Anisotropic index-near-zero metamaterials for enhanced directional acoustic emission

In this paper, we show that the enhanced directional acoustic emission can be obtained via a class of anisotropic metamaterial based on a cavity-channel network and producing a near-zero group index of refraction n(g). This material consists of two types of air cavities which are alternately arranged in layers, and the dispersion relation is studied theoretically and numerically. Additionally, a retrieval approach for the anisotropic effective constitutive parameters is presented for this material, and it is shown that one component of the density tensor in the vertical direction of the expected propagation axis is close to zero. Numerical simulations are used to verify the effectiveness of our proposal and the results show that a highly directional radiation field with a half-power angular width of less than 10 degrees can be obtained