Ultrasonic sharp autofocusing with acoustic metasurface

Ultrasound focusing manifests itself as the accumulation of ultrasound energy on the target. It attracts extensive interest owing to its substantial advantage in engineering detection and biomedical science. However, the conventional focusing technique leads to the undesirable off-target influence in the nonfocusing region, hindering its application such as ultrasound surgery. Here we propose an ultrasound beam with idiosyncratic sharp autofocusing properties and implement it with the binary metasurface. The beam shows a sharp accumulation at the focus with an abrupt increase of intensity by three orders of magnitude while keeping the unintended region intact. The sharp autofocusing is realized by the controlled transverse self-acceleration and collapse of caustic at the focus in a nonlinear fashion. The intensity contrast, width, and position of the beam are highly tunable to realize the freewheeling focusing with high efficiency. Remarkably, the possibility to synthesize the abrupt autofocusing by the phase-only modulation is revealed. As an implementation, we demonstrate that the precise control of the sharp converging beam can be realized by an extremely simple metasurface with binary elements. The sharp autofocusing beams would open an avenue to realize the diverse ultrasound focusing with promising applications in medical ultrasound imaging, therapy, and nondestructive evaluation.