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...

Dielectric chiral metasurfaces can generate harmonic waves with high efficiency and strong circular dichroism (CD), when they are supported by metallic substrates. Numerical results show that the second-harmonic generation (SHG) efficiency of about 10 −3 % for a peak pump intensity of about 5 GW cm −2 can be achieved in the blue-UV, and the SHG CD reaches up to about 1.8, from a metasurface of Z-shaped lithium niobate nanoantenna array supported by gold substrate. Highly efficient and strong circular dichroic nonlinear responses are attributed to the plasmon-assisted local field enhancement in the dielectric chiral nanoantennae adjacent to the metallic substrate.