Metamaterial-based manipulation of orbital angular momentum for sound

The existing wave-steering devices generally control acoustic beams carrying only linear momentum. But acoustic vortices imprinted with orbital angular momentum (OAM) obviously opens new degree of freedom to control wavefront, with wide applications such as in the design of acoustic “screwdrivers” capable to generate a rotation torque to rotate objects contactlessly. To overcome the drawbacks of the traditional methods for producing acoustic OAM, we have designed and fabricated an acoustic vortex emitter with multi-arm coiling slits that utilize diffraction effect to generate vortex beam, with broadband functionality and stable topological charge. Based on this, we further propose a new mechanism for producing acoustic OAM by converting acoustic resonances to OAM. As an implementation, we have designed and fabricated a thin planar device with high efficiency to verify our scheme. Compared with existing ways for OAM production by phased spiral sources that need sophisticated electronic control and by physically spiral sources that need screw profiles and may also have a bulky size, our acoustic resonance-based OAM production bears the advantages of high efficiency, planar profile, compact size, no spiral structure, and can be freely tuned to produce different orders of OAM. I will also discuss some potential applications of our proposed scheme for OAM manipulation by metamaterials.