The fluorescent lifetimes of molecules used in biological applications tend to be in the sub-ns to a few ns timescale (let's say 0.8-4). The most direct methods to measure lifetimes typically involve gating (either explicit or via time correlations).
But this is horribly wasteful as information is discarded from those photons that did not come at a selected time. If you have a delicate sample, you can destroy it before you finish your measurement; if you have a rapidly varying process, the state can have changed before you've finished making the measurement.
What is the state of the art in sustained photon counting in the visible range (perhaps 450-600 nm), and is it at the level where one could reasonably time every photon without having to drop counting rates drastically below the fluorescence lifetime itself? The photophysics admits measurement of lifetime at a MHz rate (~100 photons at 10 ns intervals), but is it practical to actually do this?
Although there are a variety of options available for photon counting (PMTs, APDs, GaAsPs), it has been difficult for me to get an accurate picture of the speeds and limitations of various devices actually available, or of the physical limits to a certain type of detection scheme. For example, do PMTs suffer from blurred readout times due to the distribution of electron path lengths in the cascade? Do thermal effects render APDs useless for sustained high-rate readout even if they are small enough so that the capacitance isn't a major barrier for recharging? (Or is capacitance impossibly bad for GHz rates even with really tiny e.g. 100 um square devices?)