Resonant Coupling between Propagating and Localized Plasmons for Enhanced Plasmonic Rulers

Plasmonics has transformed nanoscale light-matter interactions, enabling applications in sensing, imaging, and energy conversion through localized surface plasmon resonances (LSPRs) and propagating surface plasmon polaritons (SPPs). The nanoparticle-on-film (NPoF) configuration stands out as a powerful platform for achieving strong plasmonic coupling and field enhancement. Previous research has primarily focused on structural modifications, such as varying spacer layers, to tune the optical response of NPoF systems. In this work, we systematically investigate the optical properties of NPoF structures and demonstrate that resonant coupling between SPPs and NPoF modes enables modulation of the optical response without requiring any structural changes. Excitation of NPoF systems via propagating plasmon polaritons induces pronounced spectral reshaping in the far-field response, which can be controlled by adjusting the incident angle. Experimental results show excellent agreement with numerical simulations and reveal that resonant coupling significantly enhances sensitivity to particle–film spacing. This excitation scheme therefore functions as an improved plasmonic ruler, providing continuous feedback and superior resolution for dynamic distance measurements.