Topological Singularities and Edge-State Coupling Enable Robust on-Chip Slow Light.

Quantum interference (QI) between multiple excitation pathways can manipulate high-quality optical responses, like slow light, highly-sensitive sensors, and ultrafast switches. However, the enhanced light-matter interactions and reduced group velocity tend to arouse high sensitivity to the environment, challenging fabrication and operation in practical applications. In this work, the first experimental demonstration of topological QI-like effect is reported in a 1D on-chip system, where two different topological mechanisms are induced to provide robustness for slow light. Topological charges rooted in the parameter space can offer immunity to parameter deviations, such as coupling strengths, and topological edge states rooted in the momentum space can manifest robustness against structural disturbances, like width disorders and bending deformations. By incorporating bright and dark edge states into a composite waveguide, the electromagnetically induced transparency (EIT) window is observed and switching between slow light and fast light is demonstrated by measuring the transmission and group delays. The findings provide an extensible platform for exploring novel QI and topological physics, and pave the avenue for developing robust on-chip devices.