1/6/2024 0 Comments Diffraction gradientWe show that a one-dimensional lattice of chiral resonators with loss can induce differential absorption of helical sounds (i.e., acoustic vortices) carrying opposite orbital angular momentum (OAM). Here, we generalize one of the phenomena-helical dichroism-to acoustics. Our work offers opportunities for chiral sound manipulation as well as integrated and tunable acoustic OAM devices.Ĭircular dichroism and helical dichroism are intriguing chiroptical phenomena with broad applications in optical sensing and imaging. Such non-Hermitian selective excitation enables our experimental realization of acoustic vortex emission with switchable OAM but free of system reconfiguration. One of the sources satisfies the chirality-reversal condition, generating a travelling wave field fully decoupled from and opposite to the chiral eigenmode, while the other source is phase-shifted such that the wave generated by the first source can be canceled out, and the remaining sound field circulates in the same direction as the chiral eigenmode. Here we show that this restriction can be lifted by controlling the individual on-off states of two coherent monopolar sources inside a passive parity-time-symmetric ring cavity at an exceptional point where the counter-propagating waves coalesce into one chiral eigenmode. However, their flexibility in terms of chirality control has thus far been limited by the lack of reconfigurability and degrees of freedom like spin-orbit coupling. Our work provides previously unidentified opportunities for manipulating sound vortices, which can advance more versatile design for OAM-based devices.Īrtificial structures provide an efficient method to generate acoustic vortices carrying orbital angular momentum (OAM) essential for applications ranging from object manipulation to acoustic communication. To exemplify our findings, we designed and experimentally verified a PGM based on Helmholtz resonators that support asymmetric transmission of sound vortices. This diffraction law can explain and predict the complicated diffraction phenomena of sound vortices, as confirmed by numerical simulations. A sound vortex diffraction law is theoretically revealed based on the generalized conservation principle of topological charge. Here, we propose the diffraction mechanism to manipulate sound vortices in a cylindrical waveguide with phase gradient metagratings (PGMs). However, it has limited ability to manipulate sound vortices, and a more powerful mechanism for sound vortex manipulation is strongly desired. By engineering acoustic metasurfaces with phase gradient elements, phase twisting is commonly used to obtain acoustic OAM. Wave fields with orbital angular momentum (OAM) have been widely investigated in metasurfaces.
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