Emergent Berry Curvature in Inversion-Symmetric Photonic Crystals for Ultraconfined Topological States

An interface between two periodic structures is topological when the global periodicity cannot be restored through local deformations. In time-reversal symmetric photonic crystals, such interfaces typically form between structures with distinct topological phases characterized by nonzero Berry curvature, which is conventionally induced by breaking spatial inversion symmetry through the modifications of air hole sizes and positions. Here, a topological interface is formed between photonic crystals by breaking the unit cell inversion symmetry via spatial shifts ( $\Delta y=\pm \frac{a}{\sqrt{3}}$ ). This modifies the rotational symmetry of a honeycomb photonic crystal unit cell from C₆ to C₃, which induces nonzero Berry curvature in the lower photonic band. It is further shown that nonzero Berry curvature emerges in any photonic crystals with bulk inversion symmetry and C₃ symmetric unit cells, regardless of lattice structure or in-plane polarization. Using this unit cell, a glide-symmetric interface is designed to support an ultraconfined topological state that exhibits single-unit-cell (0.2λ) in-plane field confinement, subwavelength bending radius (0.24λ), and out-of-plane isolation. These findings challenge the conventional view that geometrical modifications are necessary to break the spatial inversion symmetry and generate nonzero Berry curvature in time-reversal symmetric photonic crystals, offering it as a promising platform for high density, subwavelength scale photonic integration.