Afterpulsing

Afterpulsing is a spurious detector signal that occurs after a primary detection event, caused by delayed processes within a photodetector that release charge or generate secondary electrons. It is observed in devices such as photomultiplier tubes and single-photon avalanche diodes and can degrade timing accuracy and detection efficiency.

In photomultiplier tubes, afterpulses are mainly produced by positive ions formed during the initial electron cascade. These ions drift back to surfaces and release secondary electrons, causing an afterpulse after a short delay. In solid-state detectors like SPADs, afterpulsing arises from charge carriers trapped in defect states during an avalanche; when released, they can trigger a new avalanche. The afterpulse timing distribution is typically exponential with timescales from a few nanoseconds to microseconds, depending on device design and operating conditions.

Measurement and impact: Afterpulsing adds counts correlated with the preceding event, complicating time-correlated measurements and increasing noise. The afterpulse probability, P_ap, is characterized as a function of gating, temperature, bias, and time since the initial pulse.

Mitigation: Approaches include improving vacuum and reducing residual gas in PMTs, designs that minimize ion feedback, optimized dead times and cooling for SPADs, fast quenching circuitry and gated operation, and material improvements to reduce trap densities.

Applications: Afterpulsing is a key consideration in time-resolved photon counting, LIDAR, fluorescence lifetime measurements, and quantum communication where precise timing and low dark counts are essential.