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.