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.

We theoretically proposed a topological multichannel add-drop filter (ADF) and studied its unique transmission properties. The multichannel ADF was composed of two one-way gyromagnetic photonic crystal (GPC) waveguides, a middle ordinary waveguide, and two square resonators sandwiched between them, which can be regarded as two paralleling four-port nonreciprocal filters. The two square resonators were applied with opposite external magnetic fields (EMFs) to support one-way states propagating clockwise and counterclockwise, respectively. On the basis of the fact that the resonant frequencies can be tuned by the EMFs applied to the square resonators, when the intensities of EMFs were the same, the multichannel ADF behaved as a power splitter with a 50/50 division ratio and high transmittance; otherwise, it functioned as a demultiplexer to separate two different frequencies efficiently. Such a multichannel ADF not only possesses excellent filtering performance but also has strong robustness against various defects due to its topological protection property. Moreover, each output port can be switched dynamically, and each transmission channel can operate independently with little crosstalk. Our results have the potential for developing topological photonic devices in wavelength division multiplexing systems.

We have presented adjustable enhanced Goos-Hänchen (GH) shift in a magneto-optical photonic crystal (MOPC) waveguide. The waveguide consists of a top layer of ferrite rods and a lower MOPC with opposite biased dc external magnetic fields (EMFs), and it supports both odd-like and even-like modes simultaneously. The simulation results show the odd-like mode can cause an enhanced negative GH shift, while the even-like mode can result in an enhanced positive GH shift. The physical reason for such negative and positive GH shifts is attributed to the efficient mode coupling and propagation behaviors of the electromagnetic (EM) wave in the waveguide. Furthermore, we have realized the switchable negative/positive GH shift by altering the direction combination of the EMFs. In addition, the magnitudes of both GH shifts can be adjusted by changing the strength of EMF or the width of the waveguide. These results provide new ways to control the transmission behaviors of EM wave and hold promise in applications such as detections, optical switches, and sensors.

We present mode conversion in different magneto-optical photonic crystal (MOPC) waveguides. An odd-mode waveguide (OMW) and an even-mode waveguide (EMW) are designed by adjusting the geometric parameters of the waveguide. These waveguides are constructed by adding a layer of yttrium-iron-garnet (YIG) rods with opposing magnetic fields between an MOPC and an Al2O3 photonic crystal (PC). Due to the coupling effect caused by the middle layer of YIG rods, the OMW (or EMW) only supports an odd (or even) mode within a single-mode frequency range. Simulation results demonstrate that they can convert other modes into odd or even modes, and there is almost no power loss during the conversion. Most importantly, they are robust against backscattering from perfect electric conductors (PECs) and point defects. Based on these properties, we propose a device that can efficiently separate the odd and even modes into different ports. These results offer a novel approach to controlling the transmission modes of waveguides, which facilitates the interconnection of diverse topological magneto-optical waveguides.