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Photonic topology, which is an extension of its counterpart in electron systems [1], triggers significant interest since it is relatively easier to be realized and the topological edge/interface optic transport immune to back scattering can yield novel optic properties and functionality [2, 3]. Recently, we have designed a topological LC circuit [4]. In the planar circuit, capacitors are put on the sites of honeycomb lattice, which shunt the circuit to a common ground, whereas inductors are put over the links between the sites. We clarify that grouping sites into hexagons and making the inductances on the links inside hexagons larger than those between hexagons, a texture respecting the C6v symmetry, induces a topological state. The nontrivial topology characterized by mirror winding numbers has its roots in the Dirac dispersion formed by the honeycomb structure, and is induced by the inductance texture which opens a photonic band gap accompanied by a band inversion between p-like and d-like photonic modes. We have realized the idea experimentally in microstrips, a typical transmission line, and measured accurately the unidirectional microwave transportation governed by the orbital angular momentum (OAM) hosted in hexagonal unit cells [4]. The present approach provides a novel way to generate optic modes with OAM without breaking time-reversal symmetry and space-inversion symmetry. This work is based on the collaboration with Hong CHEN, Tongji University. XH is supported by KAKENHI (17H02913, JSPS) and CREST (JPMJCR18T4, JST).
References:
[1] H.-M. Weng, R. Yu, X. Hu, X. Dai and Z. Fang: Adv. Phys. vol. 64, 227 (2015).
[2] L.-H. Wu and X. Hu: Phys. Rev. Lett. vol. 114, 223901 (2015).
[3] Y.-T. Yang, Y.-F. Xu, T. Xu, H.-X. Wang, J.-H. Jiang, X. Hu and Z.-H. Hang: Phys. Rev. Lett. vol. 120, 217401 (2018)
[4] Y. Li, Y. Sun, W.-W. Zhu, Z.-W. Guo, J. Jiang, T. Kariyado, H. Chen and X. Hu: Nat. Commun. vol. 9, 4598 (2018).
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