Pollen tube, a one-way special train for special passengers

Dynamically reconfigurable topological edge state in phase change photonic crystals

The observation of topological edge states (TESs) revolutionized our understanding of scattering and propagation of electromagnetic (EM) waves. Supported by topological robustness, the TES at the interface between trivial and non-trivial insulators was not reflected from the structural disorders and imperfections. Recently topological photonic crystals (PhCs) were demonstrated to obtain remarkable one-way propagation of the TES, having the advantages of lossless propagation, dense integration, and high fabrication tolerance over conventional PhCs. Nevertheless, the lack of reversible switching of TES possesses significant limitations in helicity/spin filtering and tunable photonic devices. We proposed a topological PhC based on a prototypical phase-change material, Ge₂Sb₂Te₅ (GST225) to solve the problem. We find that at a particular frequency, the TES within the structure can be reversibly switched between "on" and "off" by transiting the GST225 structural state between amorphous and crystalline. Moreover, the topology of the PhC was maintained since the tuning of TES was achieved by varying the refractive index of GST225 instead of the structural geometry. This provides a continuous change of the spectral position of the photonic bandgap and TES by gradually crystallising the GST225. We show that the phase change of GST225 from amorphous to crystalline and vice versa can be engineered in nanoseconds. Our proof of concept may offer a platform for dynamically tuning the TESs that might otherwise be challenging to attain in photonic systems. We expect it to have potential applications for photonic devices in topological optical circuits and scatter-free one-way light propagation.