Second-Harmonic Hyper-Mie Optical Activity Enables Closed-Loop Chiral Photochemistry.

Photochemistry promises sustainable chemical processing but typically depends on ultraviolet light with limited selectivity and penetration. We report nonlinear chiral photochemistry, where femtosecond infrared pulses are frequency-doubled to both drive and track a transformation of chiral CdTe/CdO nanohelices into CdO nanospheroids. Circularly polarized light induces a controlled oxidation sequence monitored in real time through second-harmonic scattering intensity and chiroptical contrast. As the CdO shell fractures and exposes non-centrosymmetric CdTe, second-harmonic intensity rises twenty-fold, polarization reverses, and characteristic CdTe photoluminescence emerges. These findings are enabled by the experimental observation of the second-harmonic hyper-Mie optical activity effect, which completes a suite of nonlinear chiroptical scattering phenomena predicted over 45 years ago. Our results offer a spatially confined, selective, and temporally-resolved method for material transformation.