Badreddine Assouar

Acoustic metasurface-based perfect absorber with deep subwavelength thickness

Conventional acoustic absorbers are used to have a structure with a thickness comparable to the working wavelength, resulting in major obstacles in real applications in low frequency range. We present a metasurface-based perfect absorber capable of achieving the total absorption of acoustic wave in an extremely low frequency region. The metasurface possessing a deep subwavelength thickness down to a feature size of ∼λ/223 is composed of a perforated plate and a coiled coplanar air chamber. Simulations based on fully coupled acoustic with thermodynamic equations and theoretical impedance analysis are utilized to reveal the underlying physics and the acoustic performances, showing an excellent agreement. Our realization should have an high impact on amount of applications due to the extremely thin thickness, easy fabrication, and high efficiency of the proposed structure.

Surface acoustic wave devices based on AlN/sapphire structure for high temperature applications

AlN/sapphire layered structure has been investigated as a potential substrate for surface acoustic wave (SAW) devices operating at high temperatures up to 950 °C under air atmosphere. Frequency characterizations of the SAW delay lines based on this structure indicate a slight increase of 2 dB in the insertion losses after annealing for 30 min at 900 °C. Scanning electron and atomic force microscopy as well as x-ray diffraction measurements suggest that theses losses are due to the deterioration of the Pt/Ta electrodes and to a slight oxidation of the AlN film.

Numerical investigation of the propagation of elastic wave modes in a one-dimensional phononic crystal plate coated on a uniform substrate

The propagation of wave modes in a two-layer free standing plate composed of a one-dimensional phononic crystal (PC) thin layer coated on a uniform substrate was investigated numerically by the modified plane wave expansion method. The band structures of the system with different thicknesses of the substrate were calculated. The numerical result showed that Bragg scattering by the periodic structure in a PC and wave scattering by the free surface could be coupled to each other with an added substrate layer. The properties of the confined modes in such a system, for example, the Love-wave-like mode, the confined PC mode (which is localized mainly in the PC layer) and the surface mode on the free surface of the substrate layer, were investigated.

Effect of diamond nucleation process on propagation losses of AlN/diamond SAW filter

In this work, the effect of a diamond nucleation process on freestanding aluminium nitride (AlN)/diamond surface acoustic wave (SAW) device performances was studied. Before diamond deposition, silicon (Si) substrates have been mechanically nucleated, using an ultrasonic vibration table with submicron diamond slurry, and bias-enhanced nucleated (BEN). Freestanding diamond layers obtained on mechanically scratched Si substrates exhibit a surface roughness of R/sub MS/=13 nm, whereas very low surface roughness (as low as R/sub MS//spl les/1 nm) can be achieved on a freestanding BEN diamond layer. Propagation losses have been measured as a function of the operating frequency for the two nucleation techniques. Dispersion curves of phase velocities and electromechanical coupling coefficient (K/sup 2/) were determined experimentally and by calculation as a function of normalized thickness AlN film (kh/sub AlN/=2/spl pi/h/sub AlN///spl lambda/). Experimental results show that the propagation losses strongly depend on the nucleation technique, and that these losses are weakly increased with frequency when the BEN technique is used.

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.

Opening a large full phononic band gap in thin elastic plate with resonant units

In this paper, the mechanism for opening a locally resonant band gap in a thin elastic plate is investigated. Two previously suggested structures, which are constructed by periodically drilling holes on elastic plate and then filling them with the rubber-coated masses, or just by periodically stubbing the rubber rods with mass cap on the plate, are revisited. We find that, because of the partial band gaps for in-plane and out-of-plane plate modes cannot be appropriately overlapped, the full band gaps in both of the structures are generally narrow. The reason for this phenomenon is based on the selective coupling between the different resonant patterns of the resonant units and the in-plane and out-of-plane plate modes. Based on the understanding, a new structure with the three-layered spherical resonant units is proposed. Numerical results show that, making use of such kind of resonant units, a large sub-wavelength full band gap can be opened.

Iridium interdigital transducers for high-temperature surface acoustic wave applications

Iridium is investigated as a potential metal for interdigital transducers (IDTs) in SAW devices operating at high temperatures. SAW delay lines based on such IDTs and langasite (LGS) substrate are fabricated and electrically characterized. The results show reliable frequency responses up to 1000°C. The strong increase of insertion losses beyond this temperature, leading to the vanishing of the signal between 1140 and 1200°C, is attributed to surface transformation of the LGS crystal, consisting of relevant gallium and oxygen losses, as evidenced by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and secondary ion mass spectroscopy.

Acoustic energy harvesting based on a planar acoustic metamaterial

We theoretically report on an innovative and practical acoustic energy harvester based on a defected acoustic metamaterial (AMM) with piezoelectric material. The idea is to create suitable resonant defects in an AMM to confine the strain energy originating from an acoustic incidence. This scavenged energy is converted into electrical energy by attaching a structured piezoelectric material into the defect area of the AMM. We show an acoustic energy harvester based on a meta-structure capable of producing electrical power from an acoustic pressure. Numerical simulations are provided to analyze and elucidate the principles and the performances of the proposed system. A maximum output voltage of 1.3 V and a power density of 0.54 μW/cm3 are obtained at a frequency of 2257.5 Hz. The proposed concept should have broad applications on energy harvesting as well as on low-frequency sound isolation, since this system acts as both acoustic insulator and energy harvester.

Hypersonic characterization of sound propagation velocity in AlxGa1−xN thin films

Sound propagation velocity of surface acoustic waves (SAWs) and bulk acoustic waves (BAWs) has been investigated by means of high resolution Brillouin spectroscopy. The results show a linear dependence of the BAW propagation velocity with the Al concentration. There is no relevant influence of the substrate chosen in the propagation velocity of BAWs in AlxGa1−xN thin films. SAW propagation velocity of epitaxially grown AlN is clearly lower than the observed one in AlN prepared by reactive dc magnetron sputtering. Numerical simulation results of SAW propagation velocity are compared with the experimental results.

Transmission property of the one-dimensional phononic crystal thin plate by the eigenmode matching theory

Eigenmode matching theory, which was developed originally for the band structure and the transmission property of the infinite phononic crystal (PC), is extended to deal with the PC thin plate. By this method, the transmission property of the one-dimensional PC thin plate with and without a uniform substrate is investigated. It is shown that in the PC thin plate without a substrate, the permitted band of the structure can be separated into two parts, which can be excited by the incident antisymmetric and symmetric Lamb modes, respectively. However, for the PC plate with a substrate, the energy conversion between the symmetric and antisymmetric modes can be found in the transmission spectrum. The physical origin of such an energy conversion is discussed.