Fabrication of Symmetric SOI Photonic Crystal Slabs and their Topological Bandgaps and Edge Modes

We successfully fabricated SOI (silicon-on-insulator) photonic crystal (PhC) slabs in which electromagnetic topological bandgaps and edge modes were materialized for the lowest-order symmetric TE (transverse electric)-like modes. Because the structure of our specimens can be regarded as symmetric about the middle plane of the PhC slab, their symmetric and antisymmetric eigenmodes are rigorously separated, so we could achieve genuine photonic bandgaps for the former. We fabricated those specimens by electron beam (EB) lithography of the top Si layer of SOI wafers and successive plasma-enhanced chemical vapor deposition (PECVD) of a SiO2 capping layer. We confirmed the complete common bandgap of topologically trivial and non-trivial PhCs, which we call PhC-t and PhC-n hereafter, and the topological edge modes on the boundary between them by high-resolution angle-resolved reflection spectroscopy in the mid-infrared (IR) range. To the best of our knowledge, this is the first observation of the common bandgap and edge modes materialized in symmetric PhC slabs without under-etching to increase the refractive-index contrast.