Dual band symmetry-protected terahertz bound states in the continuum inside the spoof localized surface plasmon induced-transparency windows

A new phenomenon of dual band symmetry-protected bound state in the continuum (BIC) is revealed inside the plasmon-induced transparency (PIT) windows. A metasurface of circular periodically grooved cavity integrated with a complimentary symmetric double-gap split-ring resonator (DSRR) is employed. Intrinsic spoof localized surface plasmon (SLSP) interferes destructively with dipole oscillation of DSRR. A PIT phenomenon is generated between the two bright side-modes (ν 1 = 0.49 THz, ν 2 = 0.79 THz) when the metasurface is in C 2v symmetry. The displacement of upper-gap (while keeping the lower gap fixed) of DSRR results in three dark modes inside the frequency range of induced transparency windows, two of which are quasi-BIC. At a relatively low degree of asymmetry, one anapole dark mode ν 3 = 0.55 THz dominate quasi-BIC I and another magnetic dipole coupled quadrupole dark mode ν 4 = 0.75 THz dominates quasi-BIC II. At a relatively larger degree of asymmetry, one more dark mode ν 5 = 0.75 THz occurs in the frequency spectra as is a tilted SLSP intrinsic mode. Since the dark mode ν 5 is not sensitive to the asymmetric displacement of DRSS. A coupled five oscillators’ model reveal that coupling strength with free space and the damping ratios are attributed to the asymmetry of the structure. The leaky channels of both BICs have a much lower damping ratio than the bright side-mode of PIT. The coupling coefficients indicate that quasi-BIC I is affiliated to the lower frequency bright side-mode ν 1, and quasi-BIC II is affiliated to the higher frequency bright side-mode ν 2. The measured Q factors fit well with the relation function of geometric asymmetry, among which the maximum Q factor measured of the quasi-BIC-II exceeds 20. The realization of above results paves a new way to achieve dual band terahertz quasi-BIC by tuning SLSP-induced transparency window. This provides a feasible solution for the design of multi-band terahertz thin-film sensors.