Superchiral field enhancement from chiral bound states in the continuum

Generally, achieving chiral bound states in the continuum (BICs) in achiral metasurfaces or photonic crystals (PCs) requires breaking the in-plane inversion (I) symmetry or up-down mirror (σ_z) symmetry of the structures. However, chiral BICs evolve from vanished BICs, which inherently limits their quality (Q) factors. Here, we reveal that adjusting the thickness of an achiral PC yields chiral BICs near the at-Γ BIC with Q factors up to 10⁵, which is almost two orders of magnitude higher than those from the symmetry breaking of structures. As the PC thickness increases, the Q value of the chiral BIC scales with the corresponding in-plane wavevector k ‖⁻². Upon incident wave illumination, the local optical chirality density exceeds that of circularly polarized light in vacuum by four orders of magnitude, along with a 180-fold enhancement in the local electric field amplitude. The maximum peak of the circular dichroism (CD) spectra approaches 0.9. Our findings offer a pathway to achieving superchiral field enhancement and ultra-high CD responses, possessing significant value in enhancing chiral response, chiral recognition, and sensing.