19th RQC Seminar
中田 陽介 准教授
ハイブリッド（Zoom ・ 理研 和光事業所 統合支援棟2階 大会議室）
Profound origin of surface plasmon polaritons: Hidden symmetry protection and topology in plasmonics
Surface plasmon polaritons (SPPs), which are localized waves on a metal, play a pivotal role in plasmonics for controlling light in a deep subwavelength scale. Very recently, the discovery of topological insulators stimulated a novel understanding of SPPs from a fresh perspective of topology [1,2]. The possible reasoning for SPP formation could be given by bulk-edge correspondence, which predicts the existence of boundary modes between two regions with different topological integers. However, the underlying physics in the formation of SPPs has still been veiled in mystery because the bulk-edge correspondence has not been rigorously established in plasmonic systems. This study unveils the profound origin of SPPs from the perspective of symmetry protection . First, we identify special modes with zero frequency as the origin of SPPs. Considering these zero modes are protected by hidden symmetry, they are robust against perturbation as long as the symmetry is maintained. Excitation of similar magnetic zero modes is experimentally feasible in a time-varying system. Next, we establish the bulk-edge correspondence, which generally ensures the existence of SPPs on a metal, based on the circuit theory. Finally, the topological difference between metal and dielectric is understood by a minimal circuit model.
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 Y. Nakata, T. Nakanishi, R. Takahashi, F. Miyamaru, and S. Murakami, arXiv:2210.13637 (2022).