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Photodetector
sensors of light or other electromagnetic energy
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A photodetector salvaged from a CD-ROM drive. The photodetector contains three photodiodes, visible in the photo (in center).
Thermal: Photons cause electrons to transition to mid-gap states then decay back to lower bands, inducing phonon generation and thus heat.
Polarization: Photons induce changes in polarization states of suitable materials, which may lead to change in index of refraction or other polarization effects.
Photochemical: Photons induce a chemical change in a material.
Weak interaction effects: photons induce secondary effects such as in photon drag[5][6] detectors or gas pressure changes in Golay cells.
Photodetectors may be used in different configurations. Single sensors may detect overall light levels. A 1-D array of photodetectors, as in a spectrophotometer or a Line scanner, may be used to measure the distribution of light along a line. A 2-D array of photodetectors may be used as an image sensor to form images from the pattern of light before it.
A photodetector or array is typically covered by an illumination window, sometimes having an anti-reflective coating.
Properties
There are a number of performance metrics, also called figures of merit, by which photodetectors are characterized and compared[2][3]
Spectral response: The response of a photodetector as a function of photon frequency.
Responsivity: The output current divided by total light power falling upon the photodetector.
Noise-equivalent power: The amount of light power needed to generate a signal comparable in size to the noise of the device.
Detectivity: The square root of the detector area divided by the noise equivalent power.
Gain: The output current of a photodetector divided by the current directly produced by the photons incident on the detectors, i.e., the built-in current gain.
Dark current: The current flowing through a photodetector even in the absence of light.
Response time: The time needed for a photodetector to go from 10% to 90% of final output.
Noise spectrum: The intrinsic noise voltage or current as a function of frequency. This can be represented in the form of a noise spectral density.
Nonlinearity: The RF-output is limited by the nonlinearity of the photodetector[7]
Devices
Grouped by mechanism, photodetectors include the following devices:
Photoemission or photoelectric
Gaseous ionization detectors are used in experimental particle physics to detect photons and particles with sufficient energy to ionize gas atoms or molecules. Electrons and ions generated by ionization cause a current flow which can be measured.
Microchannel plate detectors use a porous glass substrate as a mechanism for multiplying electrons. They can be used in combination with a photocathode like the photomultiplier described above, with the porous glass substrate acting as a dynode stage
Cadmium zinc telluride radiation detectors can operate in direct-conversion (or photoconductive) mode at room temperature, unlike some other materials (particularly germanium) which require liquid nitrogen cooling. Their relative advantages include high sensitivity for x-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and better energy resolution than scintillator detectors.
HgCdTe infrared detectors. Detection occurs when an infrared photon of sufficient energy kicks an electron from the valence band to the conduction band. Such an electron is collected by a suitable external readout integrated circuits (ROIC) and transformed into an electric signal.
Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity. Normally the resistance of LDRs decreases with increasing intensity of light falling on it.[8]
Bolometers measure the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. A microbolometer is a specific type of bolometer used as a detector in a thermal camera.
Pyroelectric detectors detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials.
Thermopiles detect electromagnetic radiation through heat, then generating a voltage in thermocouples.
Golay cells detect photons by the heat they generate in a gas-filled chamber, causing the gas to expand and deform a flexible membrane whose deflection is measured.
Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
A graphene/n-type silicon heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity. Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector.[16]
Frequency range
In 2014 a technique for extending semiconductor-based photodetector's frequency range to longer, lower-energy wavelengths. Adding a light source to the device effectively "primed" the detector so that in the presence of long wavelengths, it fired on wavelengths that otherwise lacked the energy to do so.[17]
^A. Grinberg, Anatoly; Luryi, Serge (1 July 1988). "Theory of the photon-drag effect in a two-dimensional electron gas". Physical Review B. 38 (1): 87-96. Bibcode:1988PhRvB..38...87G. doi:10.1103/PhysRevB.38.87.
^Bishop, P.; Gibson, A.; Kimmitt, M. (October 1973). "The performance of photon-drag detectors at high laser intensities". IEEE Journal of Quantum Electronics. 9 (10): 1007-1011. Bibcode:1973IJQE....9.1007B. doi:10.1109/JQE.1973.1077407.
^Enss, Christian (Editor) (2005). Cryogenic Particle Detection. Springer, Topics in applied physics 99. ISBN978-3-540-20113-7.CS1 maint: extra text: authors list (link)
^Yuan, Hongtao; Liu, Xiaoge; Afshinmanesh, Farzaneh; Li, Wei; Xu, Gang; Sun, Jie; Lian, Biao; Curto, Alberto G.; Ye, Guojun; Hikita, Yasuyuki; Shen, Zhixun; Zhang, Shou-Cheng; Chen, Xianhui; Brongersma, Mark; Hwang, Harold Y.; Cui, Yi (1 June 2015). "Polarization-sensitive broadband photodetector using a black phosphorus vertical p-n junction". Nature Nanotechnology. 10 (8): 707-713. arXiv:1409.4729. Bibcode:2015NatNa..10..707Y. doi:10.1038/nnano.2015.112. PMID26030655.