Fig. 1
Soft gradient-index metasurfaces with highly controlled gradients of porosity. a Measured relative refractive acoustic index n for various soft porous silicone rubbers with porosities ranging from 0% to 15%. Insets: Scanning electron microscopy images of the porous structure for Φ = 5% and 12%; These soft porous materials are fabricated with a targeted porosity controlled with an accuracy of 1%. b–e Examples of soft porous GRIN metasurfaces made of soft porous silicone rubber stripes with a thickness d = 2 mm. The corresponding acoustic index profiles are shown in the histograms below. The values mentioned at the bottom of the histograms correspond to the porosity in each porous stripe. The linear index profile shown in b is designed for beam deflection with an angle of 5°, while the hyperbolic index profile shown in c is designed for beam focusing with a focal length F = 70 mm. The radial index profile shown in d is designed for 3D sound focusing with a focal length F = 20 mm, while the azimuthal index profile shown in e is designed for the generation of an acoustic vortex beam with a topological charge l = +1
