Gold Nanohole Arrays: From Structural Optimization to Biosensing Applications

In this thesis, the optical properties of gold nanohole arrays (NHAs) are extensively investigated from an experimental perspective, focusing on biosensing applications. Gold NHAs are hybrid metallic/dielectric metasurfaces composed of periodically arranged nanoholes in a thick metal film, typically deposited on a standard glass substrate. These structures are strong candidates for biosensing applications resorting to surface plasmon resonance (SPR) technique due to their unique physical properties, allowing the coupling of electromagnetic radiation to both propagating and localized surface plasmons. Additionally, gold NHAs have only recently been explored for studying plasmon-enhanced fluorescence (PEF), which relies on the interaction between the photoluminescence features of a dye and the electric field associated with the plasmonic excitations when a fluorophore is placed near the gold NHA surface. In both cases, achieving optimal sensing performances requires precise tuning of the gold NHAs optical response. Such a coupling can be tailored by engineering the geometrical parameters of the NHAs. Fluorescence enhancement was successfully demonstrated, revealing the coupling mechanism between the dye and surface plasmons through comprehensive optical characterization and finite-difference time-domain (FDTD) simulations. The results showed that enhancement depends on the measurement configuration as well as the refractive index of the surrounding medium. Moreover, we demonstrated the influence of plasmonic mode dispersion on fluorescence emission, offering new insights into optimal dye selection and measurement configurations in relation to the gold NHA optical response. However, current nanofabrication techniques for gold NHAs rely on high-cost methods like electron- or ion-beam lithography, limiting their potential for industrial applications. This problem was addressed by developing an optimization routine within the FDTD method based on particle-swarm optimization to obtain a precise tuning of the gold NHAs optical response by tailoring of the array pitch, nanohole radius, and gold film thickness. A new lithographic technique based on the Talbot effect, known as displacement Talbot lithography (DTL) was used to nanofabricate gold NHA. This technique allowed for successful wafer-scale nanofabrication of gold NHAs with precise control over their geometrical parameters. Furthermore, the versatility of DTL enabled the fabrication of gold NHAs with elliptical nanoholes, which have recently attracted significant interest due to their ability to induce symmetry-breaking effects on the optical response of gold NHAs.