Si-an Yu

Topological phononic states of underwater sound based on coupled ring resonators

We report a design of topological phononic states for underwater sound using arrays of acoustic coupled ring resonators. In each individual ring resonator, two degenerate acoustic modes, corresponding to clockwise and counter-clockwise propagation, are treated as opposite pseudospins. The gapless edge states arise in the bandgap resulting in protected pseudospin-dependent sound transportation, which is a phononic analogue of the quantum spin Hall effect. We also investigate the robustness of the topological sound state, suggesting that the observed pseudospin-dependent sound transportation remains unless the introduced defects facilitate coupling between the clockwise and counter-clockwise modes (in other words, the original mode degeneracy is broken). The topological engineering of sound transportation will certainly promise unique design for next generation of acoustic devices in sound guiding and switching, especially for underwater acoustic devices.

Surface phononic graphene

Strategic manipulation of wave and particle transport in various media is the key driving force for modern information processing and communication. In a strongly scattering medium, waves and particles exhibit versatile transport characteristics such as localization, tunnelling with exponential decay, ballistic, and diffusion behaviours due to dynamical multiple scattering from strong scatters or impurities. Recent investigations of graphene have offered a unique approach, from a quantum point of view, to design the dispersion of electrons on demand, enabling relativistic massless Dirac quasiparticles, and thus inducing low-loss transport either ballistically or diffusively. Here, we report an experimental demonstration of an artificial phononic graphene tailored for surface phonons on a LiNbO3 integrated platform. The system exhibits Dirac quasiparticle-like transport, that is, pseudo-diffusion at the Dirac point, which gives rise to a thickness-independent temporal beating for transmitted pulses, an analogue of Zitterbewegung effects. The demonstrated fully integrated artificial phononic graphene platform here constitutes a step towards on-chip quantum simulators of graphene and unique monolithic electro-acoustic integrated circuits.

Acoustic phase-reconstruction near the Dirac point of a triangular phononic crystal

In this work, acoustic phase-reconstruction is studied and experimentally demonstrated in a triangular lattice two-dimensional phononic crystal (PnC) composed of steel rods in air. Owning to the fact that two bands of this triangular lattice PnC touch at the K/K′ point and thus give rise to a conical Dirac cone, acoustic waves transmitting through this PnC can exhibit a pseudo-diffusion transportation feature, producing a reconstructed planar wavefront in the far field away from the interface of the PnC. Such phase reconstruction effect can be utilized in many applications, and here we demonstrate experimentally two important applications: an acoustic collimator and an acoustic cloak operating at a Dirac frequency of 41.3 kHz.

Bulk acoustic wave delay line in acoustic superlattice

The bulk acoustic wave delay lines are demonstrated both theoretically and experimentally based on the acoustic superlattices. In one bulk acoustic wave delay line, two collinear acoustic superlattices are separately located, where the generation and detection of the bulk acoustic waves are executed, respectively. A basic bulk acoustic wave delay line working at 819 MHz with the delay time of 2.46 μs was realized. Furthermore, by employing two enantiomorphous aperiodic acoustic superlattices, a broadband dispersive bulk acoustic wave delay line with the center frequency at 295 MHz has also been fabricated and investigated.

Three-dimensional topological acoustic crystals with pseudospin-valley coupled saddle surface states

Topological valley states at the domain wall between two artificial crystals with opposite valley Chern numbers offer a feasible way to realize robust wave transport since only broken spatial symmetry is required. In addition to the valley, spin and crystal dimension are two other important degrees of freedom, particularly in realizing spin-related topological phenomena. Here we experimentally demonstrate that it is possible to construct two-dimensional acoustic topological pseudospin-valley coupled saddle surface states, designed from glide symmetry in a three-dimensional system. By taking advantage of such two-dimensional surface states, a full set of acoustic pseudospins can be realized, exhibiting pseudospin-valley dependent transport. Furthermore, due to the hyperbolic character of the dispersion of saddle surface states, multi-directional anisotropic controllable robust sound transport with little backscattering is observed. Our findings may open research frontiers for acoustic pseudospins and provide a satisfactory platform for exploring unique acoustic topological properties in three-dimensional structures.Valley states can be used to realise topologically protected transport. Here, He et al. show that considering additional degrees of freedom, together with glide symmetry, allow the design of 2D acoustic topological pseudospin-valley coupled saddle surface states in 3D structures.

Elastic pseudospin transport for integratable topological phononic circuits

Precise control of solid-state elastic waves’ mode content and coherence is of great use nowadays in reinforcing mechanical energy harvesting/storage, nondestructive material testing, wave-matter interaction, high sensitivity sensing, and information processing, etc. Its efficacy is highly dependent on having elastic transmission channels with lower loss and higher degree of freedom. Here, we demonstrate experimentally an elastic analog of the quantum spin Hall effects in a monolithically scalable configuration, which opens up a route in manipulating elastic waves represented by elastic pseudospins with spin-momentum locking. Their unique features including robustness and negligible propagation loss may enhance elastic planar-integrated circuit-level and system-level performance. Our approach promotes topological materials that can interact with solid-state phonons in both static and time-dependent regimes. It thus can be immediately applied to multifarious chip-scale topological phononic devices, such as path-arbitrary elastic wave-guiding, elastic splitters and elastic resonators with high-quality factors.Precise control of elastic waves is of great use in current technologies. Here, Yu et al. realize the analogue of quantum spin Hall effects for the elastic waves in a plain plate consisting of identical perforated holes in wavelength scales.