The lay out of the upconversion set-up is shown above. A first femtosecond pulse excites the dye molecule and sets the time t=0 of the dynamics. The fluorescence emission from the dye is collected and focussed on a non-linear BBO crystal, where it is mixed with a second femtosecond pulse at 800 nm (called the gate pulse) which arrives at a given time delay. The mixing of the fluorescence light and with the gate pulse gives rise to the sum frequency of the two, which can be imaged with a spectrograph and a CCD camera. The delay of the gate with respect to the pump pulse can be continuously tuned which allows to record the temporal evolution of the dye fluorescence.
A special feature in our set-up is the polychromatic detection scheme. At each time-delay the BBO is rotated over the range of phase-matching angles to convert the whole fluorescence spectrum which is recorded with the CCD camera. In a single time-scan we obtain the whole 2D- (spectral and temporal) evolution of the fluorescence spectrum over a spectral range of >200nm with a temporal resolution of <100 fs.
Of special interest is the combined coverage of a very wide spectral range with time-resolution we obtained in the UV spectral range