High-<i>Q</i> resonances governed by the quasi-bound states in the continuum in all-dielectric metasurfaces

Abstract

The realization of high-Q resonances in a silicon metasurface with various broken-symmetry blocks is reported. Theoretical analysis reveals that the sharp resonances in the metasurfaces originate from symmetry-protected bound in the continuum (BIC) and the magnetic dipole dominates these peculiar states. A smaller size of the defect in the broken-symmetry block gives rise to the resonance with a larger Q factor. Importantly, this relationship can be tuned by changing the structural parameter, resulting from the modulation of the topological configuration of BICs. Consequently, a Q factor of more than 3,000 can be easily achieved by optimizing dimensions of the nanostructure. At this sharp resonance, the intensity of the third harmonic generation signal in the patterned structure can be 368 times larger than that of the flat silicon film. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which may promote the development of the potential meta-devices for nonlinearity, lasing action, and sensing.

Efficient Second‐Harmonic Generation from Silicon Slotted Nanocubes with Bound States in the Continuum

“…[ 37–40 ] A periodic array of nanocubes without air‐slots can achieve the symmetry‐protected BIC. [ 41 ] This mode can stably exist above the lightline with infinite Q factor, as verified by the band structure calculation shown in the Supporting Information. Once the broken symmetry is introduced, the mode turns into a leaky mode with high Q factor.…”

“…Like the perturbation of the symmetry-protected BIC, [39], [40] the dimension of the air-slot would determine the Q factor of the BIC, which consequently modifies the enhanced electric field over the sidewall surface of the air-slot as well as the SHG process. In Figure 2d, we plot the enhancement factors of the electric field and the SHG as functions of the air-slot width a with respect to the values obtained with a=0 nm, i.e., nanocubes without air-slots.…”

All-dielectric χ⁽²⁾ metasurfaces: recent progress

“…Mie resonators are highly multi-mode open cavities. Their eigenmodes, being non strictly orthogonal within the resonator volume 120 , can either interfere giving rise to Fano resonances 13,15 or strongly couple to each other, originating Friedrich-Wintgen BICs 21,22 . Exploiting this approach, Koshelev et al 68 reported an experimental record quality factor in an isolated AlGaAs resonator with a maximum SHG efficiency .…”

Cylindrical vector beams reveal radiationless anapole condition in a resonant state

“…21−24 for details]. In contrast to the embedded eigenstates or bound state in the continuum which cannot be accessed by the excitation in the continuum 25 , the so-called electromagnetic anapole condition provides a nonscattering condition sustained under the excitation of external field 5,26 , which resembles a promising physical mechanism for tailoring light-matter interaction in a nonscattering manner [21][22][23][24]27 . It is generally perceived that such an anapole condition requires that all of the in-duced electromagnetic multipole moments are suppressed at the same wavelength, resembling the canonical way of realizing the radiationless anapole condition 4−20 .…”