Perfect oblique-incidence ultrasound transmission across dissimilar solids

Abstract

Perfect ultrasound transmission from one medium to another is critical in many applications, including medical treatment, imaging, and diagnostics. However, an obliquely incident ultrasonic wave through the solid-solid interface inevitably generates multiple wave modes (longitudinal waves and transverse waves). Accordingly, the well-known impedance matching theory considering only single wave mode fails to achieve the perfect oblique-incidence ultrasound transmission even from one isotropic solid to another. Here, we present the generalized matching theory that is newly established to enable the perfect transmission of ultrasonic waves across dissimilar solids at an arbitrary incidence angle and frequency. We also design a novel single-phase anisotropic solid-void metamaterial layer that can fulfill all conditions stated by the theory. It is shown that the insertion of the proposed metamaterial layer between metal (aluminum) and plastic (polyether ether ketone) plates achieves nearly perfect wave transmission over 98% energy efficiency, without which transmission reduces to 20%-40%. Our findings are validated numerically and experimentally.