Abstract
Fast frame rate complementary metal–oxide–semiconductor cameras in combination with photon counting image intensifiers can be used for microsecond resolution wide-field fluorescence lifetime imaging with single photon sensitivity, but the time resolution is limited by the camera exposure time. We show here how the image intensifier's P20 phosphor afterglow can be exploited for accurate timing of photon arrival well below the camera exposure time. By taking ratios of the intensity of the photon events in two subsequent frames, photon arrival times were determined with 300 ns precision with 18.5 μs frame exposure time (54 kHz camera frame rate). Decays of ruthenium and iridium-containing compounds with around 1 μs lifetimes were mapped with this technique, including in living HeLa cells, using excitation powers below 0.5 μW. Details of the implementation to calculate the arrival time from the photon event intensity ratio are discussed, and we speculate that by using an image intensifier with a faster phosphor decay to match a higher camera frame rate, photon arrival time measurements on the nanosecond time scale could be possible.
Highlights
Single photon detection and timing capabilities are important in a number of fields such as fluorescence spectroscopy and microscopy, lidar, optical tomography and quantum cryptography, as has been reviewed recently [1,2,3,4]
Single photon events The single photon events on the phosphor screen of the image intensifier recorded with the camera vary in size and brightness, but are approximately round [56] and stand out from the flat, nearly zero background, as shown in figures 4(a) and (b)
As the phosphor decay varies with gain voltage but not with the number of electrons creating the photon event [46], variations in the phosphor decay function with photon event intensity are negligible
Summary
Single photon detection and timing capabilities are important in a number of fields such as fluorescence spectroscopy and microscopy, lidar, optical tomography and quantum cryptography, as has been reviewed recently [1,2,3,4]. Time-correlated single photon counting (TCSPC), in particular, is a precise, reliable and mature technique to time photon arrival. Unlike the pixels in charge-coupled device (CCD) cameras, CMOS pixels have their own amplification, digitization and read-out circuitry. Direct detection of individual photons is not possible with these particular cameras. In this context, we note CMOS single photon avalanche diode (SPAD) array image sensors have been developed, incorporating picosecond timing circuitry in each pixel or chip [7,8,9]
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