NEAR-FIELD SCANNING OPTICAL MICROSCOPY OF SINGLE MOLECULES BY FEMTOSECOND 2-PHOTON EXCITATION

We present a demonstration of near-field scanning optical microscopy o f 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 m olecules. Although near-field probes are normally metal coated to achi eve superresolution, we used uncoated tips to achieve sufficiently hig h optical powers to generate acceptable fluorescence emission rates. I mages of single molecules demonstrate a resolution of similar to 175 n m (< lambda/4) on a topographically smooth surface, which surpasses th e apparent lambda/2 resolution limit for uncoated tips operating in th e linear response regime. (C) 1998 Optical Society of America.