Multibranch Elastic Bound States in the Continuum.

Constructing a highly localized wave field by means of bound states in the continuum (BICs) promotes enhanced wave-matter interaction and offers approaches to high-sensitivity devices. Elastic waves can carry complex polarizations and thus differ from electromagnetic waves and other scalar mechanical waves in the formation of BICs, which is yet to be fully explored and exploited. Here, we report the investigation of local resonance modes supported by a Lamb waveguide side-branched with two pairs of resonant pillars and show the emergence of two groups of elastic BICs with different polarizations or symmetries. Particularly, the two groups of BICs exhibit distinct responses to external perturbations, based on which a label-free sensing scheme with enhanced-sensitivity is proposed. Our study reveals the rich properties of BICs arising from the complex wave dynamics in elastic media and demonstrates their unique functionality for sensing and detection.

Observation of phononic skyrmions based on hybrid spin of elastic waves.

Skyrmions with topologically stable configuration have shown a promising route toward high-density magnetic and photonic information processing due to their defect-immune and low-driven energy. Here, we experimentally report and observe the existence of phononic skyrmions as new topological structures formed by the three-dimensional hybrid spin of elastic waves. We demonstrate that the frequency-independent spin configuration leads to ultra-broadband feature of phononic skyrmions, which can be produced in any solid structure, including chip-scale ones. We further experimentally show the excellent robustness of the flexibly movable phononic skyrmion lattices against local defects of disorder, sharp corners, and even rectangular holes. Our research opens a vibrant horizon toward an unprecedented way for elastic wave manipulation and structuration by spin configuration and offers a promising lever for alternative phononic technologies, including information processing, biomedical testing, and wave engineering.

Janus acoustic metascreen with nonreciprocal and reconfigurable phase modulations.

Integrating different reliable functionalities in metastructures and metasurfaces has become of remarkable importance to create innovative multifunctional compact acoustic, optic or mechanical metadevices. In particular, implementing different wave manipulations in one unique material platform opens an appealing route for developing integrated metamaterials. Here, the concept of Janus acoustic metascreen is proposed and demonstrated, producing two-faced and independent wavefront manipulations for two opposite incidences. The feature of two-faced sound modulations requires nonreciprocal phase modulating elements. An acoustic resonant unit cell with rotating inner core, which produces a bias by a circulating fluid, is designed to achieve high nonreciprocity, leading to decoupled phase modulations for both forward and backward directions. In addition, the designed unit cell consisting of tunable phase modulators is reconfigurable. A series of Janus acoustic metascreens including optional combinations of extraordinary refraction, acoustic focusing, sound absorption, acoustic diffusion, and beam splitting are demonstrated through numerical simulations and experiments, showing their great potential for acoustic wavefront manipulation.

Acoustic perfect absorbers via Helmholtz resonators with embedded apertures.

Acoustic perfect absorption via a structure with deep subwavelength thickness is of great and continuing interest in research and engineering. This study analytically and experimentally investigates acoustic systems based on Helmholtz resonators which have embedded-apertures. The strategy of embedding apertures greatly improves the ability to manipulate the impedance of the systems. Based on the inverted configuration, perfect absorption has been realized (reaching 0.999 in experiments) via a design whose thickness is only ∼1/50th of the operating wavelength. Moreover, a tunable resonant frequency (137-300 Hz) and tunable absorption frequency bandwidth (22%-46%) can be achieved while preserving the perfect absorption performance and constant external shape. In tuning the perfect absorbers having a constant thickness, a conservation factor is revealed experimentally and then verified analytically, which could guide absorbers' design and facilitate the tuning. In addition, the distinct features of the proposed design were evaluated and validated and were compared with those of a related structure, a metasurface with a coiled backing cavity. The results have the potential to help with the design of highly efficient, thin, and tunable acoustic absorbers.

Effects of bovine serum albumin on a single cavitation bubble.

