A time-gated large-array SPAD camera for picosecond resolution real-time FLIM

Single-photon avalanche diode (SPAD) arrays have recently emerged as promising detectors for many wide-field fluorescence lifetime imaging microscopy (FLIM) applications, thanks to their picosecond range temporal resolution, single-photon sensitivity and fast parallel readout. However, ultra-fast SPAD cameras with very large number of pixels (> 100,000) equipped with time-correlated single-photon counting (TCSPC) capabilities for each pixel have not been developed to date, due to the difficulty with implementing compact in-pixel ps-timestamping. Here, we report a time-resolved image sensor for real-time FLIM comprising the largest SPAD array reported so far. Thanks to its in-pixel time-gated architecture, this 512×512 pixels sensor can perform time-resolved photon-counting at a maximum of 97 kfps. The state-of-the-art photon detection probability and dark count rate of the SPAD allow the camera to collect enough photons for lifetime extraction in low light conditions at video rate. To reach the timing performance obtained by conventional time-to-digital converters (TDC) architectures, we used a time-gated technique based on large (5 ns) time-gates shifted by picosecond delays. Using a finite number of overlapping gates, fluorescence lifetime can be accurately recovered. This architecture and the corresponding FLIM data is ideally suited for phasor analysis, a powerful and computationally simple method to extract fluorescence lifetime information, which allows real-time monitoring of FLIM information provided by such large imagers.