Classic Principle of TCSPC
Time correlated single photon counting (TCSPC) detects single photons of a periodic light signal and determines the times of the photons after the excitation pulses.
The pulse repetition rate of the signal is much higher than the photon detection rate. Therefore, the detection of several photons per signal period is extremely unlikely. Only a single photon per signal period needs to be considered. The time of this photon can be determined at extremely high resolution.
From the times of the individual photons TCSPC builds up the distribution of the photons over the time after the excitation pulse.
Every photon seen by the detector contributes to the buildup of the photon distribution. Therefore, the recording process works at near ideal efficiency.
The photon distribution represents the waveform of the optical signal.
In practice, for a TCSPC experiment a pulsed laser source is needed to excite periodically luminescence, usually fluorescence, of a sample. The pulse duration should be in the range of femtoseconds to about 100 picoseconds, and the pulse repetition rate should usually be between 1 – 100 MHz. A single photon detector is employed to record the fluorescence emission. Finally, a fast TCSPC electronics is needed to register the photon events in relation to the laser sync pulses as shown in the figure on the right.
TCSPC is able to cover an extremely large dynamic range. Unlike analog techniques, TCSPC is not limited by the linearity range of the detector. A fluorescence decay function (shown below) can be followed over more than three orders of magnitude. From the fluorescence decay curve the fluorescence lifetime is determined. When TCSPC is combined with a scanning technique fluorescence lifetime imaging (FLIM) can be performed.
In general, TCSPC can yield an extremely high time resolution (with the right equipment). The time resolution of TCSPC is given by the transit time jitter of the detector, not by the width of the detector response. With Becker & Hickl (bh) TCSPC modules and bh detectors an instrument response of less than 20 ps full width at half maximum can be achieved. The time-channel width can be made as short as 203 fs. For more details, please read our brochure about classic TCSPC and its extension to multi-dimensional TCSPC – most popular FLIM.