Photonics Research

The prevailing research on topological photonics has provided premium recipes for realizing robust energy transport against sharp corners or defects, and those topological ideas are bringing great advancement to the functionality and performance of optical components. Recently, frequency multiplexing topological devices such as topological rainbow and wavelength division multiplexer have attracted much attention, where the ability to separate to separate different frequency channels promises great potential for application in communication systems. However, from the existing literature, the existing frequency multiplexing topological devices are generally based on the slow light effect. The resulting static spatial local mode or finely-tuned flat band has zero-group velocity, making it difficult for both experimental excitation and channel out-coupling. These shortages and difficulties are blocking frequency multiplexing topological devices from practical application.

 

To address the above shortages and difficulties, the cooperation research group led by associate Prof. Chunmei Ouyang from Tianjin University, associate Prof. Yuting Yang from China University of Mining and Technology, Dr. Dongyang Wang from University of Southampton, and Prof. Weili Zhang of Oklahoma State University propose and experimentally demonstrate the design of a new type of asymmetric frequency multiplexing devices including topological rainbow and frequency router based on floating propagating topological modes (instead of localized ones), where perfect frequency channel isolation can be achieved. Such muti-channel propagating modes can be efficiently pumped by one single point source, and asymmetric transmission is also here first-time realized for topological rainbow and frequency router. The research results were published in Photonics Research, Volume 12, Issue 6, 2024. [Jiajun Ma, Chunmei Ouyang, Yuting Yang, Dongyang Wang, Hongyi Li, Li Niu, Yi Liu, Quan Xu, Yanfeng Li, Zhen Tian, Jiaguang Han, Weili Zhang, "Asymmetric frequency multiplexing topological devices based on a floating edge band," Photonics Res. 12, 1201 (2024)]

 

The frequency channel isolation in this design is achieved by using the discovered 'floating' edge band, i.e., the topological edge band is sandwiched by the lower frequency and higher frequency band gaps. Benefiting from such sandwiched morphology, it can be designed that the propagating edge mode in one topological photonic crystal is at the same frequency of the band gap in the second photonic crystal, so that the energy transport at this frequency channel is prohibited at the interface between two crystals. Using this idea, this group has successfully designed a seven-order topological rainbow in Fig. 1, where the modes in seven channels collectively propagate from a point source and stop at distinct spatial positions to achieve isolation.

 

 

Fig. 1. Experimental characterization of the seven-order topological rainbow.

 

More interestingly, due to the existence of both upper frequency and lower frequency band gaps, the proposed topological rainbow supports respective set of seven transportation channels along opposite directions, as shown in Fig. 2. This empowered the topological rainbow with asymmetric transmissions, a feature that has not been possible for topological rainbow before.

 

 

Fig. 2. Experimental characterization of the asymmetrical channels of the topological rainbow.

 

Finally, to further demonstrate the idea of frequency multiplexing, this group also configured an asymmetric frequency router with topological domain walls. As shown in Fig. 3, the energy of different frequency channels was coupled out along different ports, which experimentally verified the asymmetric topological router.

 

Fig. 3. Asymmetric propagating topological frequency router.

 

The proposed new type of asymmetric propagating topological rainbow and frequency router based on 'floating' edge band has totally resolved the current challenges of frequency multiplexing devices in channel coupling and difficulty of excitation, it also provided the novel feature of asymmetric transmission for signal isolation or information capacity enhancement in optical communication systems. In the next step, the team will optimize the structural parameters of multi-frequency topological optical devices with asymmetric transmission and apply them to terahertz and telecommunication wavelengths, providing a new method for the development of topological photonic devices for next-generation terahertz or optical communication.