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Single Photon Counting

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Buyer's Guide

Use the RP Photonics Buyer's Guide to find suppliers for photonics products! You will hardly find a more convenient resource.

Ask RP Photonics for any advice in the context of photodetection. RP Photonics has extensive know-how e.g. on different detectors and their detection noise.

Definition: photodetection at low light levels where single photon absorption events are counted

German: Photonenzählung, Einzelphotonendetektion

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Some kinds of photodetectors are so sensitive that they allow the detection of single photons. It is then possible to register single photon absorption events, rather than measuring an optical intensity or power. It is also possible to register coincidences between two or more detectors; this is very important for many experiments in quantum optics.

Important Properties of Single Photon Counters

Photon counting detectors have characteristic properties which are somewhat different from those of other photodetectors. The most important ones are the following:

Photodetectors for Single Photon Counting

The classical way of single photon detection is to use a photomultiplier tube. Particularly with a cooled photocathode, such a device can have a very low dark count rate. The quantum efficiency can reach several tens of percent in the visible spectral region, whereas devices for infrared light achieve quantum efficiencies of at most a few percent.

Avalanche photodiodes (APDs) can be operated in the Geiger mode for photon counting. Here, the applied reverse voltage is slightly above the avalanche breakdown voltage. An electron can then be triggered by a single photon, and must be stopped by lowering the voltage for a short time interval, which determines the dead time. Depending on the wavelength, the quantum efficiency can be well above 50%. The dark count rate can be strongly reduced by cooling the diode, but this can increase the rate of after-pulses caused by trapped electrical carriers. Silicon-based APDs are used between roughly 350 and 1050 nm and can reach dark count rates of only a few hertz. A typical r.m.s. timing jitter is some tens of picoseconds. For longer wavelengths in the near-infrared region, devices based on indium gallium arsenide (InGaAs) and indium phosphide (InP) or germanium (Ge) are used. Their quantum efficiency is lower than that of silicon devices in the visible spectrum, but higher than for IR photomultipliers. Count rates are typically limited to a few megahertz, or more for silicon APDs.

Hybrid photomultipliers (see the article on photomultipliers) are essentially consisting of a vacuum tube with an integrated avalanche diode; they offer the combination of some beneficial features of photomultipliers and avalanche diodes, in particular a high speed, a high pulse height resolution and a compact setup.

For longer wavelengths, sum frequency generation in a nonlinear crystal allows one to upconvert the photons to the visible spectral range, followed by detection with a silicon APD. A less common approach is to use a superconducting single photon detector.


Single photon counters are used in various areas of science and technology:

See also: photons, photodetectors, avalanche photodiodes, photomultipliers, quantum efficiency

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