Ultrahigh-efficiency and superresolution focusing of the flexural wave enabled by a hybrid metalens with a large numerical aperture

Flexural-wave metalenses with a large numerical aperture (NA), high-efficiency focusing, and superresolution ability are desired in various applications, such as nondestructive testing and structure-health monitoring. Here, we propose a design paradigm of a hybrid metalens for the flexural wave, which is composed of a gradient metasurface in the central region and two metagratings in the surrounding region to achieve an ultrahigh focusing efficiency. With intelligently designed unit cells and deliberately matched transmissive phases, the metagrating's high deflection efficiency at larger steering angles and the metasurface's tunable phase shift and high efficiency at smaller steering angles are simultaneously and cooperatively utilized to realize an unprecedented focusing performance, where 240.9-times enhancement of wave intensity is attained at the focal spot. The ultrahigh-efficiency, large-NA (0.957), and superresolution (0.35\ensuremath{\lambda}) characteristics of the focusing performance, as well as the compact and planar configuration of the hybrid metalens, provide an alternative and promising solution to the sharp-focusing functionality of a flexural wave.