News Source: School of Physics

Editor: News Agency of BIT

Translator: Cao Xinyu, News Agency of BIT

Recently, Lu Cuicui from the School of Physics at BIT cooperated with Professor Xiao Meng of Wuhan University and Professor Chen Ziting of Hong Kong University of Science and Technology. They proposed to use the translational motion of optical lattice as the synthetic dimension to construct topological state and achieved the topological rainbow and a slow-light effect of photonic crystals. The theoretical results were published in the top journal Physical Review Letters in the field of physics.

Micro-nano all-optical devices with photons as information carriers have important applications in optical communication, optical information processing, optical computing and other fields. They are core components to the next generation of photonic chips. In recent years, the concept of topology has been extended from the study of topological state in condensed-matter physics to optics, acoustics, cold atom system and other areas, which greatly promotes the development of topological physics. In particular, topological photonics is gradually becoming an interdisciplinary frontier in optics and related areas. Compared with conventional photonic states, the topological photonic state has the advantages of robustness and anti-interference due to the topological protection. Therefore, the topological states provide a new platform for the realization of micro-nano all-optical devices. In the past few years, researchers have mainly focused on the methods of achieving topological states and the novel properties of topological states (e.g., unidirectional transmission, high order topology, topological laser, etc.). Most of the researches focus on specific energy bands and specific frequency bands. Currently, there is no good method to build devices for topological states with different frequencies.

FIG. 1. The construction of topological rainbow trapping with different frequencies using quadripartite lattice (a) The left side of the interface is a perfect photonic crystal; the right side is a photonic crystal with synthetic dimension. The light source is placed on the left side of the structure. (b) Topological patterns with different frequencies supported by structures. (c) At different frequencies, light waves are trapped in different regions on the photonic crystal interface; the higher the frequency, the lower the region.

On the basis of traditional optical wavelength routing devices (Optica,6,1367,2019) based on intelligent algorithms, Lu Cuicui et al. from the School of Physics of BIT further proposed at the physical level introducing synthetic dimension into photonic crystals, which provides a new idea for building topological optical wavelength routing devices. Their research found that when the synthetic dimension in the optical lattice gradually changes one lattice constant from zero, the Zak phase is exactly 2π, which corresponds to the emergence of topological boundary state. This synthetic dimension allows for several methods of construction, including linear gradients, sine cosine functions, etc. Different synthetic dimensions can achieve the "trapping" of topological states with different frequencies. Therefore, the maximum electric field obtained at different positions of the photonic crystal lattice corresponds to the topological state of different frequencies, that is, the "topological rainbow". The structure is simple and easy to achieve. It can apply to topology routing, topology slow light, topology optical storage and other applications. This method allows the construction of multiple topological boundary states and Chern insulators of great Chern number without breaking the time reversal symmetry. Conventional rainbow devices are easily affected and deviate from the working band or even disappear completely when encountering whole structure scaling, random error introduction, material defect, or impurity interference, but rainbows based on synthetic dimensions are protected by topology, hence will continue to exist.

FIG. 2. Different synthetic dimensions (abscissa) correspond to different frequency (ordinate) topological states, and different topological states have different group velocities, and the position of the green dotted line corresponds to zero group velocity.

This method has a good universality. It only requires that photonic crystals have a band gap (which ordinary photonic crystals satisfy) and has no requirement on lattice type, symmetry, band, or other characters. At the same time, achieving this topological state has no requirement on the material, Optical materials in the visible and near-infrared waveband in nature hardly have any magnetic response, but this method does not need the material to have a magnetic response and is feasible with most of the ordinary medium materials. Therefore, it provides a reliable solution for topological devices of nanoscale optical frequency bands and a new idea for the integrated topological photonic nanodevices on chips. Associate Research Fellow Lu Cuicui of BIT, Prof. Xiao Meng of Wuhan University and Prof. Chen Ziting of Hong Kong University of Science and Technology are co-authors of the paper. Wang Chenyang, an undergraduate of BIT, and Professor Zhang Zhaoqing of Hong Kong University of Science and Technology also made essential contributions. This work was supported by the Projects of the National Natural Science Foundation of China major research program cultivation project, youth project, emergency management project, BIT Young Teachers Academic Start-up Program, Light Field Control and Application Center of Shandong Normal University and Huawei technology co., ltd.

Article information：Cuicui Lu,* Chenyang Wang, Meng Xiao,† Z. Q. Zhang, C. T. Chan,‡ "Topological Rainbow Concentrator Based on Synthetic Dimension", Physical Review Letters 126, 113902 (2021).

Links to articles：https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.113902