Anisotropic Metamaterials for Elastic Wave Mode Conversion Over a Wide Incidence Angle Range

The constitutive relation of an elastic medium requires a fourth-order elasticity tensor, leading to multi-modal characteristics in the elastic wave regime. As a result, elastic waves in solids carry not only longitudinal (pressure) but also transverse (shear) waves. This multi-modal characteristic complicates but enriches elastic wave phenomena, the most unique of which is the mode conversion between longitudinal and transverse waves. Early studies demonstrated that full-mode conversion between longitudinal and transverse waves could be realized by using anisotropic metamaterials with the non-zero mode coupling component in the elasticity tensor. However, the previously suggested anisotropic metamaterials for the mode conversion phenomenon operate only at normal incidence or oblique incidence with a specific incidence angle. Here, we present a novel anisotropic metamaterial with a spatially dispersive anisotropic tensor that converts longitudinal waves into transverse waves with high mode conversion efficiency over a wide range of incidence angles. The proposed anisotropic metamaterial was fabricated by drilling aperiodic holes with different sizes and angles in an isotropic base material. The mode conversion performance of the proposed anisotropic metamaterial was verified using ultrasonic-guided waves in aluminum plates.