Hysteretic self-oscillatory acoustic radiation with tunable orbital angular momentum.

Vortex beams carrying orbital angular momentum (OAM) have attracted growing attention across fields, including optics and acoustics, for potential applications in particle manipulation and high-speed communication. Intracavity generation of OAM beams, such as OAM lasers, efficiently produces high-power, high-beam-quality vortices. This scheme, however, remains rarely explored in acoustics. Here, we propose and demonstrate an acoustic intracavity OAM generation mechanism with tunable topological charges via a single nonreciprocal nonlinear boundary in a compact resonator ring. In the linear regime, the boundary creates non-Hermitian complex effective magnetic fields piercing the ring, leading to a non-Hermitian Zeeman-like effect that splits clockwise and counterclockwise eigenmodes. Upon incorporation of nonlinearity to the boundary, all resonators are mutually locked, producing a single-mode self-oscillatory OAM radiation exhibiting hysteresis and bistability. Moreover, the topological charge is tunable by manipulating the boundary. Our work reveals intriguing physics related to nonlinear, non-Hermitian boundaries and offers potentials in the next generation of acoustic self-oscillatory OAM sources, switchers, and memory devices.