Pseudo‐Dichroism: Nanophotonic Induction of Chiroptical Activity in Symmetric Molecular Systems

Abstract Chiral nanostructures are widely used to enhance weak molecular chiroptical signals, but their role in generating optical activity where none intrinsically exists is less well understood. Here, pseudo‐dichroism –a helicity‐dependent optical response that arises from near‐field intensity asymmetries in resonant chiral plasmonic structures is reported. Unlike conventional dichroism and distinct from extrinsic chirality–which originates from geometric asymmetries in light–matter interactions– pseudo‐dichroism results from localized differences in near‐field intensity under circularly polarized excitation. This mechanism induces chiroptical signatures in achiral or racemic molecular systems, driven solely by the nanostructure–light interaction. Using mid‐infrared vibrational circular dichroism (VCD) spectroscopy, this phenomenon in cysteine films deposited on “shuriken” nanostructures designed to resonate in the biophysically important amide I region is observed. Numerical simulations confirm that the observed signals originate from differential absorption by localized chiral fields, not from molecular asymmetry. A correction method based on enantiomorphic pairing cleanly separates intrinsic molecular signals from structure‐induced contributions. Beyond refining the interpretation of enhanced chiroptical biophysical measurements, pseudo‐dichroism offers a photonic mechanism to impose and control optical activity in symmetric molecular environments–enabling new design strategies for CPL‐active emitters, enantioselective sensors, and reconfigurable chiral photonic materials without relying on molecular chirality.