Abstract
Recently, metasurfaces have been proven to be effective and compact devices for the design of arbitrary wavefronts. Metasurfaces are planar metamaterials with a subwavelength thickness that allows wavefront shaping by introducing in-plane variations, namely, gradients, in the spatial wave response of these flat structures. Here we report a new class of acoustic gradient-index (GRIN) metasurfaces engineered from soft graded-porous silicone rubber with a high acoustic index for broadband ultrasonic three-dimensional wavefront shaping in water. The functionalities of these soft flat lenses are illustrated through various experiments, which demonstrate beam steering and beam focusing, as well as vortex beam generation in free space. These new GRIN metasurfaces may have important applications in various domains using designed ultrasonic fields (biomedical imaging, industrial non-destructive testing, contactless particle manipulation), since their fabrication is very straightforward with common polymer science engineering.
Highlights
Metasurfaces have been proven to be effective and compact devices for the design of arbitrary wavefronts
Local phase shifts may be induced by greatly increasing the wave path length nd while keeping subwavelength thicknesses with respect to the background medium
To accurately control the value of the acoustic index n in the soft porous materials used to build GRIN metasurfaces, we employed an emulsion templating method that was developed in previous works[15,16], coupled with a supercritical drying technique[17] to avoid pore collapse during the drying process[18]
Summary
Metasurfaces have been proven to be effective and compact devices for the design of arbitrary wavefronts. We report a new class of acoustic gradient-index (GRIN) metasurfaces engineered from soft graded-porous silicone rubber with a high acoustic index for broadband ultrasonic three-dimensional wavefront shaping in water The functionalities of these soft flat lenses are illustrated through various experiments, which demonstrate beam steering and beam focusing, as well as vortex beam generation in free space. The concept of coiling space[7,8,9] follows this idea by artificially enlarging the airborne propagation distance d through tapered labyrinthine unit cells to allow for large phase shifts This approach is interesting, since these structures can be fabricated using 3D printing and may be quantized with coding bricks[10,11]. In this work we propose to utilize this great versatility to design a new class of soft porous GRIN metasurfaces for shaping acoustic wavefronts radiating in 3D open space
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