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Development of fast, background-limited transition-edge sensors for the Background-Limited Infrared/Sub-mm Spectrograph (BLISS) for SPICA
Abstract
We report experimental progress toward demonstrating background-limited
arrays of membrane-isolated transition-edge sensors (TESs) for the
Background Limited Infrared/Sub-mm Spectrograph (BLISS). BLISS is a
space-borne instrument with grating spectrometers for wavelengths
λ= 35-435 μm and with R= λ/Δλ~500. The
goals for BLISS TESs are: noise equivalent power (NEP) =
5×10-20 W/Hz1/2 and response time
τ<30ms. We expect background-limited performance from bilayers
TESs with TC=65mK and G=15fW/K. However, such TESs cannot be
operated at 50mK unless stray power on the devices, or dark power
PD, is less than 200aW. We describe criