Giant nonreciprocal chirality in photonic quasi-BIC systems.

Simultaneously achieving giant chirality, high-efficiency nonlinearity, and robust nonreciprocity remains elusive due to the inherent trade-off between the structural asymmetry required for chiroptics and the high quality (Q) factors essential for nonlinearity. Here, we overcome this fundamental limitation in silicon metasurfaces by exploiting symmetry-protected bound states in the continuum (BICs). By implementing a two-stage symmetry-breaking mechanism that combines an in-plane structural rotation with out-of-plane height perturbations, we engineer high-Q (104) resonances dominated by magnetic quadrupole modes. Linearly, the device exhibits robust asymmetric transmission with a forward circular dichroism (CD) of 0.98 and a backward CD of -0.79. Leveraging intense near-field confinement, we realize third- and fifth-harmonic generation efficiencies of 3.6 × 10-3 and 2.9 × 10-9 at a moderate pump intensity of 1 MW/cm2. Crucially, the chiral-nonlinear coupling enables perfect nonlinear nonreciprocity, yielding near-unity nonlinear CD values of ±0.99. This work establishes a compact paradigm for high-performance chiral nonlinear isolators.