Femtosecond (Fs) fluorescence up-conversion experiments are performed in order to investigate ultrafast phenomena occurring upon excitation. This method is based on cross-correlation between the fluorescence and a probe laser pulse acting as “gate”. While the sample is excited by an ultrafast laser pulse, the collected fluorescence and the probe laser pulse are routed and focused in an NLO crystal. Sum frequency photons are generated only during the “gating” time when the probe laser pulse is present in the crystal.​

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The setup uses an amplified femtosecond double OPA laser system consisting (Mai Tai SP, Spitfire Pro XP, Topas-C, Spectra Physics) to excite the sample. The fluorescence light emitted from the sample is efficiently collected using an off-axis parabolic mirror, filtered for suppressing the scattered light, focused and overlapped with a gate pulse (800 nm, ca. 10 µJ) derived from the regenerative amplifier to an LBO crystal. By tuning the incident angle of these two beams relative to the crystal plane, the sum frequency from the fluorescence light and the gate pulse is generated. The time resolved decay traces and time resolved fluorescence spectra are then recorded by detecting this sum frequency light while changing the relative delay of the gate pulse versus the sample excitation time. The prompt response of this arrangement (including laser sources) is about 90 fs (FWHM).