We design and present a switchable slow light rainbow trapping (SLRT) state in a strongly coupling topological photonic system made from a magneto-optical photonic crystal waveguide channel. The waveguide channel supports slow light states with extremely small group velocity ($v_g=2.1\times {10}^{-6}c$), low group-velocity dispersion, and a broadband operation bandwidth (3.60–4.48 GHz, near 22% of bandwidth). These slow light states originate from the strong coupling between two counter propagating topological photonic states. Under a gradient magnetic field, different frequency components of a wave packet are separated and stored at different positions for a long temporal duration with high spatial precision (without crosstalk and overlap between the electric fields of different frequencies) to form SLRT. Besides, these SLRT states can be easily switched among the forbidden state, trapped state, and releasing state by tuning the external magnetic field. The results suggest that the topological photonic state can offer a precise route of spatial-temporal-spectral control upon a light signal and find applications for optical buffers, broadband slow light systems, optical filters, wavelength-division multiplexing, and other optical communication devices.

We propose and show that antichiral edge states can be realized in a gyromagnetic photonic crystal (GPC) with a honeycomb lattice consisting of two interpenetrating triangular sublattices $A$ and $B$. When sublattices $A$ and $B$ are immersed in opposite external magnetic fields respectively, the band structure of the GPC tilts and antichiral edge states emerge. These special edge states propagate in the same direction at the two opposite parallel zigzag boundaries of the GPC, which are completely distinguished from the well-studied topological edge states in chiral photonic systems where the edge states transport in opposite directions at the opposite two parallel zigzag boundaries. We show that these unique antichiral edge states originate from the overall coupling effects of the counterclockwise energy flux vortexes of sublattice $A$ and clockwise energy flux vortexes of sublattice $B$, so that two copropagating one-way transport channels are created on the boundaries. We further demonstrate that these antichiral edge states are also strongly robust against backscattering from the obstacles at the zigzag edges. Our findings clearly indicate that deeply digging into the GPC systems can help to find rich novel and significant topological physics. Antichiral edge states are of significance not only in basic physics, but also in offering useful insights and routines to design novel electromagnetic and optical functional devices, such as the compact multichannel one-way waveguide.

We present a scheme to realize topological slow-light state with low group velocity and vanishing group velocity dispersion. By harnessing the strong interactions between two regular co-propagating topological photonic states in a magneto-optical photonic crystal waveguide, the energy flux transport of light exhibits a peculiar eight-shaped flowing loop within each unit cell of the waveguide. This permits the broadband pulse transporting with low group velocity ( n g = 13.26 ), broad bandwidth with a relative bandwidth of 3.08%, large normalized delay-bandwidth product (about 0.409), and vanishing group velocity dispersion. More importantly, they are robust against backscattering from obstacles. Our scheme paves the way to dig into the concept and physics of topology for solving the difficult problems of signal distortion and scattering loss in slow-light systems.

We propose a pseudospin-field-dependent waveguide (PFDW) by constructing a sandwiched heterostructure consisting of three magneto-optical photonic crystals (MOPCs) with different geometric parameters. The upper expanded MOPC applied with an external magnetic field has broken time-reversal symmetry (TRS) and an analogous quantum spin Hall (QSH) effect, while the middle standard and the lower compressed ones are not magnetized and trivial. Attributed to the TRS-broken-QSH effect of the upper MOPC, the topological large-area one-way transmission that uniformly distributes over the middle domain is achieved and exhibits the characteristics of a pseudospin-field-momentum-locking; i.e., pseudospin-down (or pseudospin-up) leftward (or rightward) waveguide state when the positive (or negative) magnetic field is applied on the upper MOPC. We further demonstrate the strong robustness of the PFDW against backscattering from various kinds of defects. In addition, a topological beam modulator that can compress or expand the light beam, and a large-area pseudospin beam splitter have been designed. These results have potential in various applications such as sensing, signal processing, and optical communications.

We have studied a switchable hyperbolic metamaterial composed of a graphene–dielectric periodic structure. By tuning the chemical potential of all graphene sheets simultaneously, the isofrequency curve can switch between an ellipse and a hyperbola conveniently. In particular, a special hyperbolic isofrequency curve with its asymptote perpendicular to the interface is obtained and used to realize the zero reflection effect. Furthermore, a zero-reflection-based optical switch working in the terahertz spectrum is demonstrated. Its bandwidth can be efficiently adjusted by geometric parameters such as permittivity and period. Such an optical switch possesses the merits of low loss, high transmittance contrast, high response speed, compact size, high tolerance of chemical potential, and having all incident angles (0°–90°) simultaneously. Such an optical switch holds great potential in many fields, such as data storage, beam steering, and integrated photonic circuits.

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.