Jiuyang Lu

Observation of topological valley transport of sound in sonic crystals

Valleytronics — exploiting a system’s pseudospin degree of freedom — is being increasingly explored in sonic crystals. Now, valley transport of sound is reported for a macroscopic triangular-lattice array of rod-like scatterers in a 2D air waveguide.

Valley vortex states in sonic crystals

Summary form only given. Valleytronics is quickly emerging as an exciting field in fundamental and applied research. In this Letter, we study the acoustic version of valley states in sonic crystals and reveal a vortex nature of such states. Besides the selection rules established for exciting valley polarized states, a mimicked valley Hall effect of sound is proposed further. The extraordinary chirality of valley vortex states, detectable in experiments, may open new possibility in sound manipulations. This is appealing to scalar acoustics that lacks spin degree of freedom inherently. Besides, the valley selection enables a handy way to create vortex matter in acoustics, in which the vortex chirality can be controlled flexibly. Potential applications can be anticipated with the exotic interaction of acoustic vortices with matter, such as to trigger the rotation of the trapped microparticles without contact.

Making sound vortices by metasurfaces

Based on the Huygens-Fresnel principle, a metasurface structure is designed to generate a sound vortex beam in airborne environment. The metasurface is constructed by a thin planar plate perforated with a circular array of deep subwavelength resonators with desired phase and amplitude responses. The metasurface approach in making sound vortices is validated well by full-wave simulations and experimental measurements. Potential applications of such artificial spiral beams can be anticipated, as exemplified experimentally by the torque effect exerting on an absorbing disk.

Focusing and directional beaming effects of airborne sound through a planar lens with zigzag slits

Based on the Huygens-Fresnel principle we design a planar lens to efficiently realize the interconversion of the point-like source and Gaussian beam in the air ambience. The lens is constructed by a planar plate drilled elaborately with a nonuniform array of zigzag slits, where the slit exits act as subwavelength-sized secondary sources carrying desired sound responses. The experiments operated at audible regime agree well with the theoretical predictions. This compact device could be useful in daily life applications, such as for medical and detection purposes.

Acoustic transmission enhancement through a stiff plate drilled with subwavelength side openings

We study the acoustic transmission through a water-immersed brass plate drilled with a one-dimensional periodical array of rectangular side openings. This new geometry can reveal well the unique properties in acoustic systems, which are rooted in the complex coupling between the longitudinal and transverse waves. Besides the transmission peaks related with regular cavity modes, we observe two additional types of subwavelength peaks. One stems from the resonant excitation of the coupled nonleaky Lamb modes, and the other corresponds to the excitation of the standing plate modes. In particular, the latter is robust in the subwavelength region and has attracted no attention previously.

Acoustic Dirac degeneracy and topological phase transitions realized by rotating scatterers

The artificial crystals for classical waves provide a good platform to explore the topological physics proposed originally in condensed matter systems. In this paper, acoustic Dirac degeneracy is realized by simply rotating the scatterers in sonic crystals, where the degeneracy is induced accidentally by modulating the scattering strength among the scatterers during the rotation process. This gives a flexible way to create a topological phase transition in acoustic systems. Edge states are further observed along the interface separating the two topologically distinct gapped sonic crystals.

Directional excitation of the designer surface acoustic waves

We propose an efficient design route to realize directional excitation of the structure-induced surface waves for airborne sound. The whole system consists of a periodically corrugated rigid plate combining with a pair of asymmetric narrow slits. The directional excitation of the mimicked surface waves stems from the destructive interference between the evanescent waves emitted from the double slits. The directionality can be switched conveniently by tuning the external frequency. The theoretical prediction is validated well by simulations and experiments. Promising applications can be anticipated such as in designing compact devices for airborne sound.

On-chip valley topological materials for elastic wave manipulation

Valley topological materials, in which electrons possess valley pseudospin, have attracted a growing interest recently. The additional valley degree of freedom offers a great potential for its use in information encoding and processing. The valley pseudospin and valley edge transport have been investigated in photonic and phononic crystals for electromagnetic and acoustic waves, respectively. In this work, by using a micromanufacturing technology, valley topological materials are fabricated on silicon chips, which allows the observation of gyral valley states and valley edge transport for elastic waves. The edge states protected by the valley topology are robust against the bending and weak randomness of the channel between distinct valley Hall phases. At the channel intersection, a counterintuitive partition of the valley edge states manifests for elastic waves, in which the partition ratio can be freely adjusted. These results may enable the creation of on-chip high-performance micro-ultrasonic materials and devices.Topologically protected edge states can be observed when combining two Si-based phononic crystals of opposite phases, as well as on-chip elastic wave splitting via partition of edges states at the intersection of topological channels.

Exotic optic edge states created by inversed photonic valley pseudospins

Summary form only given. Recently, the successful observation of topological transport of light boosts great enthusiasm to search new topological phases in artificial crystals for classical waves. By introducing anisotropic scatterers, here we propose a simple route to design topologically distinct photonic insulating phases that are characterized by opposite valley Chern numbers. The topological transition can be easily realized by rotating the orientation of scatterers. The continuum model predicts that gapless chiral edge modes are robustly hosted by the interface separating different valley Hall insulators, where the number of valley-projected photonic edge states can be understood according to the bulk-boundary correspondence. Many exceptional properties of such gapless edge states are unveiled by full-wave simulations, e.g., the selective excitation and anti-reflection from sharp corners.

Highly efficient blazed gratings based on gradient-comb-like units

Here we propose a multi-scaled reflective grating with excellent blazed performance (nearly perfect blazed effect at the well-predicted frequency and orientation). The blazed grating consists of a periodical array of metallic super-cells, each made of several equal-distant subwavelength slits with linearly reduced depth. A simple model based on Huygens-Fresnel principle is established to forecast the microwave response for the incidence of different polarizations: for transverse-electric polarization, the structure provides only the ordinary total reflection (i.e., without orientation deflected); for transverse-magnetic (TM) polarization, the waves are deflected to specific orientation due to the linear phase delay of the slit exits. Similar design route can be extended to acoustic systems, considering the mathematic similarity between the acoustic wave and the electromagnetic wave of TM-polarization.