Customizing acoustic dirac cones and topological insulators in square lattices by topology optimization

Dirac point, the cornerstone of topological insulators, has been attracting ever-increasing attention due to its extraordinary properties. In this paper, a bottom-up topology optimization approach is established to systematically design the acoustic Dirac cones with customized double, triple and quadruple degeneracies at different wavelength scales. Using the proposed methodology, novel square-symmetric, chiral and orthogonal-symmetric sonic crystals (SCs) are constructed in a square lattice with tailored Dirac cones. The proposed design approach offers a unified framework to tailor SCs with exotic functionalities which are being widely researched in acoustic metamaterial community. As illustrative examples, zero-index acoustic cloaking and Talbot effect near the Dirac points of the optimized SCs are demonstrated numerically. Moreover, a novel acoustic pseudo-spin topological insulator is obtained, which entails a robust zigzag wave propagation and broadband, unidirectional, and topologically protected transport with a record-breaking relative bandwidth of 30.51%. The proposed design methodology shows promise and opens new horizons for customizing topological acoustics and conceiving high-efficiency wave devices.