Technical Note: Time-Correlated Single Photon Counting

The Principle of Time-Correlated Single Photon Counting
Time-resolved fluorescence spectroscopy is a powerful analysis tool in fundamental physics as well as in the life sciences. Implementing it in the time domain requires recording the time dependent intensity profile of the emitted light upon excitation by a short flash of light, typically a laser pulse. While in principle, one could attempt to record the time decay profile of the signal from a single excitation-emission cycle, there are practical problems preventing such a simple solution in most cases. First of all, the decay to be recorded is very fast. Typical fluorescence from important organic fluorophores lasts only some hundred picoseconds to some tens of nanoseconds. In order to recover not only fluorescence lifetimes but also the decay shape, one must be able to resolve the recorded signal at least to such an extent, that the decay is represented by some tens of samples. For decay of, e.g., 500 ps this means the transient recorder required would have to sample at e.g. 50 ps time steps.