Compact topological edge modes through hybrid coupling of orbital angular momentum modes

Topological waveguide arrays, which support robust light propagation through edge modes, offer a promising solution for fault-tolerant photonic chips. However, these arrays commonly require a considerable number of waveguide elements to ensure their topological protection, leading to increased footprint and fabrication costs. Here, we propose a topological waveguide array with only a few cells based on a photonic Aharonov-Bohm (AB) cage. This design is advantageous because the edge modes become compactly localized at the boundary waveguides. We achieve this by utilizing the orbital hybridization of the fundamental mode and first-order orbital angular momentum (OAM) modes to construct the AB cage, where arbitrary flux of artificial gauge fields (AGFs) can be generated and controlled by adjusting the angle between adjacent waveguides. Notably, these edge modes exhibit robustness against disorders of on-site potential and coupling, even within a small lattice. Furthermore, we demonstrate the extension of this mechanism to high-order OAM modes. Our work paves the way for miniaturizing topological devices using the orbital degree of freedom, holding the potential for exploring other AGF-enriched phenomena.