Three-dimensional ultrasound subwavelength arbitrary focusing with broadband sparse metalens

Ultrasound focusing in three-dimensional (3D) space is of crucial and enduring significance in a variety of biomedical and industrial applications. Conventional ultrasound focusing based on active phase array or passive geometry of bulky size is unable to realize the 3D arbitrary focusing with subwavelength resolution. Acoustic metamaterial of complex deep-subwavelength microstructure has facilitated the advanced airborne-sound-focusing but is inevitably not applicable for underwater ultrasound, restricted by the law between the multi-modes coupling/thermal viscosity and the feature size of the structure. Here, we aim to circumvent the restriction by increasing the feature size of the metamaterial while keeping the compact overall geometry, and realize the robust subwavelength ultrasound focusing with the sparse metalens of the wavelength-scale meta-atom. We theoretically propose and demonstrate numerically and experimentally the broadband arbitrary ultrasound focusing in 3D space. The axial and off-axis ultrasound focusing with the subwavelength resolution (FWHM<0.58λ) are achieved by the spatially sparse and compact metalens within one-octave bandwidth. With advantages of 3D freewheeling focusing, subwavelength resolution, spatial sparsity, geometric simplicity, and broadband, the sparse metalens would offer more initiatives to advanced researches in ultrasound focusing and empower applications such as precise biomedical imaging and therapy, nondestructive evaluation, integrated and multiplexed ultrasound devices.