Modulating the motions of photons through topological structures plays a primarily vital role in both scientific research and practical applications, which leads to a new but thriving study direction, namely topological photonics. Flexible topological phases and robust topological states provide an unprecedented perspective to the abundant physics phenomena generated by vector electromagnetic fields with spin-1. On the other hand, photonic artificial microstructures, such as metamaterials and photonic crystals, can be gradually perceived as substitutes and even upgrades of some complex topological models in condensed matter physics, which mainly rely on their rich state control mechanisms and highly customized design degrees of freedom. In this research process, some properties of optical topological states are utilized to overcome some engineering problems, including exploiting robustness to eliminate the scattering losses caused by defects and disorders. In view of the early success of Hermitian topological systems, recent focus has been laid on non-Hermitian topological systems described by non-Hermitian Hamiltonians. Especially, when the Hamiltonian of the system satisfies the parity-time (PT) symmetry, its eigenvalues are pure real, which corresponds to a unique non-Hermitian system with highly sensitive exceptional points (EPs) in the parameter space and novel skin effects in edge modes.
In the past decade, wireless power transfer (WPT) and sensing become a hotspot, which triggers immense research interest in practical applications, including mobile phones, logistic robots, medical-implanted devices, and electric vehicles. For a standard WPT system, it is mainly composed of two coupled coil resonators, which are placed on the source and receiver sides, respectively. However, there are some aspects of these conventional WPT applications that should be noted. For example, the limitation of the coupling of evanescent waves and the inherent sensitivity to the transmission distance or structural disturbance restrict the structure sizes and application scenarios. With the development of WPT devices, efficient long-range and robust WPT is highly desirable but challenging. Recently, the non-Hermitian topological edge mode provides a powerful tool for near-field robust control of WPT. Therefore, it is critical to review recent works on high-performance near-field wireless power transfer and sensing systems with topological protection characteristics inspired by non-Hermitian topological effects.
Topological edge states of dimers can provide a suitable platform for the study of robust WPT in the radio frequency (RF) regime. On the one hand, similar to the Domino structure composed of coupled resonators for long-range WPT, the topological dimer chain can be used to realize efficient long-range WPT. On the other hand, the edge modes in nontrivial dimer chains are topologically protected, and thus the corresponding WPT is robust against the disorders and fluctuations (Figs. 3-8).
At the same time, a long-range WPT can be realized through a finite quasiperiodic Harper chain based on the ultra-subwavelength coil resonators. In addition, the distribution of the asymmetric topological edge states (TESs) in the Harper chain is observed from the local density of states (LDOS) spectrum (Fig. 10). Especially, using the asymmetric topological edge states, two Chinese characters composed of light-emitting diode (LED) lamps are selectively lighted up at both ends of the chain, which intuitively show the directional WPT in the topological Harper chain (Fig. 11). Moreover, in view of the robustness of topological edge states, the designed WPT device can be robust to the disorder perturbation inside the structure. The topological edge states for directional WPT not only extend previous research work on long-range WPT but also have a circuit structure that is easier to integrate and for active control. As a result, by adding electrical variable capacitance diodes into the system, the actively tuned transmission direction by modulating the external voltages applied in variable capacitance diodes (VCDs) is experimentally observed (Fig. 12).
Moreover, the properties of the EP exist in a finite non-Hermitian topological circuit-based dimer chain (Fig. 13). The coupling between two edge states is presented, which is particularly relevant to the realization of second-order EPs. By adding loss and gain to both ends of the dimer chain, the non-Hermitian topological chain and the EP that satisfies PT symmetry (Fig. 14) can be obtained. In similar systems, topological edge states are highly sensitive to disturbances in the environment before and after the EP, which lead to new highly sensitive sensors with topological protection (Fig. 15). In sharp contrast to traditional sensors, this new sensor based on non-Hermitian and topological characteristics has unique advantages. It is immune from disturbances of site-to-site couplings in the internal part of the structure and is sensitive to the perturbation of on-site frequency at the end of the structure.
In summary, high-performance near-field WPT and sensing systems are realized with topological protection characteristics inspired by non-Hermitian topological effects. Especially, the one-dimensional system composed of resonant coils provides a simple but efficient platform to utilize the advantages of topological and non-Hermitian effects in practical applications. In addition, new topology structures with higher dimensions and higher orders are promising candidates to realize multifunctional WPTs in the future.
郭志伟, 胡胜宇, 张海燕, 王宇倩, 董丽娟, 孙勇, 李云辉, 江海涛, 羊亚平, 陈鸿. 基于非厄密拓扑效应的无线传能与传感研究[J]. 光学学报, 2023, 43(16): 1623011. Zhiwei Guo, Shengyu Hu, Haiyan Zhang, Yuqian Wang, Lijuan Dong, Yong Sun, Yunhui Li, Haitao Jiang, Yaping Yang, Hong Chen. Wireless Power Transfer and Sensing Based on Non-Hermitian Topological Effects[J]. Acta Optica Sinica, 2023, 43(16): 1623011.