Abstract
It has been shown that acoustic waves with helical wavefronts can carry angular momentum, which can be transmitted towards a propagating medium. Such a wave field can be achieved by using a planar array of electroacoustic transducers, forming a given spatial distribution of phased sound sources which produce the desired helical wavefronts. Here, we introduce a technique to generate acoustic vortices, based on the passive acoustic metasurface concept. The proposed metasurface is composed of space-coiled cylindrical unit cells transmitting sound pressure with a controllable phase shift, which are arranged in a discretized circular configuration, and thus passively transforming an incident plane wavefront into the desired helical wavefront. This method presents the advantage of overcoming the restrictions on using many acoustic sources, and it is implemented with a transmitting metasurface which can be easily three-dimensionally printed. The proposed straightforward design principle can be adopted for easy production of acoustic angular momentum with minimum complexity and using a single source.
Highlights
The discovery that waves can carry angular momentum may date back to Poynting and his claim on angular momentum of the circularly polarized light [1], which was later validated experimentally [2]
We introduce a method to create acoustic helical waves based on acoustic metasurfaces with acoustic space-coiled unit cells, which decreases the thickness of acoustic orbital angular momentum (OAM) generators down to λ/3
We introduced a method to produce acoustic vortices based on acoustic metasurfaces which converts acoustic plane waves to helical waves
Summary
Hussein Esfahlani* and Herve Lissek† Signal Processing Laboratory (LTS2), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. The proposed metasurface is composed of space-coiled cylindrical unit cells transmitting sound pressure with a controllable phase shift, which are arranged in a discretized circular configuration, and passively transforming an incident plane wavefront into the desired helical wavefront. Hefner et al produced the first ultrasonic beam with helical phase and screw dislocation [7] It was shown using the concept of pseudomomentum that acoustical vortices have the same properties as their optical counterparts. Hefner et al proposed an alternative technique, based on a circular array of phase-shifted transducers [22] This method has been adopted since in most of the studies to create acoustical vortices, requiring many transducers which are individually controlled [23]. Metasurfaces are artificially engineered surfaces composed of unit cells which
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