Locally Resonant Metagrating by Elastic Impedance Modulation

The optical and acoustic metagratings have addressed the limitations of low-efficiency wave manipulation and high-complexity fabrication of metamaterials and metasurfaces. In this research, we introduce the concept of elastic metagrating and present the theoretical and experimental demonstration of locally resonant elastic metagrating (LREM). Remarkably, the LREM, with dimensions two orders of magnitude smaller than the relevant wavelength, overcomes the size limitations of conventional metagratings and offers a unique design paradigm for highly efficient wave manipulation with an extremely compact structure in elastic wave systems. Based on a distinctive elastic impedance engineering with hybridization of intrinsic evanescent waves, the proposed LREM achieves wide-angle perfect absorption. This tackles a fundamental challenge faced by all elastic metastructures designed for wave manipulation, which consists in the unavoidable vibration modes in finite structures hindering their implementations in real-world applications

Perfect anomalous splitter by acoustic meta-grating

As an inversely designed artificial device, metasurface usually means densely arranged meta-atoms with complex substructures. In acoustics, those meta-atoms are usually constructed by multi-folded channels or multi-connected cavities of deep sub-wavelength feature, which limits their implementation in pragmatic applications. We propose here a comprehensive concept of a perfect anomalous splitter based on an acoustic meta-grating. The beam splitter is designed by etching only two or four straight-walled grooves per period on a planar hard surface. Different from the recently reported reflectors or splitters, our device can perfectly split an incident wave into different desired directions with arbitrary power flow partition. In addition, because ultrathin substructures with thin walls and narrow channels are avoided in our design procedure, the proposed beam splitter can be used for waves with much shorter wavelength compared to the previous suggested systems. The design is established by rigorous formulae developed under the framework of the grating theory and a genetic optimization algorithm. Numerical simulation and experimental evidence are provided to discuss the involved physical mechanism and to give the proof-of-concept for the proposed perfect anomalous acoustic splitter.Comment: 5 figure

Study of tantalum and iridium as adhesion layers for Pt/LGS high temperature SAW devices

International audienceIn this paper, we report on the use of tantalum and iridium as adhesion layers for platinum electrodes used in high temperature SAW devices based on langasite substrates (LGS). Unlike iridium, tantalum exhibits a great adhesive strength, and a very low mobility through the Pt film, ensuring a device lifetime of at least half an hour at 1000°C. The latter is limited by morphological modifications of platinum, starting by the apparition of crystallites on the surface, and followed by important terracing and breaking of the film continuity. SNMS and XRD measurements allowed us to show that these phenomena are likely intrinsic to platinum film, whatever be the nature of the adhesion layer. Finally, after having outlined a possible scenario leading to this deterioration, we consider some solutions that could replace platinum in order to increase the lifetime of LGS-based SAW devices in high temperatures conditions

Tunable solid acoustic metamaterial with negative elastic modulus

International audienceWe report in this letter on a tunable solid acoustic metamaterial with negative elastic modulus by means of piezoelectric composite. The theoretical formulae for one-dimensional layer-stacked metamaterial embedding a piezoelectric material by means of external shunted inductors are presented. The acoustic band structure of the composite is calculated by the transfer matrix method. Results show that a band gap can be opened and tuned by the resonant behavior of the LC circuit. It is found further by the formulae that piezoelectric material with large piezoelectric constant and small elastic modulus will be beneficial for opening a wide band gap. The effective elastic constant of the system is also calculated by the unit-cell-boundary-averaging method. Result shows that the system behaves as an effective medium with a negative elastic modulus. This property is quite different from the typical solid metamaterial achieved by dispersing heavy inclusions coated with a soft layer into a matrix for which only the negative mass density can be obtained