Superresolution ultrasonic focusing through a modified Fresnel-spiral lens

Superresolution focusing has wide applications in ultrasonics. Here, we propose a modified Fresnel-spiral lens (MFSL) consisting of Fresnel spiral (FS) slits and embedded phase gratings to achieve superresolution ultrasonic focusing. The FS slits can push low spatial frequency components of the transmitted ultrasonic wave into high spatial frequencies with minimal energy loss, while the phase gratings exert additional phase lag that the overall topological charge is zero. Then, a subdiffraction hot spot with tiny sidelobes is obtained at the focal plane. The focusing resolution of the MFSL can approach the theoretical limit of 0.38\ensuremath{\lambda} with the increasing numerical aperture (NA). It is also found that the focusing resolution of the MFSL is robust to structure features and that the focusing efficiency is influenced by both the slit width and the number of spiral arms. Finally, we demonstrate the superresolution ultrasonic focusing capability of the proposed MFSL through experiments. Our work offers an alternative strategy to realize high-efficiency subdiffraction ultrasonic focusing, which may have important implications for medical imaging and treatment, remote sensing and communication, and particle manipulation.