Broadband asymmetric acoustic transmission in a gradient-index structure

We propose a scheme of realizing broadband asymmetric acoustic transmission using gradient-index structure, and demonstrate an implementation utilizing phononic-crystal-based metamaterials which can realize an idealized model with desired parameters. The asymmetric transmission is valid within a remarkably broad frequency range. This phenomenon stems from the break of the geometric symmetry of wave trajectories along two opposite directions, essentially different from the mechanisms of previous designs. The numerical results agree well with the analytical predictions using acoustic ray theory. Our design may have potential applications in many fields such as ultrasonic therapy and noise control.