High Quality Factor Studies Of Moiré Lattices And BIC Metasurfaces In Terahertz Electromagnetic Fields

Metamaterials and metasurfaces can perform various manipulations of terahertz waves,which has been a popular research topic in recent years.Especially in the interdisciplinary field combined with biological sciences,metasurfaces provide a favorable tool for realizing local field enhancement of electromagnetic fields,which will undoubtedly improve the interaction between the sample to be tested and incident terahertz waves and promote the development of terahertz biosensing detection techniques.Based on the high quality(Q)factor of metasurfaces in terahertz electromagnetic fields,this paper mainly includes the following three parts:1.We numerically investigate the terahertz superlattice mode in a moirémetasurfaces containing fringe of squares and hexagonal structures.The edge of moiréfringes works as a route of un-directional surface current driven by the incident terahertz polarization.As such,each lateral edge of moiréfringe works as dipole oscillator,which collectively resonate in-phase leading to the lattice mode.A large twisted angle leads to a relatively high resonance frequency,as well as,a high Q factor.Our finding manifests a new method to tuning the lattice mode by terahertz twistronics.2.Bound states in the continuum(BICs)can be regarded as a state that is not coupled with the outside world,or whose energy does not leak to the outside,ideally with a zero line-width corresponding to an infinite Q factor.By breaking the symmetry of the system,the BIC transforms into a quasi-BIC,making the resonance linewidth finite and measurable.Conventionally,a symmetry-protected quasi BIC becomes achievable by breaking the C₂symmetry of meta-atoms.Our work exhibits a novel approach to achieve dual band quasi-BIC by degenerating C_2v symmetry into C_ssymmetry and maintain single band quasi-BIC by degenerating C_2v symmetry into C₂symmetry.Such a metasurface of C_2v 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 observe 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 above singlequasi-BIC are close to orthogonal polarization of incident terahertz wave.However,the metasurface of DSRR array degrades down to C_s symmetry when the double gaps are displaced in the same directions.A dual band quasi-BIC 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 C_s symmetry.The realization of above locally symmetric breaking on the quasi-BIC of terahertz metasurfaces,is helpful for the innovation of multi-band terahertz biosensors.3.The observation of quasi-BIC is related to the symmetry of the structure and the polarization of the incident radiation.Herein,we propose a metasurface in a C₄symmetric layout,which exhibits polarization insensitive terahertz symmetry protected quasi-BICs.The orientations of adjacent two 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 incident terahertz wave.A large degree of asymmetry results in a deformation of electric quadrupole,which forms an energy leaky channel to the free space.Due to the translational symmetry,the wave-vector of 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 T_xx=T_yy.As such,the leaky channel of electromagnetic scattering becomes insensitive to the incident polarization.Our results manifest an approach to achieve polarization-insensitive quasi-BICs in a topologically symmetric metasurface,which will undoubtedly aid the development of terahertz biosensors.