Complex amplitude encoding metalens realizing arbitrary ultrasonic needle beams

Extending the depth of focus is necessary in many scenarios. Recent progress in ultrasonic applications demands various kinds of foci and poses challenges to science and technologies. Here, we propose to connect individual foci forming an ultrasonic needle beam (UNB) through complex amplitude encoding with super-units. Two phase distributions are encoded into one metalens through super-units, which realizes a complex-amplitude modulation and achieve multifocal points in space. The performance of the metalens can be improved by adjusting the parameters of super-units. Both simulations and experiments have demonstrated that the metalens designed through the proposed method can efficiently produce single or twin UNBs. Our work has potential applications in biomedical treatment and imaging, remote communication, and nondestructive detection.