A research paper entitled Valley Topological Phases in Bilayer Sonic Crystals was published in Physical Review Letters, a world-renowned academic journal of physics. The first signed unit of this paper is the School of Physics and Optoelectronics, South China University of Technology, and the first author is Lu Jiuyang, a postdoctoral fellow of our college. This is also the first important achievement of our college postdoctoral research platform and published in the international top journals.

Paper screenshots

  The energy valley degree of freedom marks discrete energy extremes in momentum space and is an effective electronic control method. Because of its easy regulation, energy valleys have been introduced into classical wave systems more and more recently, realizing a series of valley polarization states and chiral transport phenomena in phononic crystals and photonic crystals. Previous research by Zhengyou Liu and his team at the Institute showed that acoustic valleys are characterized by vortices, and their transport is similar to the spin Hall effect, in which sound waves with opposite vortices propagate to both sides of the crystal. The two-dimensional phononic crystal with acoustic band gap has two different acoustic insulating phases, and there is an acoustic edge state with no band gap at the interface. The edge state has many novel properties which are not found in traditional acoustic waveguides, such as anti-reflective propagation to bending. Due to the macroscopic properties of the iOL, the acoustic wave transport at the interface can be flexibly regulated.

Figure (a) Schematic diagram of phononic crystal bilayer structure; (b) Valley topological phase of two-layer phononic crystals;(c) Singular acoustic transport behavior in bilayer phononic crystals, from top to bottom, layer polarization, interlayer oscillation and layer transformation transport respectively

  Recently, the research team extended the monolayer acoustic valley to the bilayer structure (FIG. (a)), and obtained the valley topological phase of the bilayer phononic crystal by using the interlayer coupling and the change of the scattering Angle. Analogous to the spin number in the electron body, the layer polarization can be used as a new pseudospin degree of freedom to divide the different valley topological phases and obtain rich phase diagrams (FIG. (b)). There are topologically protected sound edge transmissions at the interfaces of phononic crystals with different valley topological phases. By combining the phononic crystals with different valley topological phases, a variety of singular acoustic transmissions can be obtained, such as layer polarization, interlayer oscillation, layer transformation transmission, etc. (experimental measurement results are shown in Figure (c)).

  It is understood that the research results have potential application value in the design of new acoustic devices.