Giant transparent chirality and Kerr optical bistability via polarization-conversion quasi-BICs

High-performance chiral optical devices capable of active all-optical modulation are indispensable for the emerging quantum information processing and spin-encoded photonic logic. However, realizing such devices is constrained by back-scattering noise and the fundamental trade-off between optical chirality and quality [Formula: see text] factors inherent in traditional metasurfaces. Here, we overcome these limitations by demonstrating a rationally engineered silicon metasurface that supports high-Q quasi-bound states in the continuum (q-BIC) featuring near-unity transparent chirality. We simultaneously achieve giant transparent chirality ([Formula: see text] and [Formula: see text] and a high [Formula: see text]-factor ([Formula: see text] by leveraging an efficient polarization-conversion mechanism. Furthermore, we demonstrate low-threshold ([Formula: see text]) and high-modulation-depth ([Formula: see text]) Kerr optical bistability in the telecommunication C-band ([Formula: see text][Formula: see text]nm) based on the high-Q chiral q-BIC resonance. These bistable chiroptical responses enable reconfigurable spin-encoded logic gates and an all-optical spin latch exhibiting near-unity modulation contrast ([Formula: see text]). This work establishes a novel platform for generating high-Q resonances with giant, lossless transparent chirality, offering a new paradigm for low-threshold, high-contrast all-optical quantum photonic devices.