Recently, the domain of topological insulators in condensed matter physics has been teeming with intriguing and very exciting discoveries. Notably, the capacity of guiding currents towards specific directions according to the spin of the travelling electrons has a great potential for electronic devices. This new phenomenon has been transposed to the classical domain in electromagnetics and acoustics, unveiling the pseudo-spin locking of guided waves. However, these macroscopic analogues are photonic/phononic crystals which are intrinsically wavelength scaled. In this work, we realize a genuine acoustic analogue of the valley-Hall effect in the audible regime using a lattice of soda cans. The crystalline description of this very simple metamaterial allows us to demonstrate experimentally the unidirectional excitation of sound guided at a scale much smaller than the wavelength of operation. These results not only open the tantalizing valley-topological phenomena to the audible regime but also allow to envision compact applications for acoustic manipulation.
This paper presents an overview of Unidirectional Loop Metamaterial (ULM) structures and applications. Mimicking electron spin precession in ferrites using loops with unidirectional loads (typically transistors), the ULM exhibits all the fundamental properties of ferrite materials, and represents the only existing magnetless ferrimagnetic medium. We present here an extended explanation of ULM physics and unified description of its component and system applications.
We prove the possibility of achieving unidirectional edge modes in time-modulated supercell structures. Such finite structures consist of two trimers repeated periodically. Because of their symmetry, they admit degenerate edge eigenspaces. When the trimers are time-modulated with two opposite orientations, the degenerate eigenspace splits into two one-dimensional eigenspaces described by an analytical formula, each corresponds to a mode which is localized at one edge of the structure. Our results on the localization and stability of these edge modes with respect to fluctuations in the time-mod…
Nonadiabatic holonomic quantum transformations (NHQTs) have attracted wide attention and have been applied in many aspects of quantum computation, whereas related research is usually limited to the field of quantum physics. Here we bring NHQTs into constructing a unidirectional acoustic metamaterial (UDAM) for shaping classical beams. The UDAM is made up of an array of three-waveguide couplers, where the propagation of acoustic waves mimics the evolution of NHQTs. The excellent agreement among analytical predictions, numerical simulations, and experimental measurements confirms the great appli…
Unidirectional radiation is of particular interest in high-power lasing and optics. Commonly, however, it is difficult to achieve a unidirectional profile in such a system without breaking reciprocity. Recently, assisted by metamaterials without structural symmetry, antennas that radiate asymmetrically have been developed, hence providing the possibility of achieving unidirectionality. Nevertheless, it has been challenging to achieve extremely high radiation asymmetry in such antennas. Here, we demonstrate that this radiation asymmetry is further enhanced when magnetic plasmons are present in …
We investigate elastic-wave propagation in a spatially-dispersive multilayered, totally passive metamaterial system. At oblique incidence a longitudinal (acoustic) wave can convert to transverse in the solid material comprising the layers, but when the incident wave enters the multilayer from a solid as opposed to a liquid medium, the incident transverse component supported by the solid medium indirectly causes the longitudinal transmission response to be greatly modified and similarly for the transverse wave exiting the multilayer into a solid medium in response to an incident longitudinal wa…
In lossless acoustic systems, mode transitions are always time-reversible, consistent with Lorentz reciprocity, giving rise to symmetric sound manipulation in space-time. To overcome this fundamental limitation and break space-time symmetry, nonreciprocal sound steering is realized by designing and experimentally implementing spatiotemporally-modulated acoustic metamaterials. Relying on no slow mechanical parts, unstable and noisy airflow or complicated piezoelectric array, our mechanism uses the coupling between an ultrathin membrane and external electromagnetic field to realize programmable,…
