  A simple yet effective achiral platform using elliptical nanoholes for
chiroptical analysis is demonstrated. Under linearly polarized excitation, an
elliptical nanohole in a thin gold film can generate a localized chiral optical
field for chiroptical analysis and simultaneously serve as a near-field optical
trap to capture dielectric and plasmonic nanospheres. In particular, the
trapping potential is enantioselective for dielectric nanospheres, i.e., the
hole traps or repels the dielectric nanoparticles depending on the sample
chirality. For plasmonic nanospheres, the trapping potential well is much
deeper than that for dielectric particles, rendering the enantioselectivity
less pronounced. This platform is suitable for chiral analysis with
nanoparticle-based solid-state extraction and pre-concentration. Compared to
plasmonic chiroptical sensing using chiral structures or circularly polarized
light, elliptical nanoholes are a simple and effective platform, which is
expected to have a relatively low background because chiroptical noise from the
structure or chiral species outside the nanohole is minimized. The usage of
linearly polarized excitation also makes the platform easily compatible with a
commercial optical microscope.
