A switchable and highly efficient dual-focus acoustic metalens

Acoustic metalenses with switchable and high-efficiency focusing are pivotal for advanced manipulation of underwater sound waves. This study introduces a hybrid metalens comprising a central metasurface and a peripheral metagrating. The central metasurface ensures precise focusing at small deflection angles, while the intelligently designed metagrating units, each comprising two elliptic iron cylinders, enable efficient and switchable focusing at large angles. Initially, the metasurface and metagrating operate synergistically to focus normally incident waves into a single transmission side spot with exceptional efficiency and energy confinement, achieving a peak intensity 137 times that of the incident wave and a full width at half-maximum (FWHM) of 0.33λ. Upon rotating the elliptic cylinders, the hybrid metalens generates two distinct foci: one on the transmission side from the metasurface and another on the reflection side from the metagrating, thereby realizing switchable dual focusing. This compact, high-performance design holds great promise for applications in medical ultrasound, underwater detection, and acoustic communication.