Manipulating electromagnetic waves in a cavity-waveguide system with nontrivial and trivial modes.

The coupled cavity-waveguide approach provides a flexible platform to design integrated photonic devices that are widely applied in optical communications and information processing. Topological photonic crystals that can excite the nontrivial edge state (ES) and corner state (CS) have an unprecedented capability to manipulate electromagnetic (EM) waves, leading to a variety of unusual functionalities that are impossible to achieve with conventional cavity-waveguide systems. In this Letter, two-dimensional photonic crystals consisting of an ES waveguide, a CS cavity, and a trivial cavity are proposed as a means to robustly control the transmission characteristics of electromagnetic waves. As a proof-of-principle example, the analog of electromagnetically induced transparency (EIT) that is tolerated in disorders due to the robustness of the CS is numerically demonstrated. In addition, the analog of multi-EIT is also verified by introducing a trivial cavity with two degenerate orthogonal modes. This unique approach for robustly manipulating EM waves may open an avenue to the design of high-performance filters, modulators, and on-chip processors.