Near-field scanning optical microscopy of single molecules by femtosecond two-photon excitation

We present a demonstration of near-field scanning optical microscopy of single molecules based on ultrafast two-photon-induced fluorescence. Measurements were performed by use of 100-fs pulses at 800 nm from a Ti:sapphire laser to excite the two-photon transition in Rhodamine B molecules. Although near-field probes are normally metal coated to achieve superresolution, we used uncoated tips to achieve sufficiently high optical powers to generate acceptable fluorescence emission rates. Images of single molecules demonstrate a resolution of $\sim 175 \mathrm{nm} \left(<\lambda /4\right)$ on a topographically smooth surface, which surpasses the apparent $\lambda /2$ resolution limit for uncoated tips operating in the linear response regime.