Chirality-switchable acoustic vortex emission via non-Hermitian selective excitation at an exceptional point

Artificial structures provide an efficient method to generate acoustic vortices carrying orbital angular momentum(OAM) essential for applications ranging from object manipulation to acoustic communication. 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. 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. 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. Such non-Hermitian selective excitation enables our experimental realization of acoustic vortex emission with switchable OAM but free of system reconfiguration. Our work offers opportunities for chiral sound manipulation as well as integrated and tunable acoustic OAM devices.

Conformally Mapped Multifunctional Acoustic Metamaterial Lens for Spectral Sound Guiding and Talbot Effect

We demonstrate a conformally mapped multifunctional acoustic metamaterial Mikaelian lens.Mikaelian lens is a gradient medium with a hyperbolic secant refractive index profile that can realize functions like beam self-focusing.Unlike the conventional design approaches,with a conformal transformation method,only isotropic material parameters with gradient refractive index profiles are required for the construction of such lens.To realize desired gradient index distribution,we carefully design a new type of crosschannel-shaped acoustic metamaterial,whose refractive index can be effectively modulated by simply changing the slit opening size.The distinct capabilities of the metamaterial Mikaelian lens in manipulating acoustic waves are experimentally verified with the fabricated sample.Simultaneous sound guiding and Talbot effects,which normally require respective geometrical and wave acoustic approximations,are observed in simulations and experiments.Furthermore,those effects of shaping acoustic wave propagations were validated within a relatively wide frequency range.Our study reveals how the conformal transformation method can help to bridge the ray acoustics with wave acoustics.It offers opportunities to the development of novel multifunctional acoustic devices for various applications,such as sound and particle manipulations.