Achromatic Needle Focusing with Superresolution Enabled by a Ring-Shaped Acoustic Metalens

Superresolution metalenses that can realize a far-field needle focusing effect are highly desired in various applications, such as medical diagnosing, ultrasonic detection, and underwater communication. Here, we propose a design paradigm of a metalens with superresolution for waterborne sound, which is based on metagrating and has a compact and simple configuration. The central region of the metalens is open, surrounded by a metagrating-based peripheral region. In the radial cross section, each meta-atom contains two elliptical iron cylinders that are intelligently designed to deflect the impinging wave along the desired direction towards the intended focal spot. In this way, a needle focusing effect with high intensity and superresolution is achieved in the far-field region, where the conventional Rayleigh-Abbe diffraction limit is broken due to the coherent interference of the \ifmmode\pm\else\textpm\fi{}1 order diffraction channels. Since the design principle does not rely on sharp resonances, the foci of waves carrying different frequencies partially overlap so that an achromatic focusing functionality is realized where incident waves in a continuous frequency range can be simultaneously focused around the same spots. The compact, planar, and center-open configuration of the metalens not only provides a flexible and practical solution to acoustic needle focusing functionality, but also has potential applications as a powerful planar component that can be easily integrated.