Tunable Second‐Order Topological States in Rhombic Photonic Crystals

Abstract

Topological photonic devices with dynamically tunable functions are highly on demand in practice, but the majority of previously proposed photonic systems have been limited to fixed performances, once fabricated. Although several approaches have been proposed for obtaining the tunability in topological photonic systems, they are limited to first‐order topological states and require rather complicated structures. Herein, second‐order topological properties of rhombic photonic crystals (PCs) are revealed, for the first time, enabling to realize tunable photonic devices. For this purpose, the conventional square lattice PCs composed of four rigid dielectric rods are reshaped to rhomboid ones with preserved inversion symmetry, which exhibit well‐quantized bulk polarizations. Since the eigenfrequencies of topological edge and corner states depend on the angle between the neighboring sides of unit cells, the second‐order topological systems exhibit dynamic tunability, being useful for diverse applications such as optical switching and flexible beam control. Unlike the previous results for reconfigurable routing limited to special angles, this lattice‐reshaping mechanism has the ability to realize dynamically tunable routing, extending the realm of applications of topological photonics. For its simplicity and feasibility, this mechanical lattice‐reshaping approach paves the way toward higher‐order topological photonic devices with dynamically controlled functions.