Underwater gradient metalens for broadband subwavelength focusing

• Microstructure-macrostructure-functionality integrated design of underwater metalens via is proposed. • An optimization approach for improving focusing capabilities of gradient metalens is presented. • Customized mode metamaterials with broadband single longitudinal polarization are designed. • Broadband, high-efficient, high-gain, and high-symmetrical underwater acoustic focusing is realized. • Lightweight porous underwater metalens has the hydrostatic pressure resistance. Broadband effectiveness and impedance mismatching in fluid-structure coupling remain significant obstacles in the pursuit of broadband high-efficient high-symmetrical subwavelength underwater acoustic devices by using metamaterials. Here, we systematically propose a microstructure-macrostructure-functionality integrated design of underwater gradient metalens, which facilitates the realization of broadband, high-efficient, high-gain, and high-symmetry acoustic focusing. To improve the focusing capabilities of the metalens, an optimization approach of gradient index profile is presented. The square-latticed mode metamaterial is proven to support perfect single polarization characteristic and high customizability of the effective properties. The single polarization mode and optimized index contribute the metalens to presenting an excellent acoustic high-efficient subwavelength focus behavior. The sound pressure field shows that the meta-device can convert plane waves to subwavelength focusing with FWHM ≤ 0.5λ over an ultra-wide frequency range of 5∼33 kHz, even FWHM ≤ 0.4λ of 10∼24 kHz. The metalens can amplify sound intensity over 6 times at focal spots within 10∼22 kHz, when the efficiency is above 80% in 6∼20 kHz. Meanwhile, environmental adaptability evaluation reveals good hydrostatic pressure resistance (∼1.5 MPa) of lightweight porous underwater metalens. The present design strategy and the realized high-performance underwater metalens have application potential in the fields of acoustic transmission, harvesting, and communications.