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Investigations of scattering and field enhancement effects in retardation-based plasmonic nanoantennas

Modifications in scattering strength of and local field enhancement by retardation-based plasmonic nanoantennas when being transformed from straight nanorods to split-rings are investigated. The scattering properties are monitored by linear reflection and extinction spectroscopy whereas local field enhancement is estimated from measurements on individual nanoantennas by nonlinear scanning optical microscopy in which two-photon-excited photoluminescence (TPL) is detected. The linear and nonlinear optical characterizations reveal, that the optical response of nanoantennas is dominated by constructively interfering short-range surface plasmon polaritons (SR-SPP) and that the transformation of straight nanorods into split-rings by bending significantly influences the scattering strength. Importantly, strong suppression of scattering for the fundamental SR-SPP mode is observed when the bend radius is decreased, a feature that we attribute to the decrease in the nanoantenna electric-dipole response in tact with its bending. The experimental observations are corroborated with numerical simulations using the finite-element method.