Measurement of Penrose superradiance in a photon superfluid

TL;DR

This paper demonstrates Penrose superradiance in a photon superfluid through a nonlinear optics experiment, showing amplification of a reflected beam via negative norm modes and phase relations consistent with superradiance conditions.

Contribution

First experimental observation of Penrose superradiance in a photon superfluid using nonlinear optics, revealing the role of negative frequency modes and phase conditions.

Findings

Amplification of a reflected optical beam via superradiance.

Negative norm modes are generated and trapped inside the vortex core.

Phase relation matches Zel'dovich-Misner superradiance condition.

Abstract

The superradiant amplification in the scattering from a rotating medium was first elucidated by Sir Roger Penrose over 50 years ago as a means by which particles could gain energy from rotating black holes. Despite this fundamental process being ubiquitous also in wave physics, it has only been observed once experimentally, in a water tank, and never in an astrophysical setting. Here, we measure this amplification for a nonlinear optics experiment in the superfluid regime. In particular, by focusing a weak optical beam carrying orbital angular momentum onto the core of a strong pump vortex beam, negative norm modes are generated and trapped inside the vortex core, allowing for amplification of a reflected beam. Our experiment demonstrates amplified reflection due to a novel form of nonlinear optical four-wave mixing, whose phase-relation coincides with the Zel'dovich-Misner condition for Penrose superradiance in our photon superfluid, and unveil the role played by negative frequency modes in the process.}