Topological mechanical metamaterials translate condensed matter phenomena, like non-reciprocity and robustness to defects, into classical platforms. At small scales, topological nanoelectromechanical metamaterials (NEMM) can enable the realization of on-chip acoustic components, like unidirectional waveguides and compact delay-lines for mobile devices. Here, we report the experimental realization of NEMM phononic topological insulators, consisting of two-dimensional arrays of free-standing silicon nitride (SiN) nanomembranes that operate at high frequencies (10-20 MHz). We experimentally demon…
Bianisotropy is common in electromagnetics whenever a cross-coupling between electric and magnetic responses exists. However, the analogous concept for elastic waves in solids, termed as Willis coupling, is more challenging to observe. It requires coupling between stress and velocity or momentum and strain fields, which is difficult to induce in non-negligible levels, even when using metamaterial structures. Here, we report the experimental realization of a Willis metamaterial for flexural waves. Based on a cantilever bending resonance, we demonstrate asymmetric reflection amplitudes and phase…
In an effective medium description of acoustic metamaterials, the Willis coupling plays the same role as the bianisotropy in electromagnetism. Willis media can be described by a constitutive matrix composed of the classical effective bulk modulus and density and additional cross-coupling terms defining the acoustic bianisotropy. Based on an unifying theoretical model, we unite the properties of acoustic Willis coupling with $\mathcal{PT}$ symmetric systems under the same umbrella and show in either case that an exceptional point hosts a remarkably pronounced scattering asymmetry that is accomp…
We present the concept of a feedback-based topological acoustic metamaterial as a tool for realizing autonomous and active guiding of sound beams along arbitrary curved paths in free two-dimensional space. The metamaterial building blocks are acoustic transducers, embedded in a slab waveguide. The transducers generate a desired dispersion profile in closed-loop by processing real-time pressure field measurements through preprogrammed controllers. In particular, the metamaterial can be programmed to exhibit analogies of quantum topological wave phenomena, which enables unconventional and except…
This paper investigates the pseudo-spin based edge states for flexural waves in a honeycomb perforated phononic plate, which behaves an elastic analogue of the quantum spin Hall effect. We utilize finite element method to analyse the dispersion for flexural waves based on Mindlin's plate theory. Topological transition takes place around a double Dirac cone at $Γ$ point by adjusting the sizes of perforated holes. We develop an effective Hamiltonian to describe the bands around the two doubly degenerated states and analyse the topological invariants. This further leads us to observe the topologi…
Shortcut to adiabatic passage (STAP), initially proposed to accelerate adiabatic quantum state transfer, has been widely explored and applied in quantum optics and integrated optics. Here we bring STAP into the field of acoustics to design compact couplers and functional metamaterial. The space-varying coupling strengths of acoustic waveguides (WGs) are tailored by means of dressed states in a three-level system, accounting for the desirable acoustic energy transfer among three WGs with short length. We show that the acoustic coupler has one-way feature when loss is introduced into the interme…
Topological valley photonics has emerged as a new frontier in photonics with many promising applications. Previous valley boundary transport relies on kink states at internal boundaries between two topologically distinct domains. However, recent studies have revealed a novel class of topological chiral edge states (CESs) at external boundaries of valley materials, which have remained elusive in photonics. Here, we propose and experimentally demonstrate the topological CESs in valley photonic metamaterials (VPMMs) by accurately tuning on-site edge potentials. Moreover, the VPMMs work at deep-su…
High index optical waveguide devices such as slab waveguides, strip waveguides and fibers play extremely important roles in a wide range of modern applications including telecommunications, sensing, lasing, interferometry, and resonant amplification. Yet, transposing these advantageous applications from optics to acoustics remains a fundamental practical challenge, since most materials exhibit refractive indices lower than that of air for sound waves. Here, we demonstrate the relevance of acoustic metamaterials for tackling this pivotal problem. More specifically, we consider a metamaterial bu…
