Younes Achaoui
Aix-Marseille University
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Publication
Featured researches published by Younes Achaoui.
Applied Physics Letters | 2011
Sarah Benchabane; Olivier Gaiffe; Gwenn Ulliac; Roland Salut; Younes Achaoui; Vincent Laude
We observe experimentally the propagation of surface-guided waves in a hypersonic phononic crystal, both in the radiative and nonradiative regions of the spectrum. Combining electrical measurements in reflection and transmission as well as optical maps of the surface displacement, a band gap extending from 0.6 to 0.95 GHz is identified in a square lattice array of 1 μm radius air holes milled in lithium niobate. The optical measurements reveal the transmission of surface-guided waves above the band gap, well inside the sound cone.
Journal of Applied Physics | 2013
Younes Achaoui; Vincent Laude; Sarah Benchabane; Abdelkrim Khelif
The propagation of surface acoustic waves in two-dimensional phononic crystals of pillars on a surface is investigated experimentally for hexagonal and honeycomb lattice symmetries. A random array of pillars is also compared to the periodic phononic crystals. Taking into account that the geometrical and physical characteristics of the pillars are the same in all cases, it is shown that the locally resonant band gap in the low frequency range is almost independent of periodicity and resilient to randomness. In contrast, the Bragg band gap disappears with the random array.
Journal of Physics D | 2010
Younes Achaoui; Abdelkrim Khelif; Sarah Benchabane; Vincent Laude
We investigate the polarization of Bloch waves in two-dimensional piezoelectric phononic crystals and phononic crystal waveguides managed therein. It is found that in addition to the strong coupling induced for waves polarized in the plane of the periodic structuration, a weaker but non-negligible coupling of polarization components originates from material anisotropy. Numerical illustrations are given for an array of air holes in lithium niobate arranged according to a square lattice. It is observed that when a band mostly polarized in-plane gets close to a band mostly polarized out-of-plane, a phenomenon of repelling can occur, that in some instances introduces a local band gap. This interaction is accompanied by a transfer of the polarization state from one band to the other.
Journal of Applied Physics | 2012
Abdelkrim Khelif; Younes Achaoui; Boujemaa Aoubiza
The theoretical study deals with the propagation of surface acoustic waves in two-dimensional arrays of resonant elements with different symmetry lattices. The resonant elements are cylindrical pillars on the surface of a semi-infinite substrate. The obtained band structures show the interaction of the pillars acoustic resonances with the semi-infinite medium which form additional band gaps that are decoupled from Bragg gaps. Especially, the frequency position of the lowest band gap is invariant with respect to lattice symmetries. Thereby, this position is independent of the lattice pitch, which is unexpected in band gaps based on Bragg interferences. However, the role of the period remains important for defining the non-radiative region limited by the slowest bulk mode and influencing the existence of the guided modes. Numerical simulations are based on the efficient finite element method and considered silicon pillars on a silicon substrate.
AIP Advances | 2016
Andre Diatta; Younes Achaoui; Stéphane Brûlé; Stefan Enoch; Sébastien Guenneau
Recent advances in control of anthropic seismic sources in structured soil led us to explore interactions of elastic waves propagating in plates (with soil parameters) structured with concrete pillars buried in the soil. Pillars are 2 m in diameter, 30 m in depth and the plate is 50 m in thickness. We study the frequency range 5 to 10 Hz, for which Rayleigh wave wavelengths are smaller than the plate thickness. This frequency range is compatible with frequency ranges of particular interest in earthquake engineering. It is demonstrated in this paper that two seismic cloaks’ configurations allow for an unprecedented flow of elastodynamic energy associated with Rayleigh surface waves. The first cloak design is inspired by some approximation of ideal cloaks’ parameters within the framework of thin plate theory. The second, more accomplished but more involved, cloak design is deduced from a geometric transform in the full Navier equations that preserves the symmetry of the elasticity tensor but leads to Willis’ equations, well approximated by a homogenization procedure, as corroborated by numerical simulations. The two cloaks’s designs are strickingly different, and the superior efficiency of the second type of cloak emphasizes the necessity for rigour in transposition of existing cloaks’s designs in thin plates to the geophysics setting. Importantly, we focus our attention on geometric transforms applied to thick plates, which is an intermediate case between thin plates and semi-infinite media, not studied previously. Cloaking efficiency (reduction of the disturbance of the wave wavefront and its amplitude behind an obstacle) and protection (reduction of the wave amplitude within the center of the cloak) are studied for ideal and approximated cloaks’ parameters. These results represent a preliminary step towards designs of seismic cloaks for surface Rayleigh waves propagating in sedimentary soils structured with concrete pillars.
