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[[Vol. 47]Photonic Circuits Hosting Electromagnetic Waves with Pseudospin_ WPI-MANA.pdf](https://mdr.nims.go.jp/filesets/583c7e93-a533-4654-9090-c0ddfcda4569/download)

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International Center for Materials Nanoarchitectonics (WPI-MANA)

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[[Research Highlights Vol.47] Photonic Circuits Hosting Electromagnetic Waves with Pseudospin](https://mdr.nims.go.jp/datasets/a198ffcc-d893-4f5f-994c-ad1e8fc74902)

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2022/03/31 16:28 Photonic Circuits Hosting Electromagnetic Waves with Pseudospin| MANAhttps://www.nims.go.jp/mana/research/highlights/vol47.html 1/2Previous  Index  NextResearch Highlights[Vol. 47]Photonic Circuits Hosting Electromagnetic Waves with Pseudospin18 Mar, 2019Figure: Microstrip system capable of hosting electromagnetic modes with orbital angularmomentum.Metamaterials are purposely built devices mimicking structural features of normal materials, butwith unusual physical properties. Photonic crystals, for example, are periodic nanostructuresconsisting of material components with different refractive indices. They have lattice symmetrieslike solids, but the constituents of the unit cell of a photonic crystal are ‘bits’ of the different bulkmaterials. Similar to the structure–property relationships resulting from the behavior of electronsin solids (e.g. semiconduction), photonic crystals offer ways for manipulating the propagation oflight. Now, Xiao Hu at the International Center for Materials Nanoarchitectonics, National Institutefor Materials Science, Tsukuba, Japan, and colleagues have succeeded in creating a photonicmetamaterial that displays a special property known as a topological photonic state.The researchers first considered theoretically a planar construction of microstrips organized in ahoneycomb-like way. Strip segments inside hexagons are put narrower than those betweenhexagons in one half of the device, and vice versa in the other half, because a structure withalternatingly wide and narrow strips results in a so-called photonic band gap: a range offrequencies for which electromagnetic waves cannot naturally exist in the system. The nodes of thehexagonal network are connected to capacitors; the segments linking nodes act as inductors. (Acapacitor, abbreviated ‘C’ in circuit theory, is an electric component capable of storing energy in anelectric field. An inductor, abbreviated ‘L’, is a component that stores energy in a magnetic fieldwhen an electric current flows through it.)The topological LC-circuit proposed by Hu and colleagues has a peculiar property. When excited byan electromagnetic wave with a frequency in the photonic band gap, at the intersection of the twohalves, waves in opposite directions are created. What is remarkable is that these two waves canbe assigned a quantity known as pseudospin, with values ‘up’ and ‘down’, respectively, and theyare immune to backscatter even at sharp corners and robust to defects due to the topologicalprotection.https://www.nims.go.jp/mana/research/highlights/vol46.htmlhttps://www.nims.go.jp/mana/research/highlights/index.htmlhttps://www.nims.go.jp/mana/research/highlights/vol48.html2022/03/31 16:28 Photonic Circuits Hosting Electromagnetic Waves with Pseudospin| MANAhttps://www.nims.go.jp/mana/research/highlights/vol47.html 2/2To demonstrate their theoretical finding experimentally, the scientists fabricated the topological LCcircuit from microstrips — metallic strip lengths were about 1 cm and both halves consisted of 14× 8 hexagons — and, by using microwave near-field techniques, measured the electric-fieldcomponent perpendicular to the sample. The measurements confirmed the existence of the specialtopological state.Being able to generate and manipulate electromagnetic waves with pseudospin is promising forapplications like communications and high-resolution imaging. The concept presented by Hu andcolleagues is not restricted to the microwave range, but also applicable to infrared light. The planargeometry of the device makes it easy to include other circuit elements, such as resonators orsuperconducting Josephson junctions. One difficulty, though, is to channel the pseudospin modesout of the sample. As the scientists point out: “[To] figure out a way to emit efficientlyelectromagnetic modes [with pseudospin] ... supported by the microstrip structure ... into freespace is one of the most intriguing future problems.”Reference“Topological LC-circuits based on microstrips and observation of electromagnetic modes withorbital angular momentum”Yuan Li, Yong Sun, Weiwei Zhu, Zhiwei Guo, Jun Jiang, Toshikaze Kariyado, Hong Chen, and XiaoHuJournal : Nat. Comms. 9, 4598 (2018).DOI : 10.1038/s41467-018-07084-2AffiliationsInternational Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for MaterialsScience (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, JapanContact informationInternational Center for Materials Nanoarchitectonics(WPI-MANA)National Institute for Materials Science1-1 Namiki, Tsukuba, Ibaraki 305-0044 JapanPhone: +81-29-860-4710E-mail: mana-pr[AT]ml.nims.go.jphttps://samurai.nims.go.jp/profiles/kariyado_toshikaze?locale=enhttps://samurai.nims.go.jp/profiles/hu_xiao?locale=enhttps://www.nature.com/articles/s41467-018-07084-2