Photon-Sphere Modes in Curved Optical Microcavities: A Black-Hole Analogue Laser.

The bell-like ringdown of the gravitational field in the final stage of massive black-hole mergers is now routinely detected on Earth by the latest generation of gravitational-wave detectors. Its spectrum is interpreted as a sum of damped sinusoidal vibrations of spacetime in the vicinity of the black hole. These so-called quasinormal modes are the subject of extensive current studies, yet their physical nature remains elusive. Here, we emulate in the laboratory genuine four-dimensional (3+1)D black-hole metrics using an effective (2+1)D optical metric defined on a two-dimensional curved surface that preserves the features of light-like geodesics. We analytically compute the quasinormal modes of the optical cavity and show that, in addition to conventional whispering-gallery modes (WGMs) supported near the cavity boundary, a new family of modes is confined around the photon sphere, the unstable region where spacetime curvature traps light in circular orbits. By 3D-printing non-Euclidean dye-doped microcavities, we demonstrate lasing in both WGMs and photon-sphere modes, with the latter exhibiting spatial profiles in close agreement with analytical predictions. These results place our system within the broader framework of analogue-gravity experiments, providing a complementary photonic platform to investigate black-hole photon-sphere physics under tabletop conditions and inspiring new approaches to microcavity photonics.