Plasmonic and dielectric metaplatform for chiroptical sensing

Abstract

Detection of enantiomers is essential because chiral molecules with opposing structural handedness exhibit different biochemical properties. Although circular dichroism (CD) spectroscopy identifies chiral enantiomers by measuring their absorption difference under oppositely handed circularly polarized light (CPL) excitation, the CD signal is intrinsically weak. Near-field optical chirality enhancement (OCE) using nanostructures offers a promising route to amplify CD signals. Ideally, nanostructures should exhibit spatially extended OCE distribution of the same handedness, and unobstructed molecular access to regions exhibiting this OCE. This thesis develops plasmonic and dielectric metasystems to meet these requirements. First, a two-arm 3D Au Archimedean spiral with interwire links as structural support producing broadband, spatially extended, single-handed OCE under both CPL and linearly polarized light (LPL) excitation in the visible–near-infrared spectral range is demonstrated. A deterministic fabrication strategy is developed, and experimental results confirm numerically predicted behavior. The cone like geometry works as a sieve, demonstrating the potential to trap nanoparticles coated with chiral analytes within the OCE zone for broadband chiroptical sensing. Second, a low loss, monocrystalline SiO₂–InGaP bilayer metasurface is introduced, consisting of an InGaP cylinder array integrated with a diagonal glass grating. The bilayer metasurface achieves strong single-handed OCE under LPL illumination, with handedness controlled by rotating the polarization angle. Optical characterization of fabricated metasurface agrees with numerical prediction. The exposed OCE regions provide accessible sites for enhanced chiral sensing. Overall, this thesis demonstrates practical plasmonic and dielectric metasystems capable of converting far-field light into strongly enhanced chiral near fields, providing platforms for highly sensitive, low-concentration chiral sensing. Show more