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==Functionality==
Any bolometric sensor employs three basic components: an [[Absorption_(electromagnetic_radiation)|absorber]] of incident energy, a [[thermometer|thermometer]] for measuring this energy, and a (weak) [[Thermal_conductivity|thermal link]] to base temperature to dissipate the absorbed energy and re-cool the detector.<ref name="NIST">A. Lita et.al., Optics Express 3032 Vol.16. No.5, 2008.</ref>
===Absorber===
The simplest absorption scheme can be applied to TESs operating in the near-IR, optical, and UV regimes. These devices generally utilize a [[tungsten|tungsten]] TES as its own absorber, thereby absorbing up to 20% of the input radiation. <ref name = NIST2>A.J. Miller et.al., "Demonstration of a low-noise near-infrared photon counter with multiphoton discrimination," Appl.Phys.Lett., 83, 791-793. (2003).[4]http://people.bu.edu/alexserg/APL_2003.pdf DOI: 10.1063/1.1596723]</ref> If high-efficiency detection is desired, the TES may be fabricated in a multi-layer [[Optical_cavity|optical cavity]] tuned to the desired operating wavelength and employing a backside mirror and frontside anti-reflection coating. Such tricks can decrease the transmission and reflection from the detectors to negligibly low values, resulting in a device with a device whose efficiency in principle exceeds 99%. (In fact, 95% detection efficiency has been observed [3].<ref name=NIST />) At higher energies, the primary obstacle to absorption is transmission, not reflection, and thus an absorber with high photon stopping-power and low heat capacity is desirable--a [[Bismuth|Bismuth]] film is often employed.<ref>K. D.name=IrwinHilton Irwin and G. C. Hilton, "Transition-Edge Sensors," ''Cryogenic Particle Detection'', ed. C. Enss, Springer, 2005, [http://dx.doi.org/10.1007/10933596_3 doi:10.1007/10933596_3]</ref> Any absorber should have low [[heat_capacity|heat capacity]] with respect to the TES. Higher heat capacity in the absorber will contribute to noise and decrease the sensitivity of the detector (since a given absorbed energy will not produce as large of a change in TES resistance). For far-IR radiation into the millimeter range, the absorption schemes are varied and complicated, including resonant cavities as described above, [[feedhorn|feedhorns]], and more.<ref> name=IrwinHilton< /ref>
=== Thermometer ===
The TES operates as a thermometer in the following manner: absorbed incident energy automatically increases the resistance of the voltage-biased sensor within its transition edge, and the integral of the resulting drop in current is proportional to the energy absorbed by the detector [4]<ref name=NIST2 />. The output signal is proportional to the temperature change of the absorber, thus for maximal sensitivity, a TES should have low heat capacity and a narrow transition edge. Important TES properties, including not only heat capacity but also thermal conductance and thermal noise, are strongly temperature dependent, so the choice of [[Superconductivity#Superconducting_phase_transition|transition temperature]] T<sub>c</sub> is critical to the device design. Furthermore, T<sub>c</sub> should be chosen to accommodate the available [[cryostat|cryogenic system]]. Tungsten has been a popular choice for elemental TESs as thin-film tungsten displays two phases, one with T<sub>c</sub> ~15 mK and the other with T<sub>c</sub> ~1-4 K, which can be combined to finely tune the overall device T<sub>c</sub>. [5]<ref>A. Lita et.al., "Tuning of Tungsten Thin Film Superconducting Transition Temperature for Fabrication of Photon Number Resolving Detectors." (2004). IEEE Transactions on Applied Superconductivity, Vol. 15, No. 2, June 2005.</ref> Bilayer and multilayer TESs are another popular fabrication approach, where [[thin_film|thin films]] of different materials are combined to achieve the desired T<sub>c</sub>. <ref>K. D.name=IrwinHilton Irwin and G. C. Hilton, "Transition-Edge Sensors," ''Cryogenic Particle Detection'', ed. C. Enss, Springer, 2005, [http://dx.doi.org/10.1007/10933596_3 doi:10.1007/10933596_3]</ref>
===Thermal conductance===
Finally, it is necessary to tune the [[Thermal_conductivity|thermal coupling]] between the TES and the bath; a low thermal conductance is necessary to ensure that incident energy is seen by the TES rather than being lost directly to the bath, however the thermal link must not be too weak, as it is necessary to cool the TES back to bath temperature after the energy has been absorbed. The two primary approaches to control the thermal link are by electron-phonon decoupling, and by mechanical machining. In a superconductor, heat is carried not by conduction [[electrons|electrons]] but by [[phonons|phonons]], and in some cases the electron and phonon systems of the same material may differ in temperature. Tungsten is an example of such a system, and tungsten TESs exploit the inherent weak phonon-electron coupling as their thermal link to bath [2,3]. <ref name=IrwinHilton /> <ref name=NIST /> Other devices use mechanical means of controlling the thermal conductance such as building the TES on a sub-micron membrane over a hole in the substrate or in the middle of a sparse "spiderweb" structure. <ref>J. Bock et. al., "A novel bolometer for infrared and millimeter-wave astrophysics." Space Science Reviews, 74(1-2), 229-235 (1995).</ref>
==Advantages and Disadvantages==
TES detectors are attractive to the scientific community for a variety of reasons. Among their most striking attributes are an unprecedented high detection efficiency customizable to wavelengths from the millimeter regime to gamma rays [2,3]<ref name=IrwinHilton /> <ref name=NIST />, and a theoretical negligible background dark count level (less than 1 event in 1000 s from intrinsic [[Phonon_noise|thermal fluctuations]] of the device [4]<ref name=NIST2 />). (In practice, although only a real energy signal will create a current pulse, a nonzero background level may be registered by the counting algorithm or the presence of background light in the experimental setup, such as blackbody radiation which is nonetheless seen by a TES optimized for use in the visible regime.)
TES single-photon detectors suffer nonetheless from a few disadvantages as compared to their [[Single-photon_avalanche_diode|avalanche photodiode]] (APD) counterparts. APDs are manufactured in small modules which count photons out-of-the-box with a [[dead_time|dead time]] of a few nanoseconds and output a TTL pulse corresponding to each photon with a jitter of tens of picoseconds. In contrast, TES detectors must be operated in a cryogenic environment, output a signal which must be further analyzed to identify photons, and have a jitter of approximately 100 ns [3]<ref name=NIST />. Furthermore, a single photon spike on a TES detector lasts on the order of microseconds.
==Applications==
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