Time-resolved fluorescence is a direct measure for excited states lifetimes, decay channels and corresponding
rates. Hitherto, investigations on systems exhibiting fluorescence lifetimes below approximately 10 ps have
been restricted to ensemble measurement. Ensemble measurements bear the disadvantage of averaging sample
inhomogeneities and complex distributions. However, the latter problem can be circumvented by single-molecule
experiments, without the restriction to special, typically simple systems that can be prepare with very high
homogeneity. Time-resolved single-molecule microscopy is especially powerful as it allows one to probe the
spatial, temporal and spectral inhomogeneities. At present, its most common implementation, the scanning
confocal time correlated single photon counting (TCSPC), is limited to a time resolution of 20 ps. In the
wide-field epifluorescence microscopy temporal resolution is achieved by the use of intensified CCD cameras, the
fastest of which reach resolution of 80 ps. Here we present a Kerr-gated microscope setup capable of collecting
diffraction limited 2D fluorescence images with approximately 100 fs time resolution. The concept is based on the
insertion of an optical Kerr gate into a standard wide-field microscope. In addition to the considerably improved
temporal resolution, the wide-field design will allow simultaneous tracking of several molecules or nanoparticles
and ultrafast fluorescence lifetime imaging of doped and heterogeneous surfaces. Preliminary measurements to
demonstrate the performance of the setup are presented.
Pump-probe experiments show that electron injection from a dye into mesoporous TiO2 is as fast as 1×1013 s-1. However,
the same materials exhibit residual dye emission with lifetimes in the long nanosecond range. This inhomogeneity of e-
injection rates was addressed in fluorescence lifetime microscopy experiments. The residual emission of continuous
films of TiO2 was compared with that of individual anatase nanoparticles that had undergone extensive dialysis. The
films produce intense emission with multiexponential decay. The mesoporous film contains physisorbed and trapped
dye, which is the dominant source of the emission. The distribution of emission lifetimes may reflect the mean free paths
experienced by the dye molecules diffusing within the porous TiO2. The intensity of emission from individual
nanoparticles from which the loose dye was removed is orders of magnitude lower. The lifetimes are much shorter, with
the primary components on subnanosecond time scale. The presence of residual emission with a ~200 ps lifetime shows
that even on dialyzed nanoparticles a fraction of dye does not inject electrons with the same rate as observed in ultrafast
pump-probe experiments. It is likely that the residual emission originates from the dye bound to defects.
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