Controllable one-way add-drop filter based on magneto-optical photonic crystal with ring resonator and microcavities

We have proposed a controllable topological add-drop filter (ADF) by utilizing the one-way transmission property and resonant coupling effect of topological photonic states in magneto-optical photonic crystal (MOPC) system. The ADF is symmetrically constructed by a central ring resonator (RR) with each side a MO microcavity and a topological waveguide formed by MOPC/Al₂O₃ PCs. The topological waveguide supports one-way transmission and the microcavity is used to adjust resonant frequency and improve filtering performance. Based on the symmetry of structure and the adjustability of magnetic field, the input and output ports can be reconfigured conveniently to realize the dropping and adding functions of the ADF. Such an ADF possesses merits of nearly perfect filtering performance, convenient adjustability of resonance mode, and strong robustness against various defects. Moreover, we further design an optical demultiplexer consisting of two PCRRs which is able to separate two different resonant modes independently and efficiently. These results hold promise in many fields such as optical communications and wavelength-division multiplexing.