Observation of Dual-Polarization Topological Photonic States at Optical Frequencies

Topological protection in photonics offers new prospects for confining and guiding light waves. Up to now, most reported works on topological photonics have focused on the realization of topological phases originally found in condensed matter systems. However, photonic systems can possess a distinguishing topology that differs from their quantum electronic counterparts due to the vectorial nature of light. Here, dual-polarization topological photonic states are theoretically predicted and experimentally demonstrated at optical frequencies on the all-dielectric silicon-on-insulator platform. The topological band gaps opening at K/K ' valleys for both TE- and TM-polarizations in the properly designed breathing Kagome photonic crystal slabs are demonstrated, leading to dual-polarization topological edge states below the light cone. The topological edge states are directly observed by imagining the near fields with scattering scanning near-field microscopy. In addition, the significant improvement of the quality factor of topological states via engineering the transition near the boundary along with the robustness of edge states is presented. The proposed dual-polarization topological states and the improvement of quality factor via boundary profile engineering paves the way toward novel applications of topological photonics for realizing integrated polarization-dependent applications such as polarization multiplexing photonic devices and on-chip chiral quantum optics. Topological photonics offers new prospects for confining and guiding light waves. For the first time, dual-polarization topological edge states are experimentally demonstrated on the silicon-on-insulator platform at optical frequencies. The topological edge states are directly observed by imaging the near fields with scattering scanning near-field optical microscopy.image