Loss-induced multiple modal coalescences in an acoustic ring cavity

Abstract

Chiral exceptional points refer to a class of non-Hermitian degeneracies where two counter-propagating whispering-gallery-like eigenmodes coalesce into one travelling eigenmode, which have been utilized to create orbital angular momentum in various wave systems. However, most studies in this field have so far focused on the modal coalescence for a specific azimuthal order. In this work, we investigate the eigenmodes evolution of an acoustic ring cavity under complex spatial modulation of refractive index along the azimuthal direction. We show that a careful engineering of the complex modulation, possibly realized with acoustic metamaterials, can lead to multiple modal coalescences to produce a series of travelling eigenmodes with different orbital angular momentum orders. These coalesced eigenmodes can be directly excited by a simple source at their resonant frequencies. Our work provides a straightforward approach to the generation and manipulation of acoustic orbital angular momentum.