Topological optical router and Fano filter based on asymmetric C4 photonic crystals

Higher-order topological photonics offers a robust platform for controlling light. Here, we propose and demonstrate a photonic crystal with C4 asymmetric to achieve the functionalities of topological routing and filtering on-chip. By simply reversing the stacking order of two topologically distinct crystals, we construct interface-specific edge state waveguides that function as an optical router, guiding light along designated paths at three distinct frequency bands. Furthermore, we achieve precise spatial control of zero-dimensional topological corner states and switching between single- and double-corner localization by strategically displacing an interior dielectric rod within a cavity. Crucially, we engineer a Fano resonance system by coupling a topological edge state waveguide with a corner state cavity. Through structural optimization, this system exhibits a pronounced asymmetric Fano line shape and a high-quality factor of 6.62×103. Leveraging this strong coupling, we realize a dual-band filter that efficiently localizes light at two specific frequencies. Our work provides a versatile approach based on asymmetric photonic crystals for the integrated manipulation of topological states, paving the way for advanced photonic devices.