Plasmonic Nanoparticle Lattices for Molecular Chirality Transduction and Index-Mismatch- Tolerant Surface Lattice Resonances

Abstract

Periodic plasmonic nanoparticle lattices are useful platforms for controlling light–matter interactions through particle geometry, lattice periodicity, and surrounding refractive-index conditions. Surface lattice resonances （SLRs）, formed by coupling between localized plasmon modes and lattice diffraction, provide spectrally narrow far-field optical responses. However, it remains challenging to design lattices that combine strong local molecule–field interactions with coherent lattice resonances under practical measurement conditions. In this thesis, high-quality Au nanocube cluster lattices were investigated as a hierarchical nanogap–lattice platform. Each cluster acts as a lattice unit cell containing molecule-accessible intracluster nanogaps, while the periodic cluster arrangement supports a narrow SLR. This structure connects local molecular interactions in plasmonic nanogaps with a collective optical readout mode. Molecular chirality transduction was examined by incorporating chiral molecules during cluster lattice formation. After incorporation, the extinction spectra changed only slightly, whereas a clear circular dichroism （CD） response appeared near the SLR. The CD signal increased with molecular concentration and reversed sign for opposite molecular handedness, confirming chirality transduction into the lattice-mediated optical mode. Numerical simulations indicate that this response is related to homochiral optical helicity in the nanogaps and lattice-enhanced dissipation across the cluster volume. Metal- film-assisted substrates were also explored to extend SLR operation to index-mismatched aqueous conditions. FDTD simulations and initial optical measurements suggest that a metal underlayer can retain SLR-like spectral features under water-index conditions, although further optimization is required. Overall, this thesis demonstrates a plasmonic lattice design that connects molecule-accessible nanogaps with coherent lattice resonances, enabling molecular chirality transduction and suggesting a route toward index-mismatch-tolerant SLR-based sensing platforms.