Physical Review Applied

A neuromorphic metasurface embodies mechanical intelligence by realizing physical neural architectures, enabling computational capabilities within an elastic substrate with minimal energy requirements. However, well-established constraints on wave propagation in finite media have limited such systems to a single task. This work introduces a wave-based dual classifier that utilizes a single neuromorphic metasurface to concurrently execute two completely independent classification problems, via dynamic modulation of embedded waveguides. These results point to paradigms in wave-based computing systems that have thus far been elusive.

Steering opposed elastic waves independently with a single platform is crucial for multifunctional mechanical devices, but has remained out of reach. This study presents a Janus elastic metasurface that enables independent bidirectional control of broadband flexural waves, supported by a comprehensive theoretical framework and experimental verification. Leveraging high reflection and bianisotropic phase shifts, the design overcomes the one-sided limitation of conventional elastic metasurfaces. This innovative advance opens avenues to multifunctional devices, vibration control, and energy harvesting.

Phononic crystals are artificial periodic structures that leverage phonon wave interference to control mechanical vibrations at various length scales. Their spatial and spectral limits, however, are currently unknown. The authors report experimental observations of a surprising breakdown of phonon interference in two-dimensional nanoscale phononic crystals. These findings refine the current understanding of nanoscale phonon interference, and should guide phononic crystal applications in microelectronics and acoustic quantum computing.

While the Berry phase’s influence on energy spectra in the adiabatic limit is well established, its role in resonance transitions has remained elusive. The authors investigate a three-resonator system that carries a quantized Berry phase and find that the resonance transition shows unexpected deviations from frequency conservation. This system also reveals a chiral transition effect in which the final resonant state depends on the direction of the modulation loop. Circuit simulations corroborate these findings. This work bridges topological physics with parametric oscillators, offering new insight into the interplay between topology and dynamical systems.

Materials that remain ventilating while providing airborne sound insulation are highly desirable for everyday noise controllers. This review discusses lattice metamaterials as an innovative class of advanced structures capable of fulfilling both functions. The authors survey sound-insulation performance and mechanisms, and identify the most effective types of architectures among distinct categories of lattice metamaterials. In addition, they propose numerical strategies to enable accelerated design exploration.

Mode conversion between longitudinal and flexural elastic waves allows the energy in hard-to-detect bulk waves to produce surface-readable bending motion, which is valuable for structural health monitoring, ultrasonic sensing, and nondestructive testing. Here most devices rely on dense resonator arrays or intricate architectures that operate in narrow frequency ranges and are hard to fabricate. This work offers a simpler route: monolithic beams of undulating profile forming a phononic crystal that is tuned to couple longitudinal and flexural behavior. Mode locking with inverted group velocities converts incident longitudinal waves into flexural waves via reflection.

Zero-frequency and zero-wave-number band gaps are highly desirable for controlling low-frequency elastic waves, but are fundamentally forbidden in conventional passive elastic systems. The authors show that optomechanical trapping with optical tweezers based on a metasurface introduces tunable, contactless stiffness in a mass-spring chain, enabling the realization of both types of band gaps. This approach overcomes long-standing physical constraints and opens possibilities for active and reconfigurable elastic metamaterials.

Nanomechanical waveguides are desirable components in quantum acoustics, but their applications are limited by the ability to prevent phonon loss and decoherence while coupling to electromagnetic circuits. This study solves both problems by introducing a nanomechanical waveguide that combines piezoelectric coupling to circuits with confined phonons in a silicon waveguide metamaterial. The authors discover correlated dephasing, which is relevant to quantum device applications and suggests a common source of frequency noise. These results introduce a device to the toolbox of phononic circuit elements for quantum acoustodynamics, with potential uses in information processing and sensing.

Underwater noise significantly impacts marine ecosystems, so its control becomes increasingly important. Existing solutions are often limited by insufficient low-frequency bandwidth, bulky structures, and deployment inconvenience. This work presents a foldable acoustic metamaterial that employs air-entrained panels to realize soft-boundary waveguides, suppressing wave propagation below a designed cutoff frequency. This lightweight, deep-subwavelength structure enables low-frequency noise attenuation over more than three octaves, demonstrating strong potential for large-scale offshore engineering applications, such as bridge pilings and wind farms.

