For luminescence measurements with a temporal resolution below 1 picosecond one has to utilize the light itself as a "shutter". The prevalent method for this kind of experiment is the upconversion technique, which generates the sum-frequency of the luminescence and a delayed gate-pulse in a nonlinear crystal. The basic principle is depicted in figure 1. The luminescence of a sample illuminated by an ultrafast laser pulse and a delayed gate pulse are focused into a nonlinear crystal (usually a b-barium borate crystal). Since the mixing process only takes place during the time when the pump pulse is present at the sample, the frequency mixing acts as a light gate and provides a time resolution equivalent to the duration of the gate pulse.
Figure 1: Illustration of the up-conversion principle. Sum frequency radiation is generated in a nonlinear crystal only while the delayed gate pulse is present.
Figure 2: Schematic represenation of the up-conversion setup; Mono ... double grating monochromator, PM ... photo-multiplier, PC ... peltier cooler, AMP ... amplifier, Ct ... counter, Comp ... personal computer for data-acquisition and controll of experimental parameters, S ... sample