Non-Hermitian systems can change the band topology of topological systems to achieve unique physical effects, and have broad applications prospects in the development of photonic devices since most of the optical systems are non-Hermitian in the real world due to loss (or gain). Meanwhile, topological rainbow, which can separate and steer topological photonic states with different frequencies into different positions, has the great advantage of robustness against disorders and plays a key role in broadband information processing for photonic devices. However, non-Hermitian topological photonic devices are difficult to realize due to the complexity and elusive properties of the non-Hermitian systems. Up to date, no related reports have been found to demonstrate topological rainbow based on non-Hermitian photonic crystals due to lack of both theoretical methods and experimental schemes in studying non-Hermitian systems.

Recently, Prof. Cuicui Lu from Beijing Institute of technology, Prof. Xiaoyong Hu from Peking University and Prof. Zhihong Hang from Suzhou University etc. propose a method based on lossy photonic crystals (PCs) to realize topological rainbow in non-Hermitian systems. By precisely controlling the introduced loss in PCs, band engineering is feasible which is the key to realize the topological rainbow. The microwave PCs have been fabricated based on wrapping different layers of absorbed shielding materials, and obvious topological rainbow has been observed in non-Hermitian systems. The research results are published in Chinese Optics Letters, Vol. 21, Issue 12, 2023: Cuicui Lu, Wen Zhao, Sheng Zhang, Yanji Zheng, Chenyang Wang, Yaohua Li, Yong-Chun Liu, Xiaoyong Hu, Zhi Hong Hang. Observation of topological rainbow in non-Hermitian systems[J]. Chinese Optics Letters, 2023, 21(12): 123601.

In this work, the topological interface states are first generated by splicing two photonic crystals with different translation distances. The Zak phase is calculated to show that our proposed structure which has the same translational distances and different losses for different rows is topologically satisfied in the non-Hermitian systems. Then the frequencies of topological interface states are regulated by adding gradual loss along the y direction in the structure, so that the lights with different frequencies are separated are finally stopped at different spatial positions, resulting in a topological rainbow. Loss is ubiquitous in the real optical systems which is often expected to be avoided, and here loss is also taken as a degree of freedom to control the operating frequency of the topological rainbow with modulating frequencies of interface states both theoretically and experimentally, which has brought new inspiration to "turn waste into treasure".

Non-Hermitian photonic crystals are successfully built by engineering graded loss experimentally, which propose a new method to control the material's loss effectively. The method is convenient and reconfigurable based on wrapping different layers of absorbed shielding materials, which provides an excellent platform for the study of non-Hermitian physics Obvious topological rainbow has been observed in non-Hermitian systems in an operation band range from 7.725 GHz to 8.355 GHz. Different frequencies of topological photonic states propagate and stop at different positions because of the gradually decreasing group velocities. Using near-field measurements, the researchers observed an obvious topological rainbow phenomenon, which is in good agreement with the theoretical results. Only requiring a PC with bandgap, the realization of such a topological rainbow in non-Hermitian systems is general and of great convenience, because no external magnetic field is needed along with no limitations for symmetries and lattice types.

This work provides an effective method to realize topological photonic devices in non-Hermitian systems, and will promote the practical applications of topological states, especially for slowing light, photon buffer, and broadband optical information processing. In the future, the team will further explore the physical mechanism of topological photonics in non-Hermitic systems, and try to design on-chip multi-channel information processing devices with small structure volume, excellent performance and strong anti-interference.

Samples and Experimental setup; Calculated and measured topological rainbow in non-Hermitian systems.