Ultrasensitive Refractive Index Sensing Based on the Quasi‐Bound States in the Continuum of All‐Dielectric Metasurfaces

Abstract

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 ...

Large-area silicon photonic crystal supporting bound states in the continuum and optical sensing formed by nanoimprint lithography

“…In order to obtain high Q quasi-BIC resonance in symmetry-protected conditions, asymmetry is introduced through various structure truncation, distortion, and rotation, such as nanobars, nano ellipsoids, and crescents. 27–29 They can only be obtained by advanced nanofabrication techniques, such as electron beam lithography with a small device area. Although an ultrahigh Q factor over a few thousand can be obtained in theoretical quasi-BIC dielectric resonators, a slight geometrical broadening effect introduced in electron beam lithography (EBL) can result in a typical experimental Q factor slightly greater than 100.…”

Recent advances in strongly resonant and gradient all-dielectric metasurfaces

“…In another recent experimental study of an asymmetric Si nanopillar qBIC metasurface bulk sensitivity and FoM values of 608 nm RIU −1 and 46 were demonstrated. 94 Finally, Chen et al 95 proposed a qBIC metasurface composed of chiral TiO 2 nanoposts patterned on a gold film, exploiting an A-BIC stemming from the destructive interference of a narrowband toroidal dipole mode and a surface lattice resonance. Simultaneous refractometric and chiral sensing was demonstrated, with S = 80.6 nm RIU −1 (FoM = 80.6) and a 59-fold enhancement of the circular dichroism signal, which enables integrated molar chiral sensing for enantiomer-specific analysis.…”

Recent Development in Metasurfaces: A Focus on Sensing Applications

“…To generate exceptionally surface-sensitive and spectrally sharp resonances for nanophotonic biosensors, symmetry-protected quasi-bound states in the continuum (BIC) driven by MSs with broken in-plane equilibrium are commonly employed [ 139 ]. Through the creation of Si-based asymmetric nanobar sets, a quasi-BIC mode with a dominating toroidal dipole (TD) and electric quadrupole (EQ) resonant characteristic in the NIR is stimulated, and this mode demonstrates ultrahigh sensitivity in the refractometric tracking of the local changing environment.…”

“…This is in contrast to the usual E and H Mie-type dielectric resonator resonances, which have the enhanced field mostly within the resonator volume. The experimentally determined (simulated) sensitivity and FOM for nanobar sets with a height of 450 nm are 608 nm/R.I.U and 46 (612 nm/R.I.U and 85), respectively [ 139 ]. The CMOS-compatible approach may be utilized to develop an ultrahigh-sensitive all-dielectric platform suitable for on-chip integration and sensing device reduction in a wide range of medical diagnostics.…”