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 ~175nm(< λ/4) on a topographically smooth surface, which surpasses the apparent λ/2 resolution limit for uncoated tips operating in the linear response regime.
© 1998 Optical Society of America
(110.0180) Imaging systems : Microscopy
(180.5810) Microscopy : Scanning microscopy
(190.4180) Nonlinear optics : Multiphoton processes
(190.7110) Nonlinear optics : Ultrafast nonlinear optics
(260.2510) Physical optics : Fluorescence
Michael K. Lewis, Peter Wolanin, Ari Gafni, and Duncan G. Steel, "Near-field scanning optical microscopy of single molecules by femtosecond two-photon excitation," Opt. Lett. 23, 1111-1113 (1998)