CLEO®/Europe-EQEC 2021 Penrose wave amplification in superfluids of light

The amplification of waves in the scattering with rotating black holes is a fundamental process in gravitational physics. It was introduced by Roger Penrose in 1969 as a way to extract energy from these astrophysical objects [1]. Soon after Penrose proposal, in 1971 Zel'dovich predicted a similar superradiant amplification for electromagnetic waves scattered by a metallic rotating cylinder [2]. In Zel'dovich proposal, waves get amplified if their angular frequency ω satisfies the condition ω < m Ω, where m is the waves topological charge and Ω is the cylinder angular velocity. Despite this amplification process is ubiquitous in wave scattering physics, and not limited to electromagnetic waves and astrophysics only, direct measurements are limited to an experiment with waves in a water vortex in a draining bathtub experiment [3]. The related, but not identical, Zel'dovich amplification has been observed for acoustic waves [4].