Bidirectional asymmetric acoustic focusing with two flat acoustic metasurfaces

We design an asymmetric transmission system (ATS) with two flat acoustic metasurfaces (AMs) to yield bidirectional asymmetric acoustic focusing (BAAF). The acoustic waves could be focused on both sides of the ATS with different focal lengths and intensities. To achieve high intensity energy concentration, the accelerating acoustic beams are selected to realize the BAAF. The working bandwidth of the BAAF based on our ATS could reach~0.4 kHz. It is found that by adjusting the distance between two flat AMs, the focal length and intensity of the bidirectional focusing could easily be modulated. Because the distance between two flat AMs is large enough, the BAAF even could be converted into a unidirectional acoustic focusing. The proposed BAAF may find applications in non-destructive evaluation, biomedical imaging and medical diagnosis.