Anomalous spectral shift of near- and far-field plasmonic resonances in nano-gaps

TL;DR

This paper introduces a novel optical technique to directly compare near-field and far-field spectral responses of plasmonic nanostructures, revealing significant spectral shifts and providing insights for designing enhanced optical devices.

Contribution

It develops a widely-tuneable method for correlating near- and far-field responses in plasmonic systems, uncovering spectral shifts due to interference effects.

Findings

Near-field resonances are blueshifted by >40meV compared to far-field peaks.

Spectral shifts are explained by interference between gap modes.

The technique enables precise control of optical responses in nanostructures.

Abstract

The near-field and far-field spectral response of plasmonic systems are often assumed to be identical, due to the lack of methods that can directly compare and correlate both responses under similar environmental conditions. We develop a widely-tuneable optical technique to probe the near-field resonances within individual plasmonic nanostructures that can be directly compared to the corresponding far-field response. In tightly-coupled nanoparticle-on-mirror constructs with nanometer-sized gaps we find >40meV blueshifts of the near-field compared to the dark-field scattering peak, which agrees with full electromagnetic simulations. Using a transformation optics approach, we show such shifts arise from the different spectral interference between different gap modes in the near- and far-field. The control and tuning of near-field and far-field responses demonstrated here is of paramount importance in the design of optical nanostructures for field-enhanced spectroscopy, as well as to control near-field activity monitored through the far-field of nano-optical devices.