High-frequency A1-mode Lamb-wave resonators are promising for next-generation wireless communication, due to their strong electromechanical coupling, but spurious modes limit their performance. This study presents a circular resonator design that exploits anisotropic acoustic velocities in Z-cut lithium niobate and radially varying interdigital transducers to suppress unwanted modes, while maintaining efficient excitation of the A1 mode. Unexpectedly, the spatial interference of independent spurious modes enhances suppression beyond conventional designs. This approach offers a pathway to high-performance acoustic resonators for wideband rf applications.

Magnetoelastic coupling between surface acoustic waves and spin waves in ferromagnetic thin films enables nonreciprocal signal processing and programmable wave filters, but self-consistent simulations of layered heterostructures are tricky, in terms of interfacial stress and strain discontinuities. This work presents a finite-difference time-integration scheme within an open-source micromagnetic library, to self-consistently solve the coupled magnetic and elastodynamic equations with rigorously enforced jump conditions at interfaces. Validated against analytical solutions and experimental data, this solver establishes a robust, open-source tool for designing magnon-phonon hybrid devices.

Bound states in the continuum (BICs) are of interest for sensing applications, because their sensitivity can be made very sharp (with a quality factor that diverges, in principle), but their utility is held back because coupling to radiation loss is difficult to assess. This study makes progress by considering BIC sensitivity to the true boundary conditions, and clarifying the relation of BICs to the quasinormal modes of open systems. This insight is expected to impact the practical design of sensing solutions based on wave propagation.

High-$Q$ acoustic resonators are important for sensing and wave control, but compact airborne designs are limited by radiation leakage and unclear elastic-acoustic coupling. Combining full-wave simulations with temporal coupled-mode theory, the authors reveal a Friedrich-Wintgen quasi–bound state in the continuum arising from shear-acoustic interplay in a silicon frame. This coupling enhances destructive interference, enabling an ultrahigh-$Q$ resonance in a simple, compact structure. The result provides a practical route to high-performance acoustic devices.

Microscale ultrasound acoustofluidics has become an important tool for handling cells and microparticles in lab-on-a-chip technology, but it fails in separation and focusing of submicrometer particles, due to dissipation processes in the viscous boundary layer. Here a theoretical proof-of-concept analysis shows that by embedding the microfluidic channel in a properly designed fused-silica metamaterial, the vibrational motion of the fluid and the metamaterial can be matched, which prevents formation of the viscous boundary layer. This increases the $Q$ factor of the ultrasound resonance modes by several orders of magnitude, reduces acoustic streaming, and enables nanoparticle focusing.

Surface-acoustic-wave radio-frequency (SAW-RF) devices are important for wireless telecommunication, yet show limited functionality, large footprint, and lack of reconfigurability. This study integrates thin films of phase-change materials (PCMs) with existing SAW-RF technology to overcome these limitations. The authors discover that PCM-based thin-film superlattices can endow a SAW-RF device’s acoustic waves with topological character. Using reconfigurable PCM significantly improves device performance without increasing the footprint, opening technological avenues for low-loss next-generation radio-frequency telecommunication.

Contactless actuation with acoustic waves is attractive for applications in robotics, programmable matter, and active structures, but the ability to control motion is typically limited to small, subwavelength objects. This study presents frequency-detuned acoustic metasurfaces that overcome this limitation by converting small changes in wave frequency into large, reversible acoustic forces and torques on objects larger than the wavelength. Motion is programmed into the surface and commanded through frequency alone, opening opportunities for battery-free robotics, programmable materials, soft robotic systems, and remote mechanical control across a broad range of wave-based technologies.

This work introduces a unified platform supporting topological states for both electromagnetic and elastic waves, within the same artificial crystal. While topological photonic and phononic systems typically are investigated independently, the approach here enables direct comparison of their topological properties and transport behaviors in the selfsame geometry. Breaking a specific spatial symmetry in a graphenelike lattice opens valley-polarized topological band gaps in both physical domains, yielding interfacial states between topologically distinct crystals. The authors establish a general, unifying framework for topological photonics and phononics.