In order to get the highest possible time resolution for fluorescence detection, it is common to use indirect nonlinear optical detections principles. In our approach, we use the principle of fluorescence upconversion shown schematically below.

The fluorescence of the sample is not detected directly, but instead sent to an appropriate nonlinear optical crystal. In this crystal, it is matched to a time delayed gate pulse (which we derive from the regeneratively amplified femtosecond laser system) in order to generate sum frequency light of both. This light, in turn, is then detected using a combination of a photon counting photomultiplier, boxcar averager and lock-in amplifier.

Under optimized conditions detailed here (PDF, 189k), we can achieve a temporal resolution of ca. 250 fs while still maintaining very high sensitivity.