Photomultipliers | previous | next | feedback |
Acronym: PMT
Definition: photodetection devices based on the photoelectric effect and charge multiplication by secondary emission of electrons
Photomultipliers are vacuum tubes, where light absorbed on a photocathode generates free electrons, which are subsequently accelerated with a high voltage, generate secondary electrons on other electrodes, and finally a usable photocurrent. Due to this avalanche process, the photocurrent can be orders of magnitude higher than from, e.g., a photodiode. Therefore, photomultipliers can be used for, e.g., single photon counting. Photomultiplier tubes can be highly sensitive detectors with high linearity, high speed, and a wide dynamic range.
Disadvantages of photomultipliers are the large size (compared with e.g. photodiodes), the higher cost, the requirement for a high-voltage supply, and in some cases the typically lower quantum efficiency.
A very compact form of photomultiplier tubes are microchannel photomultipliers based on a monolithic tube in doped glass. The small size allows one to achieve a very high detection bandwidth.
In some cases, photomultipliers can be replaced with avalanche photodiodes, which also exhibit an amplification mechanism, but in that case one which occurs within a solid-state (semiconductor) material, rather than in a vacuum tube. Compared with photomultiplier tubes, avalanche diodes are much more compact and robust, but have a much smaller active area. They typically exhibit a higher quantum efficiency, but also a higher amplification noise.
A larger active area of an avalanche photodiode device can be obtained by arranging multiple (even thousands) of pixels on one chip in close proximity. Such devices are sometimes called solid-state photomultipliers or silicon photomultipliers (when the diodes are based on silicon).
See also: photodetectors, photodiodes, avalanche photodiodes, photon counting
Categories: metrology, photonic devices
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