Realizing symmetry-guaranteed pairs of bound states in the continuum in metasurfaces

Abstract

Bound states in the continuum (BICs) have received significant attention for their ability to enhance light-matter interactions across a wide range of systems, including lasers, sensors, and frequency mixers. However, many applications require degenerate or nearly degenerate high-quality factor (Q) modes, such as spontaneous parametric down conversion, non-linear four-wave mixing, and intra-cavity difference frequency mixing for terahertz generation. Previously, degenerate pairs of bound states in the continuum (BICs) have been created by fine-tuning the structure to engineer the degeneracy, yielding BICs that respond unpredictably to structure imperfections and material variations. Instead, using a group theoretic approach, we present a design paradigm based on six-fold rotational symmetry (C6) for creating degenerate pairs of symmetry-protected BICs, whose frequency splitting and Q-factors can be independently and predictably controlled, yielding a complete design phase space. Using a combination of resonator and lattice deformations in silicon metasurfaces, we experimentally demonstrate the ability to tune mode spacing from 2 nm to 110 nm while simultaneously controlling Q-factor.

Optical bound states in the continuum in periodic structures: mechanisms, effects, and applications

“…The second type is known as the symmetry-protected BIC, which is located at the center of momentum space (normalΓ point) and is guaranteed by the structural symmetry [36,37] . In structures with sufficient high in-plane symmetries, this type of BIC has been extensively investigated [32,34,38–41] . For instance, the structure in Fig.…”

“…Each symmetry-protected BIC corresponds to an irreducible representation of the symmetry group [56] , and thus its possible topological charge will be determined. Especially for the degenerate case, paired symmetry-protected BICs can be supported by structure with C6 symmetry, corresponding to its E2 representation [41] . Here, we listed all the possible topological charges of the symmetry-protected BICs in structures with different rotational symmetries, as is shown in Table 1.…”

Antenna-Based Approach to Fine Control of Supercavity Mode Quality Factor in Metasurfaces

“…Deviation from geometry corresponding to the ideal BIC conditions is usually treated as an adverse property caused by disorder, 18−20 surface roughness, 21 substrate, 22 inherent fabrication mismatch with the mathematical model, and so on. However, purposely introduced defects are important for controlling the radiating part of the quality (Q) factor of the structure.…”

“…Deviation from geometry corresponding to the ideal BIC conditions is usually treated as an adverse property caused by disorder, − surface roughness, substrate, inherent fabrication mismatch with the mathematical model, and so on. However, purposely introduced defects are important for controlling the radiating part of the quality (Q) factor of the structure. − In particular, a design of metasurface composed of elliptical scatterers arranged in a square lattice was found to be useful, since it has relatively high symmetry providing several nonradiating modes. , The symmetry reduction caused by the modification of the ellipse orientations leads to the appearance of supercavity modes inheriting the properties of symmetry-protected BIC, where the angle of rotation governs the value of Q factor.…”

Acoustic bound states in continuum protected by crystalline symmetry

“…A non-Hermitian tight-binding model is also provided, exhibiting the infinitely high quality of such BICs. Note that our mechanism for achieving BICs is based on the design of crystalline symmetries, which can thus be applied to other artificial periodic systems, such as mechanics and circuits, and extended to three-dimensional systems with various representations at high symmetry points in momentum space [42,43]. Moreover, since such BICs have specific symmetry according to the group representation, selective excitation of different BICs can be anticipated, implying applications in multiplex sensing and selective energy harvesting [44,45].…”