Multifrequency achromatic metalens realizing acoustic multi-point focusing

Up to now, achromatic focusing techniques have focused on the design and improvement of single-point focusing, neglecting the wide application possibilities of multi-point focusing. Here, we propose a multifrequency achromatic metalens to achieve multiple focal points based on complex-amplitude coding. The complex-valued wavefront-shaping function is decomposed into two independent phase distributions, which are coded into the metalens through subwavelength units. The phase dispersion for multi-point focusing can be achieved in this way. Theoretical analyses predict that different coding sequences have little influence on the focal pattern, implying extra freedom in designing the metalens. Limited by available meta-atoms, achromatic multi-point focusing has been realized only at discrete frequencies in this study. By adjusting the coding carefully, a triple-frequency achromatic metalens has been constructed through space curved structures. Three achromatic focal points have been demonstrated through experiments at 3.5, 4, and 5 kHz.