Time-correlated single photon counting (TCSPC) is a common technique to measure fluorescence decays in the time domain. In principle, single photon events are detected and their time of arrival is correlated to the laser pulse, which was used for excitation of the sample. By using a pulsed laser with a high repetition rate, this process can be repeated many times so that a photon distribution over the time and the spatial coordinates is built up. Fig. 1 explains the operation principle in more detail.
Figure 1: Operation principle of time-correlated single photon counting (TCSPC) measurements. The sample is excited by a pulsed laser source with a high repetition rate. Photons emitted by the sample are detected with a high-gain photomiltiplier and the time with respect to the excitation pulse is measured. By counting many events a histogram of the photon distribution over time is built up. (Figure reproduced from: Becker, The bh TCSPC Handbook, 6th ed., 2014, p. 69 - Link)