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Single photon counting is the most sensitive optical measurement method available. The counting range is limited by photoelectron (PE) pulse width and dark count operating in Geiger-mode typical in SPAD and SiPM sensors. The PE width is determined by the recharge process after typical picoseconds avalanche and the sensor time constant by its capacitance. We achieved sub-ns PE pulses using pF range capacitance coupled with each arrayed pixel and GHz electronics. Dark count was reduced by thermoelectric cooling. Current photon counting performance shows 580ps average PE width, saturation count 500Mcps and dark count <100cps/mm2. Counting electronics can perform up to 1Gcps with ECL logic after 200ps resolution comparator. Time correlated single photon counting (TCSPC) is an important single photon application. Due to sensor deadtime issues, START-STOP period per excitation impulse becomes the determining factor for a measurement. Deadtime-less photon detection enables the counting of multiple photons within an excitation pulse, enabling simultaneous measure of fluorescence intensity and fluorescence lifetime. The PE pulse stream is captured by a digital oscilloscope and analyzed by MATLAB script, avoiding pulse pair resolution limitations using peak detection and statistical analysis. Time resolution is decided by the sampling rate even in overlapped PE signals. Experiments were performed using a commercial oscilloscope with 8M sampling/2ms at 4Gs=250ps, showing that higher bandwidth and sampling rate instruments improve the measurements. This approach is termed Time Correlated Multi-Photon Counting (TCMPC). When combined with a wide dynamic range photon counting sensor, it is a powerful tool for fluorescence analysis, laser induced photon spectroscopy (LIPS), photon flow cytometry and potentially photon communications in deep and free-space or even underwater.
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