Better FLIM and FCS data by GaAsP hybrid detectors

The principle of the hybrid PMT is known for about 15 years: Photoelectrons emitted by a photocathode are accelerated by a strong electrical field, and directly injected into an avalanche diode chip. Until recently, the gain of hybrid PMTs was too low for picosecond-resolution photon counting. Now devices are available that reach a total gain of a few 100,000, enough to detect single photons at ps resolution. Compared with conventional PMTs, multi-channel PMTs, and SPADs (single-photon avalanche photodiodes) hybrid PMTs have a number of advantages: With a modern GaAsP cathode the detection quantum efficiency reaches the efficiency of a SPAD. However, the active area is on the order of 5 mm2, compared to 2.5 10-3 mm2 for a SPAD. A hybrid PMT can therefore be used for non-descanned detection in a multiphoton microscope. The TCSPC response is clean, without the bumps typical for PMTs, and without the diffusion tail typical for SPADs. Most important, the hybrid PMT is free of afterpulsing. So far, afterpulsing has been present in all photon counting detectors. It causes a signal-dependent background in FLIM measurements, and a typical afterpulsing peak in FCS. With a hybrid PMT, FLIM measurements reach a much higher dynamic range. Clean FCS data are obtained from a single detector. Compared to cross-correlation of the signals of two detectors an increase in FCS efficiency by a factor of four is obtained. We demonstrate the performance of the new detector for a number of applications.