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[[File:NIST_Tungsten_Transition_Edge_SensorNIST Tungsten Transition Edge Sensor.png|thumb|alt=Image of four tungsten transition -edge sensors.|Optical image of four tungsten transition -edge sensors for near-infrared single-photon detection. Image credit: [[NIST]].]]
A '''transition -edge sensor''' or ('''TES''') is a type of [[cryogenic]] energy sensor or [[cryogenic particle detector]] that exploits the strongly temperature-dependent [[Electrical resistance|resistance]] of the [[Superconductor#Superconducting phase transition|superconducting phase transition]].
==History==
The first demonstrations of the superconducting transition's measurement potential appeared in the 1940's1940s, thirty30 years after [[Heike Kamerlingh Onnes|Onnes]]'s discovery of [[superconductivity|superconductivity]]. D. H. Andrews demonstrated the first a transition-edge [[bolometer|bolometer]], a current-biased [[tantalum]] wire which he used to measure an infrared signal,. andSubsequently thereafterhe demonstrated a transition -edge [[calorimeterCalorimeter (particle physics)|calorimeter]], made of [[niobium nitride]] andwhich was used to measure [[alpha- particles]]. <ref> D. H. Andrews ''et. al.m'', "Attenuated Superconductorssuperconductors iI. For Measuringmeasuring Infrainfra-Redred Radiation.radiation". ''Rev. Sci. Instrum.'', '''13''', 281 (1942), {{doi|10.1063/1.1770037}}.</ref> However, the TES detector did not gain popularity for about 50 years, due primarily to the difficulty ofin signalstabilizing readoutthe fromtemperature suchwithin athe low-impedancenarrow system.superconducting Atransition secondregion, obstacleespecially towhen themore adoptionthan ofone TES detectorspixel was inoperated optimizingat stablethe operationsame intime, and also due to the narrowdifficulty transitionof edge.signal readout from such a low-[[JouleElectrical heatingimpedance|impedance]] system. [[Joule heating]] in a current-biased TES can lead to [[thermal runaway]] that drives the detector into the normal (non-conductingsuperconducting) state, a phenomenon known as positive [[electrothermal feedback]]. AThe solutionthermal torunaway problem was solved in 1995 by K. D. Irwin by voltage-biasing the readoutTES, problemestablishing hasstable beennegative found[[electrothermal infeedback]], and coupling them to superconducting quantum interference devices ([[SQUID|SQUIDs]]) whichcurrent areamplifiers.<ref>K. nowD. designedIrwin, to"An pairapplication effectivelyof withelectrothermal thefeedback TESfor detectors;high theresolution additionalcryogenic developmentparticle ofdetection". voltage-biased''Appl. operationPhys. forLett.'', '''66''', 1998 (1995), {{doi|10.1063/1.113674}}.</ref> This TESsbreakthrough has facilitatedled wide-spreadto widespread adoption of TES detectors since the late 1990's.<ref name="IrwinHilton">K. D. Irwin and G. C. Hilton, "Transition-Edgeedge Sensors,sensors", ''Cryogenic Particle Detection'', ed. C. Enss, Springer, (2005), [http://dx.{{doi.org/|10.1007/10933596_3 doi:10}}.1007/10933596_3]</ref>
==Setup, Operationoperation, and Readoutreadout==
[[File:TES schematic.pdf|thumb|alt=Schematic of TES-SQUID circuit|Schematic of TES-SQUID circuit]]
AThe TES single-photon detector is voltage-biased by driving a current source ''I''<sub>bias</sub> through a load resistor ''R''<sub>L</sub> (see figure). The voltage is chosen to put the TES in its so-called "self-biased region" where the power dissipated in the device is constant with the applied voltage. When a [[photon|photon]] is absorbed by the TES, this extra power is removed by negative [[Electrothermal_feedback|electrothermal feedback]]: the TES [[electrical_resistanceElectrical resistance|resistance]] increases, causing a drop in TES current,; the dissipated [[Joule power]] in turn drops, cooling the whole device back to its equilibrium power dissipationstate in the self-biasbiased region. In a common [[SQUID]] readout system, the TES is operated in series with the input coil ''L'', which is inductively coupled to a SQUID series-array. Thus a change in TES current manifests as a change in the input [[flux|flux]] to the SQUID, whose output is further amplified and read by room-temperature electronics.
