# Broadband focusing of underwater sound using a transparent pentamode lens

## TL;DR

This paper presents a broadband underwater acoustic lens using a gradient index metamaterial with a hyperbolic secant profile, enabling effective focusing across a wide frequency range for applications like medical imaging and underwater communication.

## Contribution

The work introduces a novel inhomogeneous metamaterial lens with a hyperbolic secant index profile, fabricated from microstructured aluminum, achieving broadband underwater sound focusing.

## Key findings

- Broadband focusing observed in simulations and experiments (20-40 kHz).
- The gradient index lens reduces aberration and suppresses side lobes.
- The design is suitable for medical ultrasound and underwater communication.

## Abstract

We report an inhomogeneous acoustic metamaterial lens based on spatial variation of refractive index for broadband focusing of underwater sound. 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 (FEM) simulations and underwater measurements (20-40 kHz). This design approach has potential applications in medical ultrasound imaging and underwater acoustic communications.