Photomultipliers | <<< | >>> | Feedback |
Acronym: PMT; MCP = microchannel plate detector
Definition: photodetection devices based on the photoelectric effect and charge multiplication by secondary emission of electrons
Photomultipliers (sometimes called photon multipliers) are vacuum tubes, where light absorbed on a photocathode generates free electrons, which are subsequently accelerated with a high voltage (at least hundreds of volts), 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 bandwidth (>1 GHz), and good linearity in 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 (often providing well above 1 kV), and in some cases the typically lower quantum efficiency (normally <25%, sometimes even <1%).
A very compact form of photomultiplier tubes are microchannel photomultipliers (MCPs = microchannel plates) based on a monolithic tube in doped glass. The small size allows one to assemble many channels to a 2D array and 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. A large active area can be obtained with an array containing may avalanche diodes; such devices are sometimes called silicon photomultipliers. Compared with photomultiplier tubes, silicon photomultipliers can be cheaper and much more compact and robust. 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
