Finite-size effect plays a significant role in topology photonics not to mention in reality all experimental setups are in finite-size. A photonic bandgap is opened in the topological edge state dispersion if a topological photonic crystal with finite width is considered, and the bandgap size relies on the finite-size effect. Pseudospin-preserving and pseudospin-flipping processes can be realized when a selectively switch of the pseudospin of edge states are customized by our designs. Our microwave experiments also successfully demonstrate pseudospin switch-on and -off behaviors in a finite-width photonic crystal. By combining photonic crystals with finite widths, a multi-tunneling proposal of topological photonic crystals can also be achieved. Our study of the finite-size effect will provide new approaches and thoughts to improve the development of topological photonic devices in the future.

Topological photonics has made great progress from physical concept verification to new technical applications, and valley topological photonic crystal (TPCs) are one of the most important candidates for future applications in functional devices because of large bandwidth and lossless optical transport. However, due to the limitations of the design method and structure arrangement, the multichannel valley topological beam splitter (BS) has not yet been much explored. Here, we reveal the different robustness of four types of domain walls in valley TPCs. Benefiting from the differences in domain walls, we numerically present and experimentally demonstrate a highly integrated multichannel valley topological BS in the microwave regime. Compared with traditional BSs, it has the advantages of being more robust and compact and having more output ports and higher integration. The reported multichannel topological BS opens an avenue to engineer the flow of light and offers effective design approaches for integrated photonic device miniaturization.

Topological edge states have an important role in optical modulation with potential applications in wavelength division multiplexers (WDMs). In this paper, 2D photonic crystals (PCs) with different rotation angles are combined to generate topological edge states. We reveal the relationship between the edge states and the rotation parameters of PCs, and further propose a WDM to realize the application of adjustable beams. Our findings successfully reveal the channel selectivity for optical transmission and provide a flexible way to promote the development of topological photonic devices.

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Under strong magnetic field, many interesting phenomena can occur in the electronic system, for example quantization of Landau levels and quantum Hall effect. However, photons do not carry charge, and can not have many properties induced by external magnetic fields. Recently, the pseudomagnetic field being an artificial synthetic gauge field, has attracted intense research interest in classical wave systems, in which the propagation of wave can be manipulated like in real magnetic field. The photonic crystal is an optical structure composed of periodic material distributions and provides a good platform to study the control of electromagnetic waves. In this work, we construct a uniform pseudomagnetic field by introducing uniaxial linear gradient deformation of metallic rods in a two-dimensional photonic crystal. The strong pseudomagnetic field leads to the quantization of photonic Landau levels in photonic crystals. The sublattice polarization of n=0 Landau level is also demonstrated in our simulations. Unlike the real magnetic field, the pseudomagnetic fields of photonic crystal is opposite in two inequivalent energy valleys, and the time-reversal symmetry of the system is not broken. Our designed gradient photonic crystals support the transport of edge state in the gap between <i>n</i>=0 and <i>n</i>=±1 Landau levels. The edge state can propagate unidirectionally when is excited by a chiral source. When a gaussian beam impinges on the photonic crystal, the propagating path of two splitting beams can be controlled, which gives rise to the bend of two beams. Two photonic crystals with opposite pseudomagnetic fields are assembled together, and the interesting phenomenon of "snake-state" can be obtained. Our proposal opens a new way for the design of information processing devices by manipulating electromagnetic waves.