We have studied the coupling effect of topological photonic states in a double-channel magneto-optical photonic crystal waveguide by introducing a two-layer ordinary alumina (Al2O3) photonic crystal as the coupling layer. Interestingly, for the structure with the same widths of the two channels, the electromagnetic wave propagates one-way to the right side and exhibits wave-like path within the waveguide due to the coupling effect of topological photonic states. This unique property provides an effective way to achieve desired power ratio between two right outputs by terminating the structure at different length of waveguide. Moreover, the power ratio can be tuned by the external magnetic field conveniently. As for an asymmetric waveguide with different channel widths, there exist two asymmetric one-way topological photonic states (i.e., the odd-like and even-like modes) in the bandgap. The eigenfield analyses show that the electric field of odd-like mode is stronger in the lower channel, while that of even-like mode is contrary. As the old-like mode propagates rightwards, electromagnetic waves in the two channels couple with each other via the coupling layer, then the power in the upper channel gradually transfers to the lower channel, and finally reach almost 100% transmittance in the lower output. However, the case for the even-like mode is totally contrary. These results hold great promise for many application fields such as signal transmission, optical modulation, and the design of topological devices.
Photonic crystals (PCs) are artificial micro-nano structures consisting of different materials periodically arranged. They can be divided into two categories: simple and compound PCs. They have broad application prospects in many fields such as micro-nano photonics and optoelectronic integration. Laser holography method is an important method for fabricating PCs. In this work, a theoretical study on the production of compound PCs by multi-beam holographic interferometry is carried out, and a four-beam configuration with a certain symmetry is designed to produce two-dimensional compound PCs. Simulations based on MATLAB program are in good agreement with the theoretical analysis. The evolution of the unitcell and contrast of the compound PC under different polarization combinations of a single beam and double beams are further studied. The results indicate that the polarization affects the unitcell of the PC sensitively. Under different polarization combinations, a variety of rich unitcell shapes of compound PCs such as elliptical-like rods and wave-like stripes are obtained. When all interfering beams are linearly polarized within the xoy plane, the compound PC has the best contrast. These above results hold promise for the design and fabrication of compound PCs with various unitcell shapes.
Unidirectional edge modes are achieved in gyromagnetic photonic crystals. The physical reason is attributed to magnetic resonance and broken time-reversal symmetry under external magnetic fields. These edge modes propagate only along a single direction, while the backward modes are completely suppressed. The unidirectional transmittance is nearly 100% and hardly affected by perfect electric conductor (PEC) defect. However, a PEC defect has sensitive influence on both the phase delay and pattern distribution of unidirectional edge modes. These properties hold promise in designing various unidirectional devices. Here we design a three port circulator with high transmission contrast and magnetic controllability simultaneously.
We theoretically study the properties of unidirectional edge modes in a magnetic metamaterial waveguide and their applications in tunable multi-way splitters. These edge modes can only be excited at the surface to propagate along a single direction with nearly perfect transmittance of 100%. The physical origin is attributed to the combined action of magnetic resonance and time-reversal symmetry breaking under external magnetic fields. Moreover, the propagation direction can be controlled by reversing the direction of the external magnetic field. Further study reveals that a perfect electric conductor defect scarcely affects the transmittance but has sensitive influence on the phase delay and pattern distribution of the unidirectional edge modes. These properties hold promise for designing various unidirectional photonic devices. As an example, we design a tunable multi-way splitter with the advantages of high transmission contrast and convenient pathway control simultaneously.
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