Realization of edge states along a synthetic orbital angular momentum dimension

Synthetic dimensions have emerged as promising methodologies for studying topological physics, offering great advantages in controllability and flexibility. Photonic orbital angular momentum (OAM), characterized by discrete yet unbounded properties, serves as a potent carrier for constructing synthetic dimensions. Despite the widespread utilization of synthetic OAM dimensions in the investigation of topological physics, the demonstration of an edge along such dimensions has remained challenging, significantly constraining the exploration of important topological edge effects. In this study, we establish an edge within a Floquet Su–Schrieffer–Heeger OAM lattice, creating approximate semi-infinite lattices by introducing a pinhole in the optical elements within a cavity. Leveraging the spectral detection capabilities of the cavity, we directly measure the phase transitions of zero (± π ) energy edge states, elucidating the principle of bulk-edge correspondence. Furthermore, we dynamically observe the migration of edge modes from the gap to the bulk by varying the edge phase, and we reveal that interference near the surface results in the discretization of the spectrum. We offer, to our knowledge, a novel perspective for investigating edge effects and provide an important photonic toolbox in topological photonics.