We propose and experimentally demonstrate a generic framework, requiring neither active components nor complex designs, for undistorted asymmetric elastic-wave transmission in a thin plate using just a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of the +1st and −1st orders on opposite sides of the metasurface. Compared with previous loss-induced strategies, the present metasurface provides nearly total transmission on the transportation side, but total reflection on the opposite side, exhibiting a higher contrast ratio of the transmission. Moreover, we show that this strongly asymmetric behavior is robust with respect to the incidence angle, the frequency, and the effects of loss. The present work paves the way to compact rectification, high-resolution ultrasonography, and vibration and noise control in elastodynamics and acoustics.

Guiding classical waves has inspired a wealth of nontrivial physics and significant applications. To date, a robust and compact way to guide energy flux traveling along an arbitrary, prescheduled trajectory in a uniform medium is still a fundamental challenge. Here we propose and experimentally realize a generic framework of ultrathin waveguides for omnidirectional wave trapping and efficient routing. The metagrating-based waveguide can totally suppress all high-order parasitic diffractions to route guided elastic waves without leakage. The proposed waveguide protype works in a broad frequency range under a full-angle radiated source. An analytical slab-waveguide model is presented to predict and tailor the diffracted patterns. Compared with existing methods based on topological edge states or defected metamaterials, our meta-waveguide strategy exhibits absolute advantages in compact size, robust performance, and easy fabrication, which may provide a design paradigm for vibration and noise control, energy harvesting, microfluidics, wave steering in acoustics and other waves.

Nonreciprocal transmission of waves is highly desirable for the transport and redistribution of energy. However, building an asymmetric system to break time-reversal symmetry is relatively difficult because it tends to work under stringent guidelines, narrow bandwidth, or external impetus, particularly in a three-port system. Without breaking reciprocity, realizing “one-way” transmission of elastic waves by a linear and passive structure in a higher-dimensional asymmetric system, such as a three-port circulator, poses quite a challenge. Here, based on the wave-vector modulation mechanism, we propose an elastic-wave circulator that achieves this without breaking reciprocity, enabling perfect mode transition and wave trapping simultaneously. Requiring neither activated media nor relying on the nonlinearity of nonreciprocal devices, the circulator routes elastic waves purely in a clockwise direction, offering superior performance in broad bandwidth, robust behavior, and simple configuration. Our study provides a feasible platform for asymmetric wave transport in a three-port system, which can be useful in the routing, isolation, and harvesting of energy and can also be extended to other fields, such as electromagnetic and acoustic waves.

Over the past 3 decades, phononic crystals experienced revolutionary development for understanding and utilizing mechanical waves by exploring interaction between mechanical waves and structures. With the significant advances in manufacture technologies from nanoscale to macroscale, phononic crystals attract researchers from diverse disciplines to study abundant directions such as bandgaps, dispersion engineering, novel modes, reconfigurable control, efficient design algorithms and so on. The aim of this roadmap is to present the current state of the art, an overview of properties, functions and applications of phononic crystals, opinions on the challenges and opportunities. The various perspectives cover wide topics on basic property, homogenization, machine learning assisted design, topological, non-Hermitian, nonreciprocal, nanoscale, chiral, nonlocal, active, spatiotemporal, hyperuniform properties of phononic crystals, and applications in underwater acoustics, seismic wave protection, vibration and noise control, thermal transport, sensing, acoustic tweezers, written by over 40 renown experts. It is also intended to guide researchers, funding agencies and industry in identifying new prospects for phononic crystals in the upcoming years.

Metasurfaces have shown tremendous steering functionalities over acoustic waves. However, anomalous refractions of metasurfaces based on generalized Snell's law (GSL) are hindered in efficiency due to impedance mismatch, especially at steep refraction angles. In this work, we design amplitude-modulated binary metasurfaces that can perfectly redirect incident waves. The present metasurface consists of two unit cells with properly modulated transmission amplitudes, which satisfies the requirement of power distribution to totally suppress parasitic diffractions. As demonstrations, we redirect incident waves into steep refraction angles of $-70°$ and $-80°$ with efficiencies up to $0.97$ and $0.99$ , which are much higher than theoretical limits of GSL $0.77$ and $0.52$ , respectively. Present metasurfaces have advantages in simplified designs and fabrications and offer versatility for efficient manipulations of sound.