Plasmon-Driven Dimerization Reveals the Propagation Length of Surface Plasmon Polaritons

Surface plasmon polaritons (SPPs) propagate along metal-dielectric interfaces for tens of micrometers, converting light into confined electromagnetic waves. When an SPP encounters a nanoscale impeder, a part of its energy converts to localized surface plasmons (LSPs); this local conversion and energy loss are characterized by its 1/e decay distance, called the propagation length. Albeit powerful, conventional optical methods that probe radiative energy decay provide an incomplete picture of the energy loss by overlooking nonradiative pathways. Here, we introduce a chemical approach to examine the plasmon-driven nonradiative decay, using a dimerization reaction of 4-aminothiophenol on gold as a nanoscale chemical probe of this pathway. Mapping the surface-enhanced Raman spectrum of the dimerized product, 4,4'-dimercaptoazobenzene, reveals propagation lengths of 5.5-12.9 μm that shorten with increased surface roughness. Our findings illustrate how nanoscale morphology affects SPP energy propagation, providing design guidelines by controlling the nonradiative decay of surface plasmons.