We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging.

Topological Phononic Crystals with One-Way Elastic Edge Waves

We propose a framework to realize helical edge states in phononic systems using two identical lattices with interlayer couplings between them. A methodology is presented to systematically transform a quantum mechanical lattice which exhibits edge states to a phononic lattice, thereby developing a family of lattices with edge states. Parameter spaces with topological phase boundaries in the vicinity of the transformed system are illustrated to demonstrate the robustness to mechanical imperfections. A potential realization in terms of fundamental mechanical building blocks is presented for the hexagonal and Lieb lattices. The lattices are composed of passive components and the building blocks are a set of disks and linear springs. Furthermore, by varying the spring stiffness, topological phase transitions are observed, illustrating the potential for tunability of our lattices.

Helical edge states and topological phase transitions in phononic systems using bi-layered lattices

The topological properties of an object, associated with an integer called the topological invariant, are global features that cannot change continuously but only through abrupt variations, hence granting them intrinsic robustness. Engineered metamaterials (MMs) can be tailored to support highly nontrivial topological properties of their band structure, relative to their electronic, electromagnetic, acoustic and mechanical response, representing one of the major breakthroughs in physics over the past decade. Here, we review the foundations and the latest advances of topological photonic and phononic MMs, whose nontrivial wave interactions have become of great interest to a broad range of science disciplines, such as classical and quantum chemistry. We first introduce the basic concepts, including the notion of topological charge and geometric phase. We then discuss the topology of natural electronic materials, before reviewing their photonic/phononic topological MM analogues, including 2D topological MMs with and without time-reversal symmetry, Floquet topological insulators, 3D, higher-order, non-Hermitian and nonlinear topological MMs. We also discuss the topological aspects of scattering anomalies, chemical reactions and polaritons. This work aims at connecting the recent advances of topological concepts throughout a broad range of scientific areas and it highlights opportunities offered by topological MMs for the chemistry community and beyond.

/pdf/topological-metamaterials-1vq6lzod.pdf

Topological Metamaterials.

Valley interface states, resulting in acoustic valley Hall topological insulators, have recently become a hot topic in the study of acoustic systems. On the basis of structural diversity and potential applications, we construct a two-dimensional triangular-lattice phononic crystal with ${C}_{3v}$-symmetric scatterers, and obtain two distinct valley Hall phases with nonvanishing valley Chern indices by rotating the scatterers. We numerically calculate the dispersion relations of these valley Hall phases including two kinds of interfaces, and find that valley interface states exist at not only the zigzag interface but also the armchair interface. We demonstrate the acoustic splitting and merging of valley interface states in the cross-waveguides, and numerically achieve the xor and or logic functions. We also design three complicated waveguides by assembling phononic crystals with distinct valley Hall phases. By experimental measurements in these waveguides, we successfully implement one-, two-, and three-channel topological transport. This research possibly provides a design route exploiting valley interface states to fabricate multichannel acoustic communication devices.

Multichannel Topological Transport in an Acoustic Valley Hall Insulator

Ideal Weyl points, which are related by symmetry and thus reside at the same frequency, could offer further insight into the Weyl physics. The ideal type-I Weyl points have been observed in photonic crystals, but the ideal type-II Weyl points with tilted conelike band dispersions are still not realized. Here we present the observation of the ideal type-II Weyl points of the minimal number in three-dimensional phononic crystals and, in the meantime, the topological phase transition from the Weyl semimetal to the valley insulators of two distinct types. The Fermi-arc surface states are shown to exist on the surfaces of the Weyl phase, and the Fermi-circle surface states are also observed, but on the interface of the two distinct valley phases. Intriguing wave partition of the Fermi-circle surface states is demonstrated.

Ideal Type-II Weyl Phase and Topological Transition in Phononic Crystals

Observation of topological valley waveguide transport of elastic waves in snowflake plates

We show that acoustic double Weyl points are observed in a square lattice by breaking inversion symmetry. We also show that the double Weyl points are protected by the ${C}_{4}$ rotation symmetry but are unaffected by translation symmetry along the $z$ direction. When ${C}_{4}$ rotation symmetry is broken, the double Weyl point will split into two single Weyl points in the x-y plane. Gapless surface states and backscattering immune properties are demonstrated in double and single Weyl systems. The topologically protected one-way propagation of sound waves is demonstrated experimentally. The acoustic Weyl points obtained in the easily fabricated square lattice structure will provide a platform to study the topological properties and lead to potential applications in acoustic devices.

Acoustic Weyl points in a square lattice

Pressure amplification and directional acoustic sensing based on a gradient metamaterial coupled with space-coiling structure

Weak acoustic signals detection from heavy background noise plays an important role in different research fields. However, the detection limit, namely the detection capability of minimal detectable pressure, of sensitivity still hinders the performance of ordinary acoustic sensors. We propose an acoustic metamaterials-based sensor for weak signals detection. We show that the trapezoidal structure with gradient refractive index (GRIN) can effectively realize the pressure field enhancement through acoustic wave compression effect. The acoustic pressure amplitudes can be amplified more than 20 times around the maximum pressure gain frequencies even with different thicknesses of acrylic plates and measured positions, which makes it possible to achieve a broadband acoustic enhancement. Moreover, we numerically demonstrate that both harmonic and periodic impulse signals can be detected much easier based on the GRIN metamaterial due to the acoustic enhancement. In addition, we also experimentally demonstrate that harmonic and periodic impulse signals can be effectively recovered from the background noise due to the improved signal to noise ratios (SNRs). All the results indicate that acoustic metamaterials-based sensor can be well used for weak acoustic signals detection.

Weak Signals Detection by Acoustic Metamaterials-Based Sensor

Rolling bearings fault diagnosis technology plays a significant role in modern industries. In this article, a novel method to diagnose bearing faults via acoustic metamaterials is proposed and experimentally validated by selecting and enhancing the optimal resonance frequency band for demodulation. The proposed method is performed by analysing acoustic signals, which is convenient due to its noncontact measurement. Different from conventional denoising techniques, the proposed method is implemented by enhancing the resonance frequency band via acoustic metamaterials instead of constructing filters to wipe out noise. Consequently, all the useful information of fault signals are remained and the detection limits of current acoustic sensing systems are improved because of the enhanced fault signals. Additionally, the optimal resonance band is sought by changing measured positions based on the frequency-selective enhancement property of the designed metamaterial. By comparing with spectral kurtosis-based method, the proposed method is more superior to extract bearing fault features at low signal to noise ratios. All the presented results indicate that the proposed method is effective to extract bearing fault features.

A Novel Method for Enhanced Demodulation of Bearing Fault Signals Based on Acoustic Metamaterials

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