A comprehensive study of plasmonic mode hybridization in gold nanoparticle‐over‐mirror (NPoM) arrays

Hybrid plasmonic systems that combine localized and propagative surface plasmons offer new opportunities for tunable light-matter interactions at the nanoscale. This paper provides the most comprehensive study to date of hybridization between gap localized surface plasmons (gap LSP) and diffraction-mediated propagative surface plasmon polaritons (SPP) in arrays of gold nanodisks over a mirror, part of the larger class of nanoparticle-over-mirror (NPoM) devices. By systematically mapping the hybrid mode dispersion as a function of array geometry over a large parameter space, we extract the coupling strength via a coupled oscillator model and reveal its dependence on key structural parameters, with gap thickness identified as the primary tuning factor. The resulting hybrid modes enhance the optical quality factor by nearly fivefold compared to classical LSP while maintaining strong near-field confinement, combining the advantages of their constituent modes. Dephasing times were measured with interferometric time-resolved photoemission electron microscopy (ITR-PEEM). Using a scalable lithography-compatible NPoM architecture that minimizes the optical index mismatch between the dielectric between the nanodisks and the gap material (Al₂O₃), we achieved the highest coupling strength (123 meV) and dephasing time range (23-50 fs) to date in NPoM arrays.