Amplitude-modulated binary acoustic metasurface for perfect anomalous refraction

Metasurfaces have shown tremendous steering functionalities over acoustic waves. However, anomalous refractions of metasurfaces based on generalized Snell's law (GSL) are hindered in efficiency due to impedance mismatch, especially at steep refraction angles. In this work, we design amplitude-modulated binary metasurfaces that can perfectly redirect incident waves. The present metasurface consists of two unit cells with properly modulated transmission amplitudes, which satisfies the requirement of power distribution to totally suppress parasitic diffractions. As demonstrations, we redirect incident waves into steep refraction angles of - 70 ° and - 80 ° with efficiencies up to 0.97 and 0.99, which are much higher than theoretical limits of GSL 0.77 and 0.52, respectively. Present metasurfaces have advantages in simplified designs and fabrications and offer versatility for efficient manipulations of sound.