Size dependence of the plasmonic near-field measured via single-nanoparticle photoimaging

Plasmonic nanostructures are being exploited for optical and photovoltaic applications, particularly where field enhancement of optical processes is desirable. Extensive work has focused on the optimization of plasmonic near-fields by geometric tuning and interparticle coupling, but the size tunability of near-fields has received less attention. We used single-nanoparticle photochemical imaging to characterize the near-field intensity around a plasmonic nanoparticle as a function of size. The measured near-field intensity increases with nanoparticle size, reaching a maximum at a size of 50 nm, followed by a decrease at larger sizes. An electrodynamic model explains both the measured size dependence and the optimum size for field enhancement. Whereas intrinsic damping is size-independent, the smallest nanoparticles exhibit weak fields due to surface damping of electrons. On the other end, larger nanoparticles show low field enhancement due to strong radiative scattering. The measured volcano trend, however, most closely mirrors the size dependence of electromagnetic retardation. Above 50 nm size, retardation causes damping, but below a size of 50 nm, it surprisingly reduces nonradiative dissipation, a previously unknown effect. The size dependence of plasmonic field intensity described here can guide design of plasmonic nanostructures for applications in spectroscopy, photovoltaics, photocatalysis, and lithography.