Dual-band polarization-independent high quality factor Fano resonances using a twisted tetrameric nanohole slab

Abstract

The Fano profile inspired by bound states in the continuum (BICs) has emerged as an effective approach to obtain high quality factor (QF) resonances. However, achieving polarization-independent high QF Fano resonances through the excitation of BICs is still challenging. Herein, we demonstrate that dual-band polarization-independent high QF Fano resonances can be realized by using a twisted tetrameric nanohole slab (TTNS). By twisting the nanoholes of the tetramerized slab, two quasi-BICs can be transformed into dual-band high QF Fano resonances due to the Brillouin zone folding as well as the symmetry breaking from C4v to C2v of the structure. The variation of the twist angle (θ) significantly alters the location of the Fano resonance in the longer wavelength, but it has slight influence on the Fano resonance in the shorter wavelength. A larger QF corresponds to a larger average electric-field enhancement-factor (AEE) for both of the Fano resonances, but the QF of the Fano resonance at the longer wavelength is more robust to the variation of θ due to its larger scaling rule as QF∼|θ|−6. According to multipole decompositions, Fano resonances at the longer and shorter wavelength correspond to toroidal dipole (TD) mode and hybrid TD-magnetic quadrupole (MQ) mode, respectively. In addition, both the two Fano resonances survive even if the structural parameters are significantly altered, and they exhibit polarization-independent features because the rotational symmetry of the structure can be maintained as θ is varied.