Design and Observation of Topological Band Gaps and Edge Modes of SOI Photonic Crystal Slabs in the Mid-Infrared Range

We successfully materialized a complete common band gap of a 6.0% width relative to the mid-gap frequency for transverse-electric-like eigenmodes, which was shared by topologically trivial and non-trivial photonic crystals (PhCs) fabricated in the top Si layer of silicon-on-insulator wafers. We verified the presence of the complete common band gap, which is a prerequisite for the topologically protected edge mode, by angle-resolved reflection spectroscopy in the mid-infrared range. By applying polarization selection rules expected from the spatial symmetry of the electromagnetic eigenmodes, we confirmed the band inversion at the center of the first Brillouin zone (Gamma point) in a purely experimental manner. We further detected the edge modes localized on the boundary between the trivial and non-trivial PhCs, which were linearly polarized in the vicinity of the Gamma point due to the mixture of upper and lower branches with circular polarizations, and obtained their dispersion relation.