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.
In a system of C2 symmetry, symmetry-protected bound states in the continuum (SP-BICs) exist with a continuous spectrum of radiating waves that can carry energy away and enable an infinite radiative quality (Q) factor and zero linewidth. However, the SP-BICs transform into quasi-BICs by breaking the C2 symmetry of the system, where the resonance lifetime and linewidth become finite and measurable. As such, the quasi-BICs are very sensitive to the polarization of incident radiation. Owing to the application of a biosensor or a lasing device, it is unavoidable to work with an unpolarized radiative beam. Herein, we propose a metasurface in a C4 symmetric layout, which exhibits polarization-insensitive terahertz symmetry-protected quasi-BICs. The orientations of adjacent two meta-molecules (MMs) are designed to be orthogonal to each other. By tuning the degree of asymmetry along the orientation of MMs, the quasi-BICs exhibit insensitivity to the polarization of the incident terahertz wave. A large degree of asymmetry results in a deformation of an electric quadrupole, which forms an energy leaky channel to the free space. Due to the translational symmetry, the wave-vector of the lattice in C4 symmetry is conserved so that the electric components of transmitted radiation along the x axis is identical to that along the y axis, Txx = Tyy. As such, the leaky channel of electromagnetic scattering becomes insensitive to the incident polarization. Our results present an approach to achieve polarization-insensitive quasi-BICs in a topologically symmetric metasurface, which is helpful for the innovation of terahertz biosensor.
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.
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.
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