Broadband focusing of underwater sound using a transparent pentamode lens

Abstract

An inhomogeneous acoustic metamaterial lens based on spatial variation of refractive index for broadband focusing of underwater sound is reported. The index gradient follows a modified hyperbolic secant profile designed to reduce aberration and suppress side lobes. The gradient index (GRIN) lens is comprised of transversely isotropic hexagonal microstructures with tunable quasi-static bulk modulus and mass density. In addition, the unit cells are impedance-matched to water and have in-plane shear modulus negligible compared to the effective bulk modulus. The flat GRIN lens is fabricated by cutting hexagonal centimeter scale hollow microstructures in aluminum plates, which are then stacked and sealed from the exterior water. Broadband focusing effects are observed within the homogenization regime of the lattice in both finite element simulations and underwater measurements (20-40 kHz). This design approach has potential applications in medical ultrasound imaging and underwater acoustic communications.

Quasi-isotropic underwater acoustic carpet cloak based on latticed pentamode metafluid

“…The shear modulus of the latticed PM is so small that it can be neglected and regarded as fluid in some frequency bands. PM has attracted great attention in underwater acoustic wave controlling [8,9,[28][29][30][31]. It has been proved that wave equation in PM is also form-invariant under the coordinate transformations [25], thus PM can be used to design some valuable under water devices with transformation method [8,9].…”

Partitioned gradient-index phononic crystals for full phase control

“…Gradually changing indices in a focusing lens can readily be designed via the hyperbolic secant profile function 23 . However, most of the methods in the existing studies on GRIN-PC systems thus far have been confined to focusing the wave energy using the hyperbolic-secant index profile 2,4,22,[24][25][26][27][28] , because of its simplicity, thus restricting design of GRIN-PC with any other wave path profile for general purposes.…”

Design of an underwater acoustic bend by pentamode metafluid

“…PM is a kind of special solid structure with tunable effective modulus and density that only guarantees the longitudinal wave to propagate in it, and can be designed to have anisotropic modulus [10,13]. This makes it have the similar acoustic property with fluid [21] and the advantage for designing impedance matching underwater devices [25,26].…”