The photonic topological insulator, which is the electromagnetic analogy of the topological insulator in electronic systems, has attracted a great deal of attention due to its topologically protected one-way transport of edge states. In the quantum Hall effect system in which the time-reversal symmetry is broken through external magnetic fields, the topological chiral edge states and topological one-way waveguides have the best robustness due to their unique features of free backscattering and immunity against sharp bends and defects. However, the high-efficiency coupling and conversion between topological chiral edge states and classical guided waves, which are essential for feeding energies into and extracting signals from these topological waveguides, have not been well studied.

Without considering the matching techniques from source to receiver, only a small fraction of the energy from classical source excitations will be coupled into the topological waveguide, resulting in low systematical transmission efficiency. At present, coupling from the classical transmission line modes to the topological modes based on the quantum spin Hall system, and coupling from the conventional waveguide modes to the topological modes based on the quantum valley Hall system have been reported. Topological modes in the quantum Hall system exhibit the strongest robustness against defects compared to quantum spin and quantum valley Hall systems, making it a powerful platform for exploring and utilizing novel topological phenomena. However, to the best of our knowledge, how such topological modes interact with classical electricmagnetic modes has not been reported in the photonic quantum Hall system. Therefore, the matching method between conventional waveguides and reflection-free topological one-way waveguides, or high-efficiency coupling technique between excitation sources and the topological one-way waveguide needs more attention, which has great significance for realizing the applications of chiral edge states in the topological system.

Recently, the research groups led by Prof. Feng Liang and Prof. Xiangru Wang from University of Electronic Science and Technology of China proposed a highly efficient conversion method from classical guided waves to topological one-way waves based on dielectric waveguides with tapered transition and magnetic photonic crystals in the quantum Hall system. The mechanism of high-efficiency coupling and conversion between classical and topological waves is explained by matching both the momentum and impedance. Finally, experiments are conducted to verify the proposed configuration and its robustness properties with high-efficiency transmission. The research results are published in Chinese Optics Letters, Vol. 22, Issue 2, 2024: Jianfei Han, Feng Liang, Yulin Zhao, Xiao Ding, Xiangru Wang, Deshuang Zhao, Bing-Zhong Wang. Highly efficient conversion from classical guided waves to topological chiral edge states [J]. Chinese Optics Letters, 2024, 22(2): 023902.

In this work, the high-efficiency conversion structure between a classical waveguide and a topological one-way waveguide is proposed, which efficiently converts classical guided waves to topological one-way waves. The topological one-way waveguide and classical waveguide are constructed by designing different topological regions with opposite applied magnetic fields and dielectric waveguides with tapered transitions, respectively. The momentum and impedance matching mechanisms demonstrate that our proposed conversion structure can achieve high coupling efficiency from classical waves generated by a point excitation source to topological one-way waves in the quantum Hall system. Moreover, the proposed topological waveguide has major advantages of high efficiency, being reflection-free and anti-bending, and robustness against obstacles. This work makes a bridge between theoretical research and practical applications of topological electromagnetics. It opens up a new avenue for advanced topological and classical integrated functional devices and systems, which will lead to more effective practical values.

To demonstrate the smooth transition from classical guided waves to topological chiral edge states, the simulated electromagnetic field distribution has been observed for the whole waveguide structure in the x-y plane. The results showed that the electromagnetic energy from the classic waveguide is smoothly coupled into the topological waveguide by the tapered transition when the electromagnetic energy is input from the left port (port1). To experimentally verify the proposed method, a prototype of the proposed waveguide was fabricated and the measurement setup was built. Via simulated and measured S-parameters, the researchers observed an obvious low-reflection and high-efficiency transmission phenomenon, which is consistent with the matching mechanism expectations. Meanwhile, the proposed waveguide also exhibits strong nonreciprocal isolation properties and robustness for disturbance. Additionally, it is possible to apply the proposed method and design to other crystalline structures composed of magnetic photonic crystals, such as triangular or honeycomb lattices.

This work provides useful insights and routines for designing practical topological and conventional integrated functional devices and systems. In the future, the team will further explore the physical mechanism of topological photonic systems, and try to design novel topological electromagnetic devices with small structure volume, excellent performance, and strong anti-disturbance.

Photograph of the fabricated waveguide system and measurement setup in (b); Magnetic (a) and electric (c) field distributions and S-parameters (d)-(e) for the whole waveguide.