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

Abstract

A new phenomenon of dual band symmetry-protected bound state in the continuum (SP-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 (ν1=0.49 THz, ν2=0.79 THz) when the metasurface is in C2v 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 ν3=0.55 THz domiante quasi-BIC I and another magnetic dipole coupled quadrupole dark mode ν4=0.75 THz domiantes quasi-BIC II. At a relatively larger degree of asymmetry, one more dark mode ν5=0.75 THz occurs in the frequency spectra as is a tilted SLSP intrinsic mode. Since the dark mode ν5 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 ν1, and quasi-BIC II is affiliated to the higher frequency bright side-mode ν2. 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.

Large Rabi splitting energy in resonant quasi-BIC WSe2 metasurfaces

“…However, the device structure in their work is relatively complex and faces significant challenges in practical fabrication. Therefore, there is a need for a simple design that can achieve large Rabi splitting values in strong coupling studies [42][43][44][45][46][47][48][49][50][51][52][53].…”

The impact of contact and contactless interactions between the meta-atoms on terahertz bound states in the continuum

“…Therefore, the BIC is invisible in the electromagnetic transmission spectrum. The BIC that exists in a symmetric system is classified as symmetry-protected BIC (SP-BIC) [19][20][21][22][23], which can be observed experimentally while introducing perturbations that break the symmetry of the system termed as quasi-BIC. Normally, the quasi-BIC is Fano-resonance because a discrete localized state becomes coupled to a continuum of states.…”

The impact of light polarization and the nature of modes in the formation of quasi-bound states in the continuum at near-normal incidence

“…Thus, an all-dielectricbased metasurface with high index scatterers is proposed as an alternative to plasmonic metasurfaces. Dielectric structures like split ring resonators [18,19], nanorods, and inverse structures [20] have been reported to support structure-induced optical resonances by exciting various multipolar moments.…”

“…Numerous theoretical and experimental works have been conducted to realize BIC. While symmetry-protected BIC modes have been observed at high symmetric points of the structures like grooved nanodisks [19], nanofins [48], metaatoms [49] and other configurations that permit symmetric modes, whereas the accidental BICs have been observed in nanodisks.…”