Probing single-molecule fluorescence spectral modulation within individual hotspots with subdiffraction-limit image resolution.

The enhancement of the electromagnetic field on the rough metallic nanostructure has been extensively applied to obtain chemical or biological information about molecules with high sensitivity and has received much attention due to its potential applications in new types of devices based on nanoelectronics and nanophotonics. The typical size of the field enhancement area, the so-called hotspot, is approximately 1 order of magnitude smaller than the optical diffraction limit. In the present study, an optical super-resolution microscopic and spectroscopic approach is introduced to explore single-molecule fluorescence within a hotspot where nonhomogeneous spectral modulation is resolved beyond the optical diffraction limit for the first time. Distinct Stokes shifts from individual dyes were directly observed within single hotspots, which were found to be independent of the local electromagnetic field strength. The method reported here provides a robust tool to probe the optical properties of nanoresonantors with high temporal and spatial resolution.