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Vincent Laude

Publications and source records attributed to Vincent Laude.

13 recordsLinked to original sources
PubMed2007

Guided acoustic wave Brillouin scattering in photonic crystal fibers.

We experimentally investigate guided acoustic wave Brillouin scattering in several photonic crystal fibers by use of the so-called fiber loop mirror technique and show a completely different dynamics with respect to standard all-silica fibers. In addition to the suppression of most acoustic phonons, we show that forward Brillouin scattering in photonic crystal fibers is substantially enhanced only for the fundamental acoustic phonon because of efficient transverse acousto-optic field overlap. The results of our numerical simulations reveal that this high-frequency phonon is indeed trapped within the fiber core by the air-hole microstructure, in good agreement with experimental measurements.

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PubMed2006

Surface Green's function of a piezoelectric half-space.

The computation of the two-dimensional harmonic spatial-domain Green's function at the surface of a piezoelectric half-space is discussed. Starting from the known form of the Green's function expressed in the spectral domain, the singular contributions are isolated and treated separately. It is found that the surface acoustic wave contributions (i.e., poles in the spectral Green's function) give rise to an anisotropic generalization of the Hankel function H0(2), the spatial Green's function for the scalar two-dimensional wave equation. The asymptotic behavior at infinity and at the origin (for the electrostatic contribution) also are explicitly treated. The remaining nonsingular part of the spectral Green's function is obtained numerically by a combination of fast Fourier transform and quadrature. Illustrations are given in the case of a substrate of Y-cut lithium niobate.

Algorithms↗
PubMed2005

Compression of attosecond harmonic pulses by extreme-ultraviolet chirped mirrors.

In the race toward attosecond pulses, for which high-order harmonics generated in rare gases are the best candidates, both the harmonic spectral range and the spectral phase have to be controlled. We demonstrate that multilayer extreme-ultraviolet chirped mirrors can be numerically optimized and designed to compensate for the intrinsic harmonic chirp that was recently discovered and that is responsible for temporal broadening of pulses. A simulation shows that an optimized mirror is capable of compressing the duration from approximately 260 to 90 as. This new technique is an interesting solution because of its ability to cover a wider spectral range than other technical devices that have already been proposed to overcome the chirp of high harmonics.

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PubMed2005

Full band gap for surface acoustic waves in a piezoelectric phononic crystal.

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.

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PubMed2004

Three-dimensional modelling of micromachined-ultrasonic-transducer arrays operating in water.

We report on the 3-D modelling of periodic arrays of capacitive micromachined ultrasonic transducers (cMUTs) operating in fluid. Specific developments have been performed to model biperiodic transducer arrays and to take into account radiation into any stratified media at the front-side as well as the back-side of the device. The model is based on a periodic finite-element-analysis/boundary-element-method (FEA/BEM). It is applied to micromachined ultrasonic transducers (MUTs), based on silicon-nitride-circular-membrane arrays on a silicon substrate, and operating in water. The spectrum characteristics of MUTs excited in phase are investigated, showing that very-large-band emission is achievable as previously demonstrated by many authors. However, other contributions are also found, depending on the excitation conditions, that do not radiate in the fluid. These contributions are identified as guided modes that could generate significant cross-talk effects. The origin and the nature of these modes is analyzed to gain insight in the actual operation of MUTs.

Equipment Design↗
PubMed2004

Dyadic Green's functions of a laminar plate.

We introduce the concept of dyadic Green's functions of a laminar plate. These functions generalize classical Green's functions. In addition to relating displacements and stresses at the surface of a medium, they relate these quantities at both the top and the bottom surfaces of a medium of finite thickness and infinite extent in the transverse directions. We describe here the calculation of these functions in the spectral domain and provide some academic examples demonstrating their interest.

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PubMed2004

A P-matrix based model for SAW grating waveguides taking into account modes conversion at the reflection.

Several models exist for analyzing the wave-guiding effect of a reflective grating. On the one hand, there are models based on scalar waveguide theory. These models consider that a device can be described as being made of several regions having different velocities. On the other hand, an extension of the coupling of modes (COM) model taking into account the transverse dimension has been developed. This paraxial COM model predicts that guidance is possible even when there is no velocity difference between the interior and the exterior of the grating region. Guidance, under such circumstances, is due only to differences in reflectivity between regions. Following from this insight, a new approach has been developed: guided modes and the continuum of radiating modes are first determined. At each period, reflections then are considered as occurring only in the reflective regions, so that the modes are truncated. Thus, at each reflection (and transmission), each mode is converted into a distribution of all modes. Dispersion curves very similar to those shown by other researchers are obtained by this method. They show, in particular, the existence of guided modes even when the wave velocity in all regions is identical. This model can be used to more easily analyze practical devices and exhibits a good agreement with experimental results.

Journal Article↗
PubMed2004

Fast FEM/BEM simulation of SAW devices via asymptotic waveform evaluation.

The finite element method/boundary element method (FEM/BEM) computation model applied to surface acoustic wave devices requires the solution of a large linear system for each frequency point. An asymptotic waveform evaluation technique is used to obtain an approximate solution of the linear system that is valid over a large frequency bandwidth. The approximate solution was shown to be very accurate and vastly reduces the computation time.

Journal Article↗
PubMed2003

Interface acoustic waves properties in some common crystal cuts.

Interface acoustic waves (IAWs), also termed boundary waves, propagate at the interface between two solids. We present two IAW numerical analysis tools, inspired from well established surface acoustic wave (SAW) methods. First, the interface effective permittivity is derived for arbitrary piezoelectric solids and is used to estimate some basic parameters of IAWs. The harmonic admittance for an interface excitation is then derived from the interface effective permittivity, in much the same way the harmonic admittance for surface excitation is obtained from the (surface) effective permittivity. The finite electrode thickness is neglected in this problem analysis. The harmonic admittance is used to model propagation in case an infinite periodic interdigital transducer is located at the interface. Simulation results are commented upon for some usual piezoelectric material cuts and outline a modal selection specific to IAWs as compared with SAWs. The temperature dependence of the resonance frequency is also estimated.

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PubMed2003

General solution of the coupled-wave equations of acousto-optics.

The basic problem of the diffraction of an optical plane wave by an acoustic plane wave in an anisotropic homogeneous medium is considered. The acousto-optical interaction is considered indifferently of the isotropic or of the birefringent type. Coupled-wave equations are obtained rigorously and cast into an eigenvalue value problem. A general solution is obtained for the diffraction efficiency of diffracted orders, for any interaction length and diffraction regime. The theory includes the Bragg regime, the Raman-Nath regime, and all intermediate situations in the same formulation. The method of solution is both exact and computationally efficient. It is similar in character to the rigorous coupled-wave analysis of Moharam and Gaylord but differs by the choice of basis functions adapted to propagating rather than static gratings. Examples are given for acousto-optical interaction in paratellurite, TeO2.

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PubMed2002

Volume index gratings in the intermediate and form-birefringence regimes.

We present a study of volume index gratings operating in the form-birefringence regime and what we call the intermediate regime, defined as the regime between form birefringence and the onset of diffraction. The operating regime is characterized by the ratio between the measurement wavelength and the grating period: form birefringence regime below 10, diffraction around 3, and intermediate regime between 10 and 3. The behavior of the gratings in a given regime is studied by plotting the induced retardation as a function of the incidence angle. The shape of the curve fully characterizes the regime. We report on theoretical calculations of retardation by use of a rigorous theory and its experimental validation by measuring retardation of recorded gratings in photopolymers.

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PubMed2002

A full 3D plane-wave-expansion model for 1-3 piezoelectric composite structures.

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.

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