The dynamics and sonoluminescence (SL) of a single cavitation bubble in bovine serum albumin (BSA) aqueous solutions have been experimentally and theoretically investigated. A phase-locked integral imaging has been used to record the bubble pulsation evolutions. The results show that, under the optimum driving condition, the endurable driving pressure, maximum radius, radius compression ratio and SL intensity of the cavitation bubble increase correspondingly with the increase of BSA concentrations within the critical micelle concentration, which indicates that the addition of BSA increases the power capability of the cavitation bubble. In addition, BSA molecules dampen the interfacial motion, and especially the rebounds of the bubble after its collapse. BSA molecules modify the dilatational viscosity and elasticity of the bubble wall. A viscoelastic interfacial rheological model that mainly emphasizes on the description of the bubble wall has been introduced and modified to theoretically explain the measured bubble dynamics. A good consensus between the experimental observation and model calculation has been achieved.

Quasi-static stop band with flexural metamaterial having zero rotational stiffness.

Metamaterials realizing stop bands have attracted much attentions recently since they can break-through the well-known mass law. However, achieving the stop band at extremely low frequency has been still a big challenge in the fields of elastic metamaterials. In this paper, we propose a new metamaterial based on the idea of the zero rotational stiffness, to achieve extremely low frequency stop band for flexural elastic waves. Unlike the previous ways to achieve the stop band, we found that the zero rotational stiffness can provide a broad stop band at extremely low frequency, which starts from even almost zero frequency. To achieve the zero rotational stiffness, we propose a new elastic metamaterial consisting of blocks and links with the hinge connection. Analytic developments as well as numerical simulations evidence that this new metamaterial can exhibit extremely low and broad stop band, even at the quasi-static ranges. In addition, the metamaterial is shown to exhibit the negative group velocity at extremely low frequency ranges, as well as the quasi-static stop band, if it is properly designed.

Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications.

This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.

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.

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.

Computation of plate wave dispersion diagrams and surface wave velocities without explicit boundary conditions.

We discuss the computation of the band structure of plate waves using the plane wave expansion (PWE) method. This method is generally used to formulate eigenvalue problems to compute dispersion diagrams for solid-solid phononic crystals. We show how the free surface boundary condition can be included implicitly in the form of the PWE solution, thus leading to an efficient eigenvalue problem. This generic method for wave dispersion is non-iterative and does not require an initial guess for the solution. Furthermore, surface acoustic wave velocities can be estimated from the slowest wave for large wave vectors. Examples for a single plate and a multilayer plate are given, and extension to piezoelectric materials is discussed.

Phononic crystals based on LiNbO3 realized using domain inversion by electron-beam irradiation.

We report in this paper about the realization of domain inversion in z-cut lithium niobate by electron beam irradiation in order to perform phononic crystals. The fabrication of these phononic crystals on z-cut LiNbO3, which is, in our case, a periodic repetition of voids and LiNbO3, was achieved by domain inversion followed by wet etching, taking advantage of the large difference in etching rate between z+ and z- faces. A pertinent choice of irradiation conditions such as accelerating voltage, beam current, and charge density was determined and optimized. Two-dimensional structures at the micrometer scale were then realized on z-cut LiNbO3. We demonstrate the achievement of hexagons with diameters between 2 microm and 18 microm, with a very important depth close to 30 microm, which depends on the etching time and the hole size. The obtained structures, which exhibit a filling fraction varying from 1% to 64%, were characterized before etching by polarizing microscope to visualize the inverted domains. After HF etching, scanning electron microscopy was used to observe the obtained phononic structures. Taking into account the obtained filling fraction values and the size of created hexagons, the frequency band gap of these structures is expected at a range of 200 to 350 MHz. As expected in this frequency range, we have proven experimentally the existence of the phononic band gap on z-cut LiNbO3 by combination of a realized phononic crystal with a surface acoustic wave (SAW) device.

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(MS) = 13 nm, whereas very low surface roughness (as low as R(MS) < or = 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 (K2) were determined experimentally and by calculation as a function of normalized thickness AlN film (kh(AlN) = 2pi h(AlN)/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.