Topological metamaterials exhibit unusual behaviors at their boundaries, such as unidirectional chiral waves, that are protected by a topological feature of their band structure. The ability to tune such a material through a topological phase transition in real time could enable the use of protected waves for information storage and readout. Here we dynamically tune through a topological phase transition by breaking inversion symmetry in a metamaterial composed of interacting gyroscopes. Through the transition, we track the divergence of the edge modes' localization length and the change in Ch…
Routing and guiding acoustic waves around sharp corners without backscattering losses is of great interest in the acoustics community. Sonic crystals have been primarily utilized to design backscattering-immune waveguides. While conventional approaches use defects to guide waves, a considerably more sophisticated and robust approach was recently developed based on topological edge states. In this paper, we propose a radically different theoretical framework based on extremely anisotropic metamaterials for engineering backscattering-immune waveguides. We theoretically derived the exact conditio…
Chiral Landau levels induced by two in-plane pseudomagnetic fields in underwater acoustic metamaterials
Jiangsu Key Laboratory for Design and Manufacturing of Precision Medicine Equipment, School of Mechanical Engineering, Southeast University（东南大学机械工程学院精密医疗器械设计与制造江苏省重点实验室）；Key Laboratory of Underwater Acoustic Signal Processing (Southeast University), Ministry of Education（教育部水下声学信号处理重点实验室（东南大学））；Advanced Ocean Institute of Southeast University Nantong（东南大学南通海洋研究院）；State Key Laboratory of Acoustics and Marine Information, Institute of Acoustics, Chinese Academy of Sciences（中国科学院声学研究所声学与海洋信息国家重点实验室）；Modern Matter Laboratory and Advanced Materials Thrust, The Hong Kong University of Science and Technology (Guangzhou)（香港科技大学（广州）现代物质实验室先进材料课题组）
A State-Space Framework for trivial and Topological Metamaterial Stochastic Analysis
University of Campinas, School of Mechanical Engineering（坎皮纳斯大学机械工程学院）；University of São Paulo, São Carlos School of Engineering, Department of Aeronautical Engineering（圣保罗大学圣卡洛斯工程学院航空工程系）；KU Leuven, Faculty of Engineering Technology, Department of Mechanical Engineering, LMSD Division, Ghent Campus（荷语鲁汶大学工程技术学院机械工程系LMSD分部根特校区）；Instituto Universitario de Matemática Pura y Aplicada, Departamento de Matemática Aplicada, Universitat Politècnica de València（瓦伦西亚理工大学应用数学研究所应用数学系）
Time-varying guiding structures introduce an additional degree of freedom, besides spatial-variation, that enables better control over the guided wave in a device. Periodically time-modulated structures which are usually considered enable wave control over narrowband signals. However, for ultrawideband short-pulse signals, time-variation in the form of temporal discontinuities is required. Such a setup has recently been proposed as a mean to overcome the Bode-Fano bound on impedance matching. While hard (abrupt) temporal discontinuities are relatively simple to analyze by employing continuity …
We investigate the dispersion characteristics and the effective properties of acoustic waves propagating in a one-dimensional duct equipped with periodic thermoacoustic coupling elements. Each coupling element consists in a classical thermoacoustic regenerator subject to a spatial temperature gradient. When acoustic waves pass through the regenerator, thermal-to-acoustic energy conversion takes place and can either amplify or attenuate the wave, depending on the direction of propagation of the wave. The presence of the spatial gradient naturally induces a loss of reciprocity. This study provid…
Topological features embedded in ancient braiding and knotting arts endow significant impacts on our daily life and even cutting-edge science. Recently, fast growing efforts are invested to the braiding topology of complex Bloch bands in non-Hermitian systems. This new classification of band topology goes far beyond those established in Hermitian counterparts. Here, we present the first acoustic realization of the topological non-Hermitian Bloch braids, based on a two-band model easily accessible for realizing any desired knot structure. The non-Hermitian bands are synthesized by a simple bina…
Topological systems are inherently robust to disorder and continuous perturbations, resulting in dissipation-free edge transport of electrons in quantum solids, or reflectionless guiding of photons and phonons in classical wave systems characterized by Chern or spin-Chern topological invariants. These established examples of topological physics, however, do not exhaust all possible topological phases, and recently a new class of topological metamaterials characterized by bulk polarization has been introduced. In addition to edge conduction, these systems have been shown to host higher-order to…
