A transition-edge sensor (TES) is a type of cryogenic energy sensor or

cryogenic particle detector Cryogenic particle detectors operate at very low temperature, typically only a few degrees above absolute zero. These sensors interact with an energetic elementary particle (such as a photon) and deliver a signal that can be related to the type of p ...

that exploits the strongly temperature-dependent resistance of the superconducting phase transition.

History

The first demonstrations of the superconducting transition's measurement potential appeared in the 1940s, 30 years after Onnes's discovery of superconductivity. D. H. Andrews demonstrated the first transition-edge

bolometer A bolometer is a device for measuring radiant heat by means of a material having a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley. Principle of operation A bolometer ...

, a current-biased

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

wire which he used to measure an infrared signal. Subsequently he demonstrated a transition-edge

calorimeter A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimete ...

made of niobium nitride which was used to measure

alpha particles Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...

. However, the TES detector did not gain popularity for about 50 years, due primarily to the difficulty in stabilizing the temperature within the narrow superconducting transition region, especially when more than one pixel was operated at the same time, and also due to the difficulty of signal readout from such a low- impedance system. Joule heating in a current-biased TES can lead to thermal runaway that drives the detector into the normal (non-superconducting) state, a phenomenon known as positive electrothermal feedback. The thermal runaway problem was solved in 1995 by K. D. Irwin by voltage-biasing the TES, establishing stable negative electrothermal feedback, and coupling them to superconducting quantum interference devices ( SQUID) current amplifiers. This breakthrough has led to widespread adoption of TES detectors.K. D. Irwin and G. C. Hilton, "Transition-edge sensors", ''Cryogenic Particle Detection'', ed. C. Enss, Springer (2005), .

Setup, operation, and readout

The TES is voltage-biased by driving a current source ''I''_bias through a load resistor ''R''_L (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 A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...

is absorbed by the TES, this extra power is removed by negative electrothermal feedback: the TES resistance increases, causing a drop in TES current; the

Joule power Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor produces heat. Joule's first law (also just Joule's law), also known in countries of former U ...

in turn drops, cooling the device back to its equilibrium state in the self-biased 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 to the SQUID, whose output is further amplified and read by room-temperature electronics.

Functionality

Any

bolometric A bolometer is a device for measuring radiant heat by means of a material having a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley. Principle of operation A bolometer ...

sensor employs three basic components: an

absorber In high energy physics experiments, an absorber is a block of material used to absorb some of the energy of an incident particle. Absorbers can be made of a variety of materials, depending on the purpose; lead, tungsten and liquid hydrogen are c ...

of incident energy, a

thermometer A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer ...

for measuring this energy, and a thermal link to base temperature to dissipate the absorbed energy and cool the detector.A. Lita ''et al.'', "Counting near-infrared single-photons with 95% efficiency", ''Optics Express'' 16, 3032 (2008), .

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, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

TES as its own absorber, which absorbs up to 20% of the incident radiation.A. J. Miller ''et al.'', "Demonstration of a low-noise near-infrared photon counter with multiphoton discrimination", ''Appl. Phys. Lett.'', 83, 791–793. (2003), . 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. 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 is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...

film is often employed. Any absorber should have low

heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). 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

antennas In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...

feedhorn A feed horn (or feedhorn) is a small horn antenna used to couple a waveguide to e.g. a parabolic dish antenna or offset dish antenna for reception or transmission of microwave. A typical application is the use for satellite television recep ...

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. 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

transition temperature Transition temperature is the temperature at which a material changes from one crystal state (allotrope) to another. More formally, it is the temperature at which two crystalline forms of a substance can co-exist in equilibrium. For example, when ...

''T''_c is critical to the device design. Furthermore, ''T''_c should be chosen to accommodate the available 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 K, which can be combined to finely tune the overall device ''T''_c. Bilayer and multilayer TESs are another popular fabrication approach, where

thin film A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...

s of different materials are combined to achieve the desired ''T''_c.

Thermal conductance

Finally, it is necessary to tune the 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. 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–phonon coupling and by mechanical machining. At cryogenic temperatures, the

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

and phonon systems in a material can become only weakly coupled. The electron–phonon thermal conductance is strongly temperature-dependent, and hence the thermal conductance can be tuned by adjusting ''T''_c. Other devices use mechanical means of controlling the thermal conductance such as building the TES on a sub-micrometre membrane over a hole in the substrate or in the middle of a sparse "spiderweb" structure.J. Bock ''et al.'', "A novel bolometer for infrared and millimeter-wave astrophysics", ''Space Science Reviews'', 74, 229–235 (1995), .

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 and a theoretical negligible background dark count level (less than 1 event in 1000 s from intrinsic

thermal fluctuations In statistical mechanics, thermal fluctuations are random deviations of a system from its average state, that occur in a system at equilibrium.In statistical mechanics they are often simply referred to as fluctuations. All thermal fluctuations b ...

of the device). (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. Even thermal

blackbody radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spe ...

may be 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

avalanche photodiode An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of phot ...

(APD) counterparts. APDs are manufactured in small modules, which count photons out-of-the-box with a

dead time For detection systems that record discrete events, such as particle and nuclear detectors, the dead time is the time after each event during which the system is not able to record another event. An everyday life example of this is what happens when ...

of a few nanoseconds and output a 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 that must be further analyzed to identify photons, and have a jitter of approximately 100 ns. 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 SCUBA-2, the HAWC+ instrument on the Stratospheric Observatory for Infrared Astronomy, the

Atacama Cosmology Telescope The Atacama Cosmology Telescope (ACT) is a cosmological millimeter-wave telescope located on Cerro Toco in the Atacama Desert in the north of Chile. ACT makes high-sensitivity, arcminute resolution, microwave-wavelength surveys of the sky in or ...

, the

Cryogenic Dark Matter Search The Cryogenic Dark Matter Search (CDMS) is a series of experiments designed to directly detect particle dark matter in the form of Weakly Interacting Massive Particles (or WIMPs). Using an array of semiconductor detectors at millikelvin temperatur ...

, the

Cryogenic Rare Event Search with Superconducting Thermometers The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) is a collaboration of European experimental particle physics groups involved in the construction of cryogenic detectors for direct dark matter searches. The participating in ...

, the E and B Experiment, the

South Pole Telescope The South Pole Telescope (SPT) is a diameter telescope located at the Amundsen–Scott South Pole Station, Antarctica. The telescope is designed for observations in the microwave, millimeter-wave, and submillimeter-wave regions of the electrom ...

, the Spider polarimeter, the X-IFU instrument of the

Advanced Telescope for High Energy Astrophysics Advanced Telescope for High-ENergy Astrophysics (''Athena'') is an X-ray observatory mission selected by European Space Agency (ESA) within its Cosmic Vision program to address the Hot and Energetic Universe scientific theme. ''Athena'' will ...

satellite, the futur
LiteBIRD
Cosmic Microwave Background polarization experiment, the Simons Observatory, and the CMB Stage-IV Experiment.

References

{{reflist Superconducting detectors Radiometry Sensors Particle detectors