Molecular fluorescence in the vicinity of a nanoscopic probe

Abstract

The dramatic modifications of molecular fluorescence in the proximity of a sharp nanoscopic probe is investigated by an apertureless or antenna-based near-field scanning optical microscope, which exploits the interactions between a fluorescent sample and a laser illuminated Si atomic force microscope probe. Specifically, luminescence is monitored from evanescently excited, dye-doped polystyrene nanospheres ${\mathrm{(R}}_S\text{=20–80}$ nm) on a fused silica prism surface as a function of probe-sample geometry. The incident laser field is enhanced in the near-field of the probe tip, resulting in images with high sensitivity ${\mathrm{(\sigma }}_{\min }\text{≈2\hspace{0.5em}}{\mathrm{\AA }}^{\mathrm{2}}$ in a 1 Hz detection bandwidth) and strongly subdiffraction-limited spatial resolution. At probe-sample distances greater than ≈λ/2, the images are dominated by far-field interference between (i) direct fluorescence from the molecular sample and (ii) indirect fluorescence from image dipoles induced in the atomic force microscope probe. Near-field “shadowing” of the molecular fluorescence by the probe also occurs and is studied as a function of probe-sample-detector geometry. Finally, effects of probe-sample proximity on the fluorescence emission spectrum are investigated. In summary, the data elucidate several novel near- and far-field molecular fluorescence enhancement effects relevant to further development of molecular and nanostructural spectroscopic methods with spatial resolution well below the diffraction limit.