Symmetry-broken dielectric metasurfaces for high- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Q</mml:mi> </mml:math> circular dichroic absorption based on quasibound states in the continuum

High-quality ($Q$) all-dielectric chiral absorbers are crucial for enhancing spin-selective light-matter interactions by enabling differential absorption of left- and right-circularly polarized (LCP and RCP) light. Conventional approaches typically require back reflectors, which cause heat dissipation and reduce out-of-band transparency. We propose an all-dielectric chiral metasurface composed of zigzag-shaped silicon nanoparticles embedded in a uniform silica background. Each nanoparticle, formed by etching two diagonal corners from a square base, supports two bound states in the continuum (BICs) associated with vortex polarization singularities. By breaking the in-plane ${C}_{2}$ symmetry through asymmetric corner widths, these BICs evolve into chiral quasi-BICs (QBICs) that couple selectively to LCP light. The resulting orthogonal modes exhibit opposite parity and are dominated by a toroidal dipole (TD) and an electric quadrupole (EQ), respectively. Although each QBIC contributes less than 50% absorption due to critical coupling, spectral degeneracy between them boosts the total absorption to 90.3% at 910.3 nm, with a $Q$ factor of 824 and circular dichroism of 79.3%. The design is extendable to RCP absorption via corner-length asymmetry. This approach provides a thermally efficient, planar platform for high-$Q$ chiral absorption, offering promising applications in photodetection and chiral optoelectronics.