Transition-edge sensor: Difference between revisions

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==Functionality==

Any [[bolometer|bolometric]] sensor employs three basic components: an [[Absorption (electromagnetic radiation)|absorber]] of incident energy, a [[thermometer]] for measuring this energy, and a [[Thermal conductivity|thermal link]] to base temperature to dissipate the absorbed energy and cool the detector.<ref name="NIST">A. Lita ''et al.'', "Counting near-infrared single-photons with 95% efficiency,", ''Optics Express'' '''16''', 3032 (2008), {{doi|10.1364/OE.16.003032}}.</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]] TES as its own absorber, which absorbs up to 20% of the incident 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), {{doi|10.1063/1.1596723}}.</ref> If high-efficiency detection is desired, the TES may be fabricated in a multi-layer [[optical cavity]] tuned to the desired operating wavelength and employing a backside mirror and frontside anti-reflection coating. Such techniques can decrease the transmission and reflection from the detectors to negligibly low values; 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]] film is often employed.<ref name=IrwinHilton/> Any absorber should have low [[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 commonly employ [[Antenna (radio)|antennas]] or [[feedhorn]]s.<ref name=IrwinHilton />

===Thermometer===

The TES operates as a thermometer in the following manner: absorbed incident energy increases the resistance of the voltage-biased sensor within its transition region, 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. Important TES properties including not only heat capacity but also thermal conductance are strongly temperature dependent, so the choice of [[Superconductivity#Superconducting phase transition|transition temperature]] ''T''_c is critical to the device design. Furthermore, ''T''_c 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''_c ~15  mK and the other with ''T''_c ~1-4 1–4 K, which can be combined to finely tune the overall device ''T''_c.<ref>A. Lita ''et al.'', "Tuning of tungsten thin film superconducting transition temperature for fabrication of photon number resolving detectors,", ''IEEE Trans. Appl. Supercond.'', '''15''', 3528 (2005), {{doi|10.1109/TASC.2005.849033}}.</ref> Bilayer and multilayer TESs are another popular fabrication approach, where [[thin film]]s of different materials are combined to achieve the desired ''T''_c.<ref name=IrwinHilton />

===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. Two approaches to control the thermal link are by electron-phononelectron–phonon coupling and by mechanical machining. At cryogenic temperatures, the [[electron]] and [[phonon]] systems in a material can become only weakly coupled. The electron-phononelectron–phonon thermal conductance is strongly temperature-dependent, and hence the thermal conductance can be tuned by adjusting ''T''_c.<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''', 229-235229–235 (1995), {{doi|10.1007/BF00751274}}.</ref>

==Advantages and disadvantages==