High‐QChiroptical Resonances by Quasi‐Bound States in the Continuum in Dielectric Metasurfaces with Simultaneously Broken In‐Plane Inversion and Mirror Symmetries

Abstract

Most of preceding works for quasi-BICs in all-dielectric metasurfaces dealt with in-plane inversion symmetry-broken photonic systems (see, e.g., refs. [8,14]) rather than mirror symmetry-broken ones.Chiral metamaterials [17][18][19] and metasurfaces, [20][21][22][23][24][25][26][27][28][29] which are characterized by broken mirror symmetry, [30][31][32] have found broad photonic applications for circular dichroic nonlinear generations, [33][34][35][36] spinselective wavefront shaping and manipulating, [37][38][39] and biosensing. [40][41][42] Up to date, a number of results for chiral metasurfaces and metamaterials have been reported, while they dealt with wide bandwidth circular dichroism. For photonic applications, circular dichroism accompanying with narrow bandwidth and high quality factor is required, for example, for chiral sensing with high spectral resolution, simultaneous spectral, and spin selective wavefront shaping and secure optical communications, [43] efficient circular dichroic nonlinear generations, and circularly polarized lasing with low threshold. Employing quasi-BIC for mirror symmetry-broken systems might be a potential solution. Recent reports for metasurfaces exhibiting chiral quasi-BIC, however, have proposed to use double layered structure [43] or to introduce out-of-plane perturbations for obtaining high-Q chiroptical responses, [44] requiring complicated fabricating processes.In this work, we numerically show that chiral quasi-BIC with high Q-factor can be obtained by simultaneously breaking the in-plane inversion and mirror symmetries in dielectric metasurfaces. Taking metasurfaces with unit cells containing dielectric meta-atoms of different shapes as examples, we reveal that strong circular dichroism can be obtained for such a simultaneous in-plane symmetry breaking, sustaining high Q-factor and highly enhanced local field. When the in-plane inversion and mirror asymmetries gradually increase, the circular dichroism for the transmission and reflection of the metasurface increases and the Q-factor decreases. Dielectric metasurfaces with mirror symmetry supporting symmetry-protected BIC ubiquitously exhibit chiral quasi-BIC by simultaneously broken in-plane inversion and mirror symmetries. The results presented in this work can find important photonic applications such as chiral biosensing with high spectral resolution, Bound states in the continuum (BIC) have recently attracted great attention in photonics. Metasurfaces with broken inversion symmetry exhibit high-Q resonances via quasi-BIC, enabling low threshold lasing, sensing, and efficient nonlinear generation. Recently, chiral BICs have been investigated by using double layer structure or out-of-plane perturbation, requiring complicated fabrication processes. This work presents chiral quasi-BIC by simultaneously breaking the in-plane inversion and mirror symmetries in dielectric metasurfaces. For such a simultaneous symmetry breaking, the metasurfaces exhibit quasi-BIC with strong chirality, resulting in near-unity circular dich...

All-dielectric bilayer complementary metasurfaces supporting quasi-bound states in the continuum induced by intrinsically broken out-of-plane symmetry

“…Strongly resonant circular dichroism has recently attracted much attention. For this purpose, however, the metasurfaces must have broken mirror symmetry for exhibiting the chirality, being hardly compatible with the in-plane inversion symmetry breaking necessary for generating the quasi-BIC, and thus several specific designs should have been taken [38][39][40] with unavoidable limitations for their performances and complicated fabrication processes. As the above-shown complementary bilayer metasurfaces exhibit high-Q resonances via the quasi-BIC due to the inherently broken out-of-plane symmetry, we do not need further symmetry breaking for the in-plane structure of the unit cells, implying that the complementary bilayer structure plus the arbitrarily broken in-plane mirror symmetry results in a high-Q chiroptical quasi-BIC.…”

“…[14][15][16] The typical applications include lasing, 21,22 nonlinear photonics, [23][24][25][26][27][28][29] imaging and sensing, [30][31][32][33] strongly resonant polarization-independent optical responses [34][35][36][37] and circular dichroism. [38][39][40] The BICs appearing in dielectric metasurfaces can be classified into two main categories: symmetry-protected BICs and accidental ones (see, e.g., ref. 1 and 16).…”

“…14–16 The typical applications include lasing, 21,22 nonlinear photonics, 23–29 imaging and sensing, 30–33 strongly resonant polarization-independent optical responses 34–37 and circular dichroism. 38–40…”

Tailoring accidental double bound states in the continuum in all-dielectric metasurfaces

“…Nonetheless, under perturbation in the parameter space around the BIC condition, it is possible (from the far field) to excite resonances (quasi-BICs) with arbitrary large Q-factors and huge enhancement of the electromagnetic field at the near field, holding promise (both BICs and quasi-BICs) of unprecedented planar devices in Nanophotonics. These interesting properties have been extensively investigated for diverse photonic applications such as enhanced sensing [27,28,29,30,31], filtering [32], lasing [17,33,34,35,36,37,38], electromagneticallyinduced transparency [39], chirality [40,41], and non-linear conversion [42,43,44,45]. Depending on the mechanism that prevents the coupling of BIC with the continuum of radiation, they are classified as symmetry-protected or accidental BICs [18,26,46].…”

Plasmonic Sensing and Switches Enriched by Tailorable Multiple Fano Resonances in Rotational Misalignment Metasurfaces

“…In this sense, optical metasurfaces can be optimized to manipulate the phase, amplitude, and polarization of the wave owing to their ability to locally control geometric parameters [ 5 ]. In comparison to the natural materials, the optical metasurfaces can showcase the enhanced and engineered optical responses by their lack of rotational symmetry or in-plane mirror-symmetry [ 6 , 7 ]. As a consequence, there have been enormous efforts devoted to achieving high-sensitivity optical sensing [ 8 ], metalens [ 9 ], optical switching [ 10 ], etc., in optical metasurfaces.…”