Penrose black hole super radiance in photon fluids [In-person]

Rotational Superradiance is the amplification of energy in the scattering off a rapidly rotating object. This process was first proposed over 50 years ago by Roger Penrose [1] who predicted that particles, scattered by a rotating black hole, could gain energy from the body. However, measuring such astrophysics amplification is still challenging for nowadays technology. In this context, analogue gravity studies have attracted major attention due to their versatility in proposing laboratory tests of otherwise astrophysical inaccessible phenomena [2] and provided the first measurements of rotational superradiance first in water waves [3] and then in photon fluids [4,5], showing the ubiquity of this phenomenon also in wave-physics.
In this talk, I will show the observation of Penrose superradiance in a 2D rotating nonlinear fluid of light [4,5]. Making use of the analogy with curved spacetime, we scatter a weak optical beam with angular momentum from the core of a strong vortex beam, playing the role of the rotating spacetime. We then measure the amplification of the scattered positive modes outside the so called ergoregion, i.e. the analogue region to that of rotating black holes, where a body cannot be at rest as seen by an observer at infinity. I will also explain and show the advantage of using photon fluids as analogue systems, being that they allow to observe the generation and trapping of negative norm modes inside the pump vortex core.

[1] R. Penrose and R. M. Floyd, “Extraction of rotational energy from a black hole,” Nat. Phys. Sc. 229, 177 (1971).
[2] C. Barcelo’, S. Liberati, and M. Visser, “Analogue gravity,” Liv. Rev. 8, 12 (2005).
[3] T. Torres, S. Patrick, A. Coutant, M. Richartz, E.W. Tedford, and S.Weinfurtner, “Rotational superradiant scattering in a vortex flow,” Nat. Phys. 13, 833–836 (2017).
[4] M.C. Braidotti, D. Faccio, and E. M. Wright, “Penrose Superradiance in Nonlinear Optics”, Phys. Rev. Lett. 125, 193902 (2020).
[5] M.C. Braidotti, R. Prizia, C. Maitland, F. Marino, A. Prain, I. Starshynov, N. Westerberg, E. M. Wright, and D. Faccio, “Measurement of Penrose Superradiance in a Photon Superfluid”, Phys. Rev. Lett. 128, 013901 (2022).