Broadband localization of light at the termination of a topological photonic waveguide.

Localized optical field enhancement enables strong light-matter interactions necessary for efficient manipulation and sensing of light. Specifically, tunable broadband energy localization in nanoscale hotspots offers many applications in nanophotonics and quantum optics. We experimentally demonstrate a mechanism for the local enhancement of electromagnetic fields based on strong suppression of backscattering. This is achieved at a designed termination of a topologically nontrivial waveguide that nearly preserves the valley degree of freedom. The symmetry origin of the valley degree of freedom prevents edge states to undergo intervalley scattering at waveguide discontinuities that obey the symmetry of the crystal. Using near-field microscopy, we reveal that this leads to strong confinement of light at the termination of a topological photonic waveguide, even without breaking reciprocity. We emphasize the importance of symmetry conservation by comparing different waveguide termination geometries, confirming that the origin of suppressed backscattering lies with the near conservation of the valley degree of freedom, and show the broad bandwidth of the effect.