Terahertz inner and outer edge modes in a tetramer of strongly coupled spoof localized surface plasmons.

Photonic edge mode confining light in cavities of surface plasmons is beneficial in image and biosensor applications. In the terahertz band, however, the edge mode in a cavity of spoof localized surface plasmons has not matured sufficiently. Herein, a cost-effective strategy to achieve a terahertz photonic edge mode using a metasurface of strongly coupled fourfold spoof localized surface plasmons in a tetramer layout is demonstrated. The quality factors of edge modes decrease when the tetramer shrinks, as revealed by the terahertz dielectric functions. The edge modes that emerge can be categorized as inner and outer edge modes, as deduced from the simulated electric field distribution. Our results show that the edge modes are due to the interaction of spoof localized surface plasmons in the terahertz band.

Tuning symmetry-protected quasi bound state in the continuum using terahertz meta-atoms of rotational and reflectional symmetry.

Conventionally, a symmetry-protected quasi bound state of the continuum (BIC) becomes achievable by breaking the C2 symmetry of meta-atoms. Our work exhibits a novel approach to achieving dual band quasi-BIC by breaking the C2v symmetry into Cs symmetry. Also, we show that a single band quasi-BIC can be achieved by breaking the C2v symmetry into C2 symmetry. Our metasurface of C2v symmetry is composed of double gaps split ring resonator (DSRR), and it degrades to C2 symmetry when the double gaps are displaced in opposite directions. One band quasi-BIC can be observed occurring at around 0.36 and 0.61 THz respectively with the metasurface excited by x- and y-polarized terahertz radiation, respectively. A couple of dark dipole oscillator dominates the quasi-BIC at 0.36 THz, while a quadruple-like oscillator dominates the quasi-BIC at 0.61 THz. The damping ratio and coupling coefficients of the above single quasi-BIC are close to the orthogonal polarization of the incident terahertz wave. However, the metasurface of the DSRR array degrades down to Cs symmetry when the double gaps are displaced in the same directions. A dual band quasi-BIC (0.23 THz and 0.62 THz) is found to be sensitive to the y-polarized terahertz radiation. It is found that the inductive-capacitive (LC) resonance results in quasi-BIC at 0.23 THz, while a quadrupole-like oscillation results in quasi-BIC at 0.62 THz. The quasi-BIC at 0.62 THz has a higher coupling coefficient and lower damping ratio than quasi-BIC at 0.23 THz in a metasurface of Cs symmetry. The realization of the above locally symmetric breaking on the quasi-BIC of terahertz metasurfaces is helpful for the innovation of multi-band terahertz biosensors.