  Fluorescence Lifetime Imaging Microscopy in the time domain is typically
performed by recording the arrival time of photons either by using electronic
time tagging or a gated detector. As such the temporal resolution is limited by
the performance of the electronics to 100's of picoseconds. Here, we
demonstrate a fluorescence lifetime measurement technique based on
photon-bunching statistics with a resolution that is only dependent on the
duration of the reference photon or laser pulse, which can readily reach the
1-0.1 picosecond timescale. A range of fluorescent dyes having lifetimes
spanning from 1.6 to 7 picoseconds have been here measured with only ~1 second
measurement duration. We corroborate the effectiveness of the technique by
measuring the Newtonian viscosity of glycerol/water mixtures by means of a
molecular rotor having over an order of magnitude variability in lifetime, thus
introducing a new method for contact-free nanorheology. Accessing fluorescence
lifetime information at such high temporal resolution opens a doorway for a
wide range of fluorescent markers to be adopted for studying yet unexplored
fast biological processes, as well as fundamental interactions such as lifetime
shortening in resonant plasmonic devices.
