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

Recently, quasi-bound states in the continuum (quasi-BICs) of localized spoof surface plasmons (LSSPs) have been developed on several metasurfaces with broken symmetry, which have high quality factors ($Q$-factors) because of low radiation loss. They are transverse electric (TE) modes. In this paper, the quasi-BICs of LSSPs transverse magnetic (TM) modes are realized on a nonparallel circular grating metasurface with a linearly polarized plane wave. When the circular gratings are placed in parallel, the LSSPs TM modes on the metasurface cannot be excited by the plane wave, because they are true-BICs protected by rotational symmetry; when the circular gratings are mechanically reconfigured to be nonparallel, owing to the asymmetry of the structure, the LSSPs TM modes turn into quasi-BICs from the true-BICs and can be excited. Owing to the strong coupling between the circular gratings in the vertical coupling configuration, the quasi-BICs of the LSSPs TM modes have higher$Q$-factors compared with the quasi-BICs of the LSSPs TE modes. An experiment was conducted and the$Q$-factor of$\text{TM}_{2.1}$was measured to be 305.0. This study provides a new idea for developing LSSPs with low radiation loss. It also extends the LSSPs from two-dimensional to three-dimensional, and brings new potential of transformation.