News Resource: School of Physics

Editor: News Agency of BIT

Translator: Shao Yikang, News Agency of BIT

Recently, Professor Lu Cuicui and others from School of Physics, Beijing Institute of Technology, proposed a method to construct a multi-frequency synthesis dimensional topology by means of employing translational and rotation deformation, which lead to the effectuation of the on-chip topological rainbow nanophotonic devices The relevant achievements were published in Nature Communications.

Micro-nano all-optical devices with photons as the information carriers have crucial applications in optical communications, optical information processing, optical computing and other fields, playing a role of core components for realizing the next generation of photonic chips. The smaller the device size is, the better the integration is whereas the performance of smaller devices is usually affected by the error of structural parameters more profoundly. Topological photonic states have advantages such as robustness and anti-interference over traditional photonic states due to their topological protection, so that the state can provide a new platform for the implementation of micro-nano all-optical devices. Frequency, the degree of freedom of photons, is the basic carrier of information transmission. Therefore, multifrequency is considered as the key to realizebig data information processing. Almost all existing topological photonics reports depicted their study on specific energy bands and frequency bands such as unidirectional transmission, higher-order topologies, topological lasers, and so onwhile confronting difficulties in acquiring implements of multiple frequency micro-nano topological devices.

Professor Lu Cuicui and others from School of Physics, BIT, proposed a dimensional method for photonic crystal synthesis based on translational and rotation deformation in constructing multiple frequency on-chip nanophotonic topological devices, extending the research direction of nanophotonic topological rainbow. Invited by the chief editor of Applied Physics Letters, the team wrote the perspective article on "Perspective on the topological rainbow” （Sayed Elshahat, Chenyang Wang, Hongyu Zhang, Cuicui Lu,* Appl. Phys. Lett. 119, 230505, 2021）. Because of the topological protection, the nanophotonic topological rainbow devices based on the synthesis dimension will be free from damage and maintain stable when experiencing structural scaling, random errors, material defects or impurity interference as long as the band gap of the photonic crystal closed.

Fig.1(a): SEM images of the sample; (b) sample test diagram; (c) scattering scanning near-field optical microscope

By developing the near-field optical microscope technology, the surface electric field of synthetic-dimensional photonic crystal topological rainbow nanodevices is directly characterized., the electric field distribution of each photonic lattice is clearly visible and the maximum value of the topological state electric field at different frequencies appears at different lattice positions, a significant topological rainbow effect is realized on the nanoscale. The development of non-porous scattered near-field optical characterization technology has significant advantages: the topography of the sample and the optical signal can be measured simultaneously, so that the amplitude signal of the electric field at different positions of the photonic crystal can be directly provided and the non-porous Atomic Force Microscopy (AFM) probe is capable of penetrating deeply into the air pores of a single photonic crystal with extremely high resolution due to its tip of only 20 nm in size. In addition, the waveguide end-face coupling excitation method is used to improve the excitation efficiency by optimizing the advantages of high collection efficiency and low background noise aroused by colleting fiber signals on the surface of the sample. This is also the first on-chip integrated photonic topological rainbow device at the nanoscale, which establishes a bridge between the frontier research of topological photonics and the mature process of silicon-based photonics, providing ideas and opportunities for promoting the transformation of the physical concept of topological photonics to the application of photonic chip devices.

Fig.2(a) sample design legend; (b) The light intensity distributions (|E|2)of the calculated results for different wavelengths; (c) topographic image of the the atomic force of sample; (d) The light intensity distribution of different wavelengths measured by the experiment on the surface of the synthetic dimension photonic crystals. Different wavelengths are imprisoned in different lattice positions at the interface.

The study also found that using photonic crystal lattice rotation can also achieve on-chip topological rainbow and the topological state has a more compact electric field distribution and the topological state electric field distribution of different frequencies is not covered. In addition, by regulating structural or material parameters, it is also possible to achieve a distribution of different frequency topologies without overlapping. Further, after extending the two-dimensional synthetic dimensional photonic crystal structure on the chip to high-dimensional space photonic crystals, the topological rainbow phenomenon will exist stably and the topological states of different frequencies will be separated and imprisoned in different positions spatially. The study of topological rainbow micro-nano photonic devices provides a reliable method to solve the robustness of nano-devices such as on-chip router devices, wavelength-division multiplexing devices, photonic cache devices, and slow optical devices.

Fig.3 A topological rainbow of photonic crystal lattice rotations, where topological states of different frequencies are imprisoned in different positions on the chip (a-c); Topological rainbows of photonic crystals in the spatial synthesis dimension, where topological states of different frequencies are imprisoned in different locations in space(d-f).

The joint corresponding authors of this project are Professor Lu Cuicui of BIT, Professor Hu Xiaoyong of Peking University and researcher Ding Wei of Jinan University. The joint lead authors on this project are Professor Lu Cuicui of BIT, research assistant Sun Yizhi of Jinan University and Wang Chenyang, the 2017 Grade undergraduates of BIT. Zhang Hongyu, a 2021 Grade ph.D. of BIT and Zhao Wen, a 2021 Grade master also contributed to experiments and theories. Co-authors of this project includes Professor Xiao Meng of Wuhan University, associate Professor Liu Yongchun of Tsinghua University, Professor Chen Ziting of the Hong Kong University of Science and Technology. The first unit of this work is the School of Physics of BIT, which is supported by the National Natural Science Foundation of China, Beijing Institute of Technology Young Scholars Academic Launch Program, National Key R&D Program, Guangdong Provincial Basic and Applied Basic Research Fund and other projects.

Professor Lu is engaged in the research of nanophotonic topological. She has published more that 50 papers in PRL, NC, Light, etc. Professor Lu has served as an editorial board member of Optics Letters, an editorial board member and guest editor of Frontiers in Physics, a guest editor of Frontiers in Materials, and a consultant to Huawei since 2020. She has won the PIERS2021 Young Scientist Award(15 people in the world) and the Outstanding Undergraduate Thesis/Design Instructor in Beijing.

Article information: Cuicui Lu,* Yi-Zhi Sun, Chenyang Wang, Hongyu Zhang, Wen Zhao, Xiaoyong Hu,* Meng Xiao, Wei Ding,* Yong-Chun Liu, C. T. Chan, "On-chip nanophotonic topological rainbow", Nature Communications, 13, 2586 (2022).

Paper link:https://www.nature.com/articles/s41467-022-30276-w