High-efficiency and large-numerical-aperture focusing with subdiffraction resolution using a hybrid elastic metalens

Metalenses have been widely studied and used because of their high design freedom and ultrathin configurations. For various practical applications, such as geological exploration, energy harvesting, and structural health monitoring, it is highly desirable to maximize the focusing efficiency of elastic waves. However, it is a great challenge to realize both high-efficiency focusing and subdiffraction resolution for elastic waves, especially when the mode coupling and conversion between longitudinal and transverse elastic waves is ubiquitous and inevitable. Here, we adopt a hybrid design approach for an elastic metalens to realize a high-efficiency (54.6%) and subdiffraction resolution (0.39\ensuremath{\lambda}) focusing performance with a large numerical aperture (NA = 0.931). The hybrid metalens is composed of a gradient metasurface in the central region and metagratings in the peripheral region to achieve a dramatic lensing functionality, where 112.5 times enhancement of wave intensity is attained at the focal spot. The high-efficiency, large-NA, and superresolution characteristics of the focusing performance, together with the planar and compact configuration of the metalens, are expected to find potential applications in various fields.