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 (Q) 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 (CD co ≈ 1.00 and CD cross ≈ −1.00) and a high Q-factor (Q ≈ 6.7 × 10 5 ) by leveraging an efficient polarization-conversion mechanism. Furthermore, we demonstrate low-threshold (I on ≈ 1.5 kW/cm 2 ) and high-modulation-depth (ΔT ≈ 80%) Kerr optical bistability in the telecommunication C-band (~1551 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 (𝛥CD co ≈ 0.97). 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.