Superradiant scattering of orbital angular momentum beams

TL;DR

This paper explores superradiant amplification of orbital angular momentum beams interacting with rotating disks, demonstrating new configurations and experimental approaches, especially using evanescent sound waves to probe novel acoustic regimes.

Contribution

It introduces a novel wave-structure coupling setup showing superradiance with vortex beams aligned parallel to rotation, expanding the understanding of rotational superradiance in acoustics.

Findings

Superradiant amplification occurs with vortex beams parallel to rotation axis.

Evanescent sound waves offer a promising experimental route.

Probing new parameter regimes in rotating porous media acoustics.

Abstract

We consider the wave-structure coupling between an orbital angular momentum beam and a rapidly rotating disk, and present a new configuration exhibiting the wave amplification effect known as rotational superradiance. While initially envisioned in terms of the scattering of an incident wave directed perpendicular to an object's rotation axis, we demonstrate in the context of acousto-mechanics that superradiant amplification can also occur with a vortex beam directed parallel to the rotation axis. We propose two different experimental routes: one must either work with rotations high enough that the tangential velocity at the outer edge of the disk exceeds the speed of sound, or use evanescent sound waves. We argue that the latter possibility is more promising, and provides the opportunity to probe a previously unexamined parameter regime in the acoustics of rotating porous media.