CD-Enhanced Plasmonic Metasurfaces Based on Achiral and Dimer Gratings for Biosensing Applications

Distinguishing the chirality of biomolecules is essential in biophysics and pharmaceutics, yet their intrinsic circular dichroism (CD) signals are typically weak and lie in the ultraviolet (UV) region, demanding costly instrumentation. To address this limitation, this thesis introduces tunable plasmonic and photonic metasurfaces that dramatically enhance CD signals in the visible–NIR range, where optical components are widely accessible. In the first part of this work, tilted gold nanohole arrays are employed to generate extrinsic chirality in fully achiral structures, enabling strong chiroptical responses through geometric symmetry breaking. By integrating these arrays into a Metal-Dielectric-Metal (MDM) cavity, we create a hybrid plasmonic–photonic resonator that produces intense, asymmetric near fields and enhances the chirality enhancement factor, χ, by an additional order of magnitude. Compared to a simple glass-supported biolayer, commonly used in earlier studies, we demonstrate a 50-fold CD enhancement due to the synergistic interplay between the cavity’s magnetic-dipole mode and the plasmonic nanohole resonance. In the second part of the thesis, we investigate an emerging class of intrinsically chiral architectures based on Nanoparticle–on Mirror (NPoM) cavities. Here, cross-stacked nanoparticle dimer gratings are integrated above a thin-film Au/Al2O3 cavity to create tunable superchiral hotspots through hybrid coupling of Rayleigh anomalies, guided-mode resonances, and localized plasmons. Unlike the extrinsic-chirality nanoholes, the NPoM structure provides inherent three-dimensional chirality and extremely confined optical fields within accessible nanogaps, offering a powerful platform for boosting molecular CD signals. Together, these two metasurface approaches demonstrate that cavity-assisted chiroptical enhancement can substantially improve enantiomer discrimination and biosensing performance, while offering spectral flexibility through geometric and cavity design—significantly surpassing the capabilities of conventional glass- based plasmonic substrates.