Two-photon fluorescence imaging and spectroscopy of nanostructured organic materials using a photon scanning tunneling microscope

Photon scanning tunneling microscopy and spectroscopy using femtosecond two-photon excitation are demonstrated. The measurement of both intensity dependence and spectral dependence is performed on a two-photon chromophore. A subdiffraction-limited resolution is obtained, and the domain-size dependence of spatial and spectral features is observed, which indicates the high degree of molecular order in the isolated nanoparticle. It is shown that the light confinement due to a quadratic dependence of the fluorescence intensity leads to an optical contrast enhancement with a coated probe. (C) 2000 American Institute of Physics. [S0003-6951(00)02401-3].