A physical interpretation of coupling chiral metaatoms

The physical origins of chiroptical responses from artificial optically active media are significant for developing high-performance circular dichroism (CD) spectroscopic techniques. Here, we present a biorthogonal approach based on temporal coupled-mode theory to unravel the underlying physics of chiral metasurfaces. Equipped with physically meaningful parameters, this approach inherits the intrinsic properties of open optical cavities, including time-reversal symmetry and non-Hermitian Hamiltonians, which are found to be in excellent agreement with numerical results. Remarkably, it identifies that the intrinsic chirality of coupled chiral nanocavities arises from (i) the asymmetric coupling between interlayer cross-polarized resonant modes and (ii) a coherent interference between doubly degenerate states. Based on this formalism, a critical coupling condition capable of achieving zero transmission for circularly polarized light is proposed.