AIP Advances | 2011
Abdelkrim Khelif; Younes Achaoui; Boujemaa Aoubiza
We present a theoretical analysis of an in-plane confinement and a waveguiding of surface acoustic waves in pillars-based phononic crystal. The artificial crystal is made up of cylindrical pillars placed on a semi-infinite medium and arranged in a square array. With a well-chosen of the geometrical parameters, this pillars-based system can display two kinds of complete band gaps for guided waves propagating near the surface, a low frequency gap based on locally resonant mode of pillars as well as a higher frequency gap appearing at Bragg scattering regime. In addition, we demonstrate a waveguiding of surface acoustic wave inside an extended linear defect created by removing rows of pillars in the perfect crystal. We discuss the transmission and the polarization of such confined mode appearing in the higher frequency band gap. We highlight the strong similarity of such defect mode and the Rayleigh wave of free surface medium. An efficient finite element analysis is used to simulate the propagation of guided waves through silicon pillars on a silicon substrate.
Innovative Infrastructure Solutions | 2017
Stéphane Brûlé; Bogdan Ungureanu; Younes Achaoui; Andre Diatta; Ronald Aznavourian; Tryfon Antonakakis; Richard V. Craster; Stefan Enoch; Sébastien Guenneau
Viewed from the sky, the urban fabric pattern appears similar to the geometry of structured devices called metamaterials; these were developed by Physicists to interact with waves that have wavelengths in the range from nanometers to meters (from electromagnetic to seismic metamaterials). Visionary research in the late 1980s based on the interaction of big cities with seismic signals and more recent studies on seismic metamaterials, made of holes or vertical inclusions in the soil, has generated interest in exploring the multiple interaction effects of seismic waves in the ground and the local resonances of both buried pillars and buildings. Here, we use techniques from transformational optics and theoretically validate, by numerical experiments, that a district of buildings could be considered as a set of above-ground resonators, purely elastic, interacting with an incident seismic signal. We hope that our proposal will contribute to all theoretical and experimental efforts in design of cities of the future, from a metamaterial standpoint.
Applied Physics Letters | 2015
Gautier Lefebvre; Marc Dubois; Romain Beauvais; Younes Achaoui; Ros Kiri Ing; Sébastien Guenneau; Patrick Sebbah
We experimentally demonstrate that a Duraluminium thin plate with a thickness profile varying radially in a piecewise constant fashion as h(r)=h(0)(1+(r/Rmax)), with h(0)=0.5 mm, h(Rmax)=2 mm and Rmax=10 cm behaves in many ways as Maxwells fish-eye lens in optics, since its imaging properties for a Gaussian pulse with central frequencies 30~kHz and 60~kHz are very similar to those predicted by ray trajectories (great circles) on a virtual sphere (rays emanating from the North pole meet at the South pole). However, refocusing time depends on the carrier frequency as a direct consequence of the dispersive nature of flexural waves in thin plates. Importantly, experimental results are in good agreement with Finite-Difference-Time-Domain simulations.
Applied Physics Letters | 2014
Damien Fouan; Younes Achaoui; Serge Mensah
A microbubble sizing method based on the use of the odd harmonics of square-like excitations is presented. The microbubble resonance signature is determined by measuring the backscattered signals using the Dual Frequency Method combined with a time-frequency representation. The efficiency and the limitations of this method are described in the case of sine-like excitations. It is then established that the harmonics of square-like excitations can be used to significantly enlarge the range of microbubble detection and sizing. These findings were confirmed and explained by theoretical studies on microbubble dynamics based on the Keller-Miksis formulation.
Physical Review Letters | 2017
Gautier Lefebvre; Tryfon Antonakakis; Younes Achaoui; Richard V. Craster; Sébastien Guenneau; Patrick Sebbah
Periodic structures can be engineered to exhibit unique properties observed at symmetry points, such as zero group velocity, Dirac cones, and saddle points; identifying these and the nature of the associated modes from a direct reading of the dispersion surfaces is not straightforward, especially in three dimensions or at high frequencies when several dispersion surfaces fold back in the Brillouin zone. A recently proposed asymptotic high-frequency homogenization theory is applied to a challenging time-domain experiment with elastic waves in a pinned metallic plate. The prediction of a narrow high-frequency spectral region where the effective medium tensor dramatically switches from positive definite to indefinite is confirmed experimentally; a small frequency shift of the pulse carrier results in two distinct types of highly anisotropic modes. The underlying effective equation mirrors this behavior with a change in form from elliptic to hyperbolic exemplifying the high degree of wave control available and the importance of a simple and effective predictive model.