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.
Highlights
An orbital angular momentum (OAM) beam is a traveling wave with angular momentum in the direction of its propagation
Rather than angular momentum associated with spin degrees of freedom, OAM beams result from spatial wave distributions that have a helical structure
OAM beams can be formed in nearly any effective field theory with propagating modes, such as electromagnetism and fluid dynamics, and as such have facilitated the widespread application of rotation for science and industry alike
Summary
An orbital angular momentum (OAM) beam is a traveling wave with angular momentum in the direction of its propagation. OAM beams can be formed in nearly any effective field theory with propagating modes, such as electromagnetism (i.e., light beams [1]) and fluid dynamics (i.e., sound beams in air or water), and as such have facilitated the widespread application of rotation for science and industry alike. We propose to direct an incident wave (carrying angular momentum) parallel to the object’s rotation axis. Within this configuration it is possible to utilize the arguably advantageous OAM beams to probe for superradiance. We restrict our attention to a more idealized impedance condition, and calculate the amplification spectra for experimentally feasible parameters This is followed by a discussion of vortex beam. We comment on the relevance of our proposal for electromagnetic systems, which could potentially allow quantum aspects of superradiance to be observed
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