Up-conversion experiment: A pump beam (blue) excites the molecule and the fluorescence is collected by two off-axis parabolic mirrors. The fluorescence (green) corresponding to excitation volume  is imaged  and overlapped with a gate pulse (red) in a b-BBO crystal. For the duration of the gate beam, a sum-frequency signal is generated at the crystal, collected, and  dispersed by a monochromator, and detected by a PMT.  By varying the delay between excitation and gate pulses we can map the temporal behavior of the photoluminescence.