NONLINEAR SPECTROSCOPY ON A SINGLE QUANTUM SYSTEM - 2-PHOTON ABSORPTION OF A SINGLE-MOLECULE

Two-photon fluorescence excitation spectra of single diphenyloctatetra ene molecules trapped in an n-tetradecane matrix were measured at cryo genic temperatures. The purely electronic zero-phonon line (transition at 444 nanometers) of these single molecules with a width of about 60 megahertz was excited by a continuous-wave, single-mode laser at 888 nanometers. Even though the two-photon absorption cross section is ext remely small, a high photon count rate and low background allowed nonl inear spectroscopy to be extended to the single-molecule level. This e xperiment also suggests the possibility of two-photon single-molecule scanning microscopy.