysis

A finite-element model is proposed for the time-domain analysis of electrostrictive materials. Homs material model, developed for lead magnesium niobate (PMN) ceramics, is used. It includes the quadratic dependence of strain with polarization, the saturation of polarization, assumes constant temperature, and excludes hysteresis. The theoretical formulation is justified by the principle of virtual works. The numerical model is obtained after discretization in space and time. The validation is performed by comparing numerical results with semianalytical results for an electrostrictive spherical shell subjected to a step in voltage or in charge. From these results, a method to compute the coupling coefficient of electrostrictive materials, based on Ikedas definition, is proposed and applied to a bar with parallel electric field.