From Ideal to Real: Chiroptical Consequences of Asymmetries in Plasmonic Nano-Slit Metasurfaces

Achiral plasmonic metasurfaces promise to enhance molecular chiroptical signals, but their performance is often undermined by fabrication imperfections. This study investigates the discrepancy between ideal theory and experimental reality for nanoslit arrays. Using microspectropolarimetry and numerical simulations on nanoslits in a gold film, we observe significant circular extinction (CE) and residual linear extinction (LE′) signals, even after precise alignment. These results contradict the expected null signal from a perfectly symmetric structure, indicating systematic asymmetries. Our numerical analysis models geometric asymmetries like in-plane skews and out-of-plane tilts. We demonstrate that nanometer-scale in-plane asymmetries are the primary source of these far-field artifacts. The imperfections induce linear birefringence and dichroism, creating a large, spurious CE signal orders of magnitude greater than a true molecular signal, masking the desired measurement. Critically, the “superchiral” near-field, which would govern the sensing mechanism, remains remarkably robust against these structural flaws. This highlights a crucial challenge: the far-field measurement is highly sensitive to fabrication errors that do not compromise the near-field sensing capability. Realizing reliable chiral biosensors therefore necessitates exceptional nanofabrication control and advanced polarimetric protocols to distinguish the faint analyte signal from the large, structure-induced background.