Continuous-wave two-photon excitation of individual CdS nanocrystallites

By use of low-temperature confocal microscopy, continuous-wave two-photon f luorescence images are obtained of individual CdS nanocrystallites embedded in a polymer matrix. The quadratic dependence of the emission rate on the applied laser power proves that the observed fluorescence originates from t he simultaneous absorption of two photons. From the experimental data the t wo-photon absorption cross-section sigma ((2)) could be determined, resulti ng in a value smaller than that known from literature. The work presented i s a first step towards high-resolution fluorescence-excitation spectroscopy on the electronic states in the band edge, inaccessible by conventional on e-photon spectroscopy. (C) 2001 American Institute of Physics.