New J. Phys. 17, 023032 (2015)

Credit: IOP

The 'afterglow' from the phosphor found in an image intensifier can be usefully employed to dramatically improve the temporal resolution of time-correlated single photon counting microscopy, say scientists from Kings College London in the UK. Usually the temporal resolution of such an imaging system is limited by the camera's exposure time, which hinders the ability to measure photon arrival times precisely. However, Liisa Hirvonen and co-workers report how taking ratios of the intensity of photon events in two subsequent frames allows photon arrival times to be determined with a precision of just 300 ns — far faster than the camera's exposure time of 18 μs. Tests with samples, including living cells, containing ruthenium and iridium fluorescent complexes demonstrated the viability of the approach. Images can be acquired in a few seconds and require an excitation power far smaller than time-gating schemes. The team says that in the future using an image intensifier with a faster phosphor decay time than the P20 phosphor used in their initial tests could potentially make the measurement of photon arrival times on the nanosecond scale possible. Applications that may benefit from the approach include biological imaging, LIDAR and ion velocity mapping.