give rise to a high-Q resonance along with its characteristic asymmetric spectral line shape. [15] Recently, a generalized concept known as bound states in the continuum (BIC) has been proposed to unify metaatoms with broken in-plane inversion symmetry, and the relationship between the structural asymmetry and the Q-factor of resonances was found to be well described by a characteristic inverse-square law. [16] Such symmetry-protected BIC originates from the forbidden coupling between the eigenmodes of resonators and the external propagating modes due to their symmetry mismatch, thus resulting in a localized state embedded in the continuum. Ideally, the BIC exhibits infinite Q-factor in a symmetry-preserved nanostructure so that it is not observable in the spectrum due to its vanishing spectral linewidth. [16] In practice, by introducing the symmetry breaking into the in-plane geometry, the quasi-BIC mode with the finite Q-factor and linewidth can be tailored by adjusting the degree of structural asymmetry, thus providing a useful platform to readily access extremely high Q-factor resonances.Recently, the quasi-BICs with ultrahigh Q-factor have been realized in various photonic systems including waveguides, [17] photonic crystals, [18] plasmonic nanostructures, [19][20][21][22] and dielectric resonators. [23] Among these demonstrations, a variety of metallic and dielectric nanostructures were used in refractive index (RI) sensing applications as the narrower resonant linewidth is beneficial for measuring small resonant shifts (Table S1, Supporting Information). [24] Generally, plasmonic sensors based on the usage of either surface plasmon polaritons or localized surface plasmons benefit from their intense optical field on the surface of metallic nanostructures [25] and thus are capable of performing significant spectral variation when external RI changes. However, the accompanied dissipative loss leads to the broadening of the resonant linewidth simultaneously. In contrary, high-index dielectric metasurfaces have emerged as a promising alternative to exhibit a much narrower resonant linewidth owing to the lack of Ohmic loss. [26,27] Thus, various asymmetric dielectric nanostructures, such as tilted elliptical nanodisk pairs, [28,29] asymmetric paired nanorods, [30][31][32] asymmetric crescent shape, [33] nanodisks with asymmetrically distributed holes, [7,34] nanobricks with symmetric or asymmetric defects, [8,35] have been demonstrated to support high-Q Symmetry-protected quasi-bound states in the continuum (BIC) controlled by metasurfaces with broken in-plane symmetry are widely exploited to achieve highly surface-sensitive and spectrally sharp resonances for nanophotonic biosensors. Through the engineering of silicon-based asymmetric nanobar pairs, a quasi-BIC mode is excited showing a dominant toroidal dipole (TD) and electric quadrupole (EQ) resonant feature in the near-infrared and performs ultrahigh sensitivity in the refractometric monitoring of local environment changes. Contrary to the typical electric ...

No abstract