Hidden Giant Third‐Order Nonlinear Circular Dichroism in Chiral Two‐Dimensional Halide Perovskite

Chiral nonlinear optical (NLO) responses provide unique opportunities for manipulating circularly polarized light (CPL) and are attracting increasing interest for advanced photonic and optoelectronic technologies. However, investigations of chiral nonlinear optical materials have largely focused on second‐order processes, while the role of chirality in third‐order NLO responses, particularly in third‐harmonic generation (THG), remains largely unexplored. Here, we demonstrate hidden giant THG circular dichroism (THG‐CD) in the chiral two‐dimensional halide perovskite (R‐MBACl) 2 PbI 4 through polarization‐resolved THG measurements, and further examine the THG‐CD in several structurally different Pb‐based two‐dimensional perovskites. Under excitonic resonant conditions, incident right‐ and left‐handed CPL generates nearly orthogonally polarized THG emission, resulting in a reversal of the THG‐CD response between the two orthogonal output polarization components. Consequently, nearly perfect CPL discrimination is achieved under polarization‐resolved detection, with dissymmetry factors exceeding | g THG − CD | = 1.9. In contrast, these chiral NLO responses are largely canceled in conventional polarization‐integrated THG measurements, resulting in apparently weak circular dichroism and masking the existence of giant chiral optical responses. These unique optical responses establish a versatile platform for polarization‐encoded photonic technologies, including circularly polarized light detection, optical information encryption, and anti‐counterfeiting systems operating at technologically relevant telecommunication wavelengths.