Quasi-Bound States in the Continuum of Localized Spoof Surface Plasmons

Localized spoof surface plasmons (LSSPs) have favorable characteristics of subwavelength confinement and nearfield enhancement but suffer from relatively low-quality factors (Q-factors), partly because of radiation loss. In this study, quasi-bound states in the continuum (quasi-BICs) of the LSSPs with high Q-factors are realized on a metasurface with eccentric circular grating as the unit when a plane wave is incident normally. Owing to the broken symmetry, LSSPs TE2,1 and above turn into quasi-BICs from true-BICs and arise with weak energy leakage, which have the advantage of a high Q-factor. The behavior for the Q-factor of a quasi-BIC as a function of the asymmetry parameter is investigated. An experiment was carried out, and the results were in good agreement with the simulation ones. This work greatly reduces the radiation loss of the LSSPs and provides some inspiring applications of the LSSPs, such as remote detection, high-precision spectroscopy, and wireless sensing.