Plasmon-Enhanced Fluorescence Near Single Gold Nanoplates Studied By Scanning Near-Field Two-Photon Excitation Microscopy

Plasmonic optical fields have been applied for surface-enhanced spectroscopy, chemical sensing, and bioimaging. Spatial distributions of optical fields are critical for optimizing their functionalities. In plasmon-enhanced fluorescence, both incoming and outgoing fields excited by the plasmon should contribute to the enhancement of the fluorescence. Spatial characteristics of plasmons are critical not only for the fundamental understanding of the plasmon but also for their practical applications. Here, we investigate the spatial characteristics of the excitation and relaxation processes near the gold nanoplate using time-resolved near-field two-photon microscopy. We reveal from near-field optical microscopy that the incident field is locally enhanced by the plasmon resonance effect and the lightning rod effect. Near-field time-resolved fluorescence imaging demonstrates that the fluorescence decay is accelerated entirely over the surface of the plate regardless of the spatial distribution of the incident field. These results provide deep insight into plasmonic optical fields and are of great importance for designing plasmon-based substrates for surface-enhanced spectroscopy and photochemical reactions.