Electric Field-Assisted Immobilization of Anisotropic Nanoparticles to Excite Surface Plasmon Polaritons

This project introduces a novel approach to excite surface plasmon polaritons (SPP) in plasmonic thin films through electric field-assisted vertical immobilization of elongated colloidal plasmonic nanoparticles (NPs) in a Nanoparticle-on-Mirror (NPoM) configuration. The phenomenon, termed Nanoparticle-Induced Surface Plasmon Resonance (NPI-SPR), overcomes key limitations of conventional NPoM systems by exposing the strong field enhancement region across the plasmonic film surface. Unlike traditional setups that confine intense electromagnetic fields to inaccessible sub-nanometric gaps. Thus, this NPI-SPR approach presents significant potential to enhance sensitivity and usability for a wide range of sensing applications. Building on our recent report on the experimental validation of NPI-SPR using spherical NPs ?(Mendes et al., 2024)?, this work leverages the superior polarizability and dipolar efficiency of elongated nanoparticles, such as nanorods (NRs) and nanobipyramids (BPs). The project introduces a disruptive method for achieving vertical alignment of these NPs, critical for their role as vertically oriented dipoles to excite NPI-SPR. Encouraged by the demonstration of dynamic electric field alignment with metal–organic framework microrods and gold nanorods ?(Chandra et al., 2020; Cheng et al., 2019; Fukagawa et al., 2020; Ruda & Shik, 2010)?, we apply the same principles to vertical oriented immobilization of NRs and BPs. Namely, by applying a uniform time-varying vertical electric field and leveraging increased surface charge density at the tips of elongated NPs, we induce torque for alignment. This method will be adapted by the introduction of a field offset towards the substrate to promote immobilization. The electric-field assisted immobilization will be combined with modifications to the NRs, or BPs, such as surface side-blocking approaches using silica shells, and specific tip-oriented thiol functionalization ?(Botequim et al., 2020; Kim et al., 2024)?. Through the combination of these three approaches, we impose a controlled oriented immobilization and consequently its operation as a dipole oriented perpendicularly to the film This approach addresses challenges posed by conventional deposition techniques, resulting in horizontal NP alignment inhibiting NPI-SPR excitation. To put this into context, current NPoM systems focus on extreme sub-nanometric light confinement, limiting their sensing applications due to inaccessible intense electromagnetic fields ?(Barreda et al., 2021; Lawless et al., 2023)?. Other authors view such NPoM systems through the lens of metal-insulator-metal (MIM) waveguides ?(Tserkezis et al., 2015)?. Recent work by Compaijen et al. ?(Compaijen et al., 2016)? proposed treating NPs as point dipoles, which can excite SPP when placed nearby a plasmonic material, opening new avenues for sensing applications. Inspired by this, our experimental validation of the NPI-SPR configuration was accomplished using an optical fiber to excite at glancing angles the plasmonic dipolar resonances of gold nanospheres over a gold thin film, using the same fiber to collect the antenna-like emission profile into its core ?(Mendes et al., 2024)?. This method demonstrated significant sensitivity enhancements, including a near doubling in bulk refractive index sensitivity (RIS) and a twenty-fold improvement as a biosensor for the detection of Thrombin when compared to traditional SPR scheme. However, the work was limited to the usage of spherical NPs, and their poor polarizability efficiency when compared to elongated NPs. The efficiency of the dipole is a core parameter as it will extensively determine the strength and figure-of-merit of the NPI-SPR. Also, to date the theoretical framework, developed in the context of MIM waveguides is limited, not allowing the complete study of NPI-SPR system parameters. This project aims to overcome the current theoretical framework limitations in the MIM NP-film waveguides, expanding it to understand the role of NP geometry, orientation, film thickness, and NP-film distance and their relation to optical response, near-field and the directional emission in the far-field regime. Experimentally, we plan the development of a disruptive electric-field driven oriented NP immobilization to enable NPI-SPR operation. By bridging fundamental research with practical implementation, this work will establish NPI-SPR as a superior alternative to current NPoM-based plasmonic sensors. The outcomes are expected to have a significant technological impact, fostering advancements in high-performance plasmonic sensing devices capable of addressing critical challenges in biosensing and beyond.