Broadband and high-numerical-aperture sharp focusing for waterborne sound with metagrating-based lens

Abstract

Metalens with broadband and high-efficiency focusing functionality is desired in various underwater acoustic applications such as sonar and oceanography. Here we design and demonstrate a metagrating-based lens consisting of spatially sparse and wavelength-scale meta-atoms with optimized structures. With the help of grating diffraction analysis and intelligent optimization algorithm, the reflective metalens enables broadband and high-numerical-aperture focusing for waterborne sound over a 40 kHz-bandwidth for working frequency at 200 kHz. Full-wave numerical simulations unambiguously verify a sharp and high-efficiency focusing of sound wave intensity, with the full width at half maximum (FWHM) at the focal spot being smaller than 0.5λ and thus beating the Rayleigh-Abbe diffraction limit. Our work not only provides an intelligent design paradigm of high-performance metalens, but also presents a potential solution for the development of planar acoustic devices for high-resolution applications.

Underwater acoustic self-focusing and bending in conformal Mikaelian lens by pentamode metafluid

“…Another area of research focus in underwater acoustic control is acoustic lenses [13][14][15]. The emergence of acoustic metamaterials has allowed for rapid development of acoustic lens technology, which has resulted in compact, flat and high-resolution lenses [16][17][18]. Recently, there has been much attention given to acoustic Mikaelian lenses [19][20][21].…”

Acoustic metalens with switchable and sharp focusing

“…3,4) It is induced by the far-field interference of coherent propagating waves scattered by the metalens, and is related to the fact that band-limited functions are able to oscillate arbitrarily faster than the highest transverse Fourier components they contain, a phenomenon called super-oscillation in the literature. [3][4][5][6] In parallel, tunable or reconfigurable metalens has received more attention recently, where the focusing functionality can be controlled in a switchable way, which is crucial in application scenarios such as imaging, displays, and augmented and virtual reality devices. [7][8][9][10][11] Usually, the studies of dynamically tunable or switchable acoustic metalens were based on phase-gradient metasurfaces, and were realized through the continuous tuning of the resonant-cavity volume, [12][13][14][15][16] the dynamic modulation of a rotating airflow, 17) or integrating piezoelectric materials into the meta-atoms.…”

“…Later we will show that such a center-opened configuration does not compromise the focusing performance and resolution ability of the lens. [3][4][5][6] For the 1st meta-atom that is closer to the lens' axis, the scattered wave has a diffraction angle q 1 and converges to the focus with a focal length of ( ) F . t r The width W of the central region is determined by the geometric relationship shown in Fig.…”