==Functionality==
Any [[bolometer|bolometric]] sensor employs three basic components: an [[Absorption_Absorption (electromagnetic_radiationelectromagnetic radiation)|absorber]] of incident energy, a [[thermometer|thermometer]] for measuring this energy, and a (weak) [[Thermal_conductivityThermal conductivity|thermal link]] to base temperature to dissipate the absorbed energy and re-cool the detector.<ref name="NIST">A. Lita ''et. al.'', "Counting near-infrared single-photons with 95% efficiency", ''Optics Express'' '''16''', 3032 Vol(2008), {{doi|10.1364/OE.16. No.5, 2008003032}}.</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, therebywhich absorbingabsorbs up to 20% of the inputincident 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''', 791-793791–793. (2003).[http://people.bu.edu/alexserg/APL_2003.pdf DOI:, {{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 trickstechniques 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.<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|Bismuthbismuth]] film is often employed.<ref name=IrwinHilton /> 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 arecommonly variedemploy and[[Antenna complicated,(radio)|antennas]] including resonant cavities as described above,or [[feedhorn|feedhorns]], and mores.<ref name=IrwinHilton />
=== 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 edgeregion, and the integral of the resulting drop in current is proportional to the energy absorbed by the detector .<ref name=NIST2 />. The output signal is proportional to the temperature change of the absorber, and 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_transitionSuperconducting 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> ~1-41–4 K, which can be combined to finely tune the overall device ''T''<sub>c</sub>. <ref>A. Lita ''et. al.'', "Tuning of Tungstentungsten Thinthin Filmfilm Superconductingsuperconducting Transitiontransition Temperaturetemperature for Fabricationfabrication of Photonphoton Numbernumber Resolvingresolving Detectors.detectors", (2004). ''IEEE TransactionsTrans. onAppl. Applied SuperconductivitySupercond.'', Vol. '''15''', No.3528 2(2005), June {{doi|10.1109/TASC.2005.849033}}.</ref> Bilayer and multilayer TESs are another popular fabrication approach, where [[thin_film|thin filmsfilm]]s of different materials are combined to achieve the desired ''T''<sub>c</sub>. <ref name=IrwinHilton />
===Thermal conductance===
Finally, it is necessary to tune the [[Thermal_conductivityThermal conductivity|thermal coupling]] between the TES and the bath of cooling liquid; a low thermal conductance is necessary to ensure that incident energy is seen by the TES rather than being lost directly to the bath,. howeverHowever, 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 primaryTwo approaches to control the thermal link are by electron-phononelectron–phonon decoupling,coupling and by mechanical machining. InAt acryogenic superconductortemperatures, heat is carried not by conductionthe [[electrons|electronselectron]] but byand [[phonons|phononsphonon]], andsystems in somea casesmaterial thecan electronbecome andonly phononweakly systemscoupled. ofThe theelectron–phonon samethermal materialconductance mayis differ instrongly temperature. Tungsten is an example of such a system-dependent, and tungsten TESs exploithence the inherentthermal weakconductance phonon-electroncan couplingbe astuned theirby thermaladjusting link to bath''T''<sub>c</sub>. <ref name=IrwinHilton /> <ref name=NIST /> Other devices use mechanical means of controlling the thermal conductance such as building the TES on a sub-micronmicrometre 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)''', 229-235229–235 (1995), {{doi|10.1007/BF00751274}}.</ref>
==Advantages and Disadvantagesdisadvantages==
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 <ref name=IrwinHilton /> <ref name=NIST />, and a theoretical negligible background dark count level (less than 1 event in 1000 s from intrinsic [[Phonon_noisePhonon noise|thermal fluctuations]] of the device <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,. suchEven asthermal [[blackbody radiation]] whichmay is nonethelessbe 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_diodephoton 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 whichthat must be further analyzed to identify photons, and have a jitter of approximately 100 ns .<ref name=NIST />. Furthermore, a single -photon spike on a TES detector lasts on the order of microseconds.
==Applications==
TES arrays are becoming increasingly common in physics and astronomy experiments such as [[James Clerk Maxwell Telescope#SCUBA-2|SCUBA-2]], the HAWC+ instrument on the [[Stratospheric Observatory for Infrared Astronomy]], the [[Atacama Cosmology Telescope]], the [[Cryogenic Dark Matter Search]], the [[Cryogenic Observatory for Signatures Seen in Next-Generation Underground Searches]], the [[Cryogenic Rare Event Search with Superconducting Thermometers]], [[the E and B Experiment]], the [[South Pole Telescope]], and the [[Spider (polarimeter)|Spider polarimeter]], the X-IFU instrument of the [[Advanced Telescope for High Energy Astrophysics]] satellite, the future [https://link.springer.com/article/10.1007/s10909-019-02329-w LiteBIRD] [[Cosmic microwave background|Cosmic Microwave Background]] polarization experiment, the [[Simons Observatory]], and the CMB Stage-IV Experiment.
{{commons category|Transition edge sensor}}
* [[Cryogenic particle detectors]] ▼
==References==
{{reflist}}
▲*[[Cryogenic particle detectors]]
<!--- Categories --->
[[Category:Superconducting detectors]]
[[Category:Radiometry]]
[[Category:Sensors]]
[[Category:Particle detectors]]
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