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 ...

Strong coupling provides a powerful way to modify the nonlinear optical properties of materials.The coupling strength of the state-of-the-art strongly coupled systems are restricted by weak field confinement of the cavity, which limits the enhancement of the optical nonlinearity. Here, we investigate a strong coupling between Mie resonant modes of high-index dielectric nanocavities and an epsilon-near-zero mode of an ultrathin indium tin oxide film and obtain an anti-crossing splitting of 220 meV. Static nonlinear optical measurements reveal a large enhancement in the intensity-independent effective optical nonlinear coefficients, reaching more than three orders of magnitude at the coupled resonance. In addition, we observe a transient response of ~300 fs for the coupled system. The ultrafast and large optical nonlinear coefficients presented here offer a new route towards strong coupling-assisted high-speed photonics.

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Dynamic toroidal dipole (TD) with its peculiar characteristic of broken space‐inversion and time‐reversal symmetries plays an important role in the fundamental physics of light–matter interaction. Here, TD metamaterials comprised of amorphous silicon nanopillar arrays embedded in spin‐on‐glass layer are experimentally demonstrated. Upon normal incidence of plane wave, the transverse toroidal moment and the associated anapole‐like state are excited in optical regime. The strong TD response stems from a complete head‐to‐tail configuration of the magnetic dipole moments within each individual nanopillar. Both the experimental and simulation results show that such TD mode sustains a large structural tolerance and can be spectrally tuned by elongating the cylindrical axis perpendicular to the light polarization, corresponding to a cross‐sectional variation from circular to elliptical shapes. The excited TD mode is found to exhibit ultrahigh refractive index sensitivity compared to other multipoles, resulting in a sensitivity of 459 nm (470 nm) per external refractive index change in the experiment (calculation). This approach provides a simple and straightforward path in realizing toroidal metamaterials and establishes a new flat‐optics platform for realizing active metadevices, sensors, and nonlinear nanophotonics.

The abundant multipolar resonances in all‐dielectric metasurfaces provide a new paradigm to simultaneously induce strong near‐field confinement in the interior of a nanocavity as well as to manipulate the far‐field scattering property, which is beneficial for the enhancement of nonlinear effects. Here, third‐harmonic generation (THG) of all‐dielectric silicon metasurfaces that sustain dominant electric dipole (ED), toroidal dipole (TD), and magnetic dipole (MD) moments in near‐infrared is numerically and experimentally studied. The effect of the interplay of these resonant modes on THG is investigated, and a pronounced THG enhancement is observed when these modes become spectrally overlapped, corresponding to the generalized Kerker condition. The constructive interference of the total electric dipole (refers to the summation of the ED and TD scattered fields) and MD modes results in the suppression of the backward scattering along with a strong local‐field enhancement inside the dielectric resonators. The simulation (experimental) results show a 214‐fold (17‐fold) THG enhancement in the vicinity of the generalized Kerker condition compared with the signals of the spectrally separated TD and MD resonances. The silicon‐based metasurfaces with their simple geometry are facile for large‐area fabrication and open new possibilities for the optimization of upconversion processes to achieve efficient nonlinear devices.