Observation of topological vortex solitons on disclinations

Vortex-carrying wave fields play a crucial role in photonics due to unusual propagation properties and interactions with matter, which enable numerous practical applications ranging from optical tweezers and imaging to information encoding and transmission. Localized vortex-carrying beams propagating in nonlinear optical media may form self-sustained excited states—vortex solitons—which are however usually prone to instabilities and require high powers for their stabilization in nontopological materials. Using fs-laser written aperiodic waveguide arrays, we demonstrate that photonic topological insulators (TIs) with disclinations admit the formation of stable and thresholdless vortex solitons with tunable shapes. These unique materials belong to a class of higher-order topological insulators and allow the propagation of localized, topologically protected excitations at the disclination core, enabling disorder-resistant transmission of signals and energy. We show that vortex solitons bifurcate from the superposition of topologically protected linear edge states at the disclination core and remain stable in the entire forbidden topological gap. Realized topological vortex solitons with symmetries that are inaccessible in periodic lattices are the first example of excited soliton states with nontrivial phase structure in a TI. Our findings shine a light on the interplay between nonlinearity, the angular momentum degree of freedom of light, and the material topology.