Valley topological photonic crystals (TPCs), which are robust against local disorders and structural defects, have attracted great research interest, from theoretical verification to technical applications. However, previous works mostly focused on the robustness of topologically protected edge states and little attention was paid to the importance of the photonic bandgaps (PBGs), which hinders the implementation of various multifrequency functional topological photonic devices. Here, by systematically studying the relationship between the degree of symmetry breaking and the working bandwidth of the edge states, we present spoof surface plasmon polariton valley TPCs with broadband edge states and engineered PBGs, where the operation frequency is easy to adjust. Furthermore, by connecting valley TPCs operating at different frequencies, a broadband multifunctional frequency-dependent topological photonic device with selectively directional light transmission is fabricated and experimentally demonstrated, achieving the functions of wavelength division multiplexing and add–drop multiplexing. We provide an effective and insightful method for building multi-frequency topological photonic devices.

The pseudo-magnetic field, an artificial synthetic gauge field, has attracted intense research interest in the classical wave system. The strong pseudo-magnetic field is realized in a two-dimensional photonic crystal (PhC) by introducing the uniaxial linear gradient deformation. The emergence of the pseudo-magnetic field leads to the quantization of Landau levels. The quantum-Hall-like edge states between adjacent Landau levels are observed in our designed experimental implementation. The combination of two reversed gradient PhCs gives rise to the spatially nonuniform pseudo-magnetic field. The propagation of the large-area edge state and the interesting phenomenon of the snake state induced by the nonuniform pseudo-magnetic field is experimentally demonstrated in a PhC heterostructure. This provides a good platform to manipulate the transport of electromagnetic waves and to design useful devices for information processing.

Unlike conventional topological edge states confined at a domain wall between two topologically distinct media, the recently proposed large-area topological waveguide states in three-layer heterostructures, which consist of a domain featuring Dirac points sandwiched between two domains of different topologies, have introduced the mode width degree of freedom for more flexible manipulation of electromagnetic waves. Until now, the experimental realizations of photonic large-area topological waveguide states have been exclusively based on quantum Hall and quantum valley-Hall systems. We propose a new way to create large-area topological waveguide states based on the photonic quantum spin-Hall system and observe their unique feature of pseudo-spin-momentum-locking unidirectional propagation for the first time in experiments. Moreover, due to the new effect provided by the mode width degree of freedom, the propagation of these large-area quantum spin-Hall waveguide states exhibits unusually strong robustness against defects, e.g., large voids with size reaching several unit cells, which has not been reported previously. Finally, practical applications, such as topological channel intersection and topological energy concentrator, are further demonstrated based on these novel states. Our work not only completes the last member of such states in the photonic quantum Hall, quantum valley-Hall, and quantum spin-Hall family, but also provides further opportunities for high-capacity energy transport with tunable mode width and exceptional robustness in integrated photonic devices and on-chip communications.

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.

A high repetition rate, picosecond terahertz (THz) parametric amplifier with a LiNbO₃ (LN) crystal has been demonstrated in this work. At a 10 kHz repetition rate, a peak power of 200 W and an average power of 12 μW have been obtained over a wide range of around 2 THz; at a 100 kHz repetition rate, a maximum peak power of 18 W and an average power of 10.8 μW have been obtained. The parametric gain of the LN crystal was also investigated, and a modified Schwarz–Maier model was introduced to interpret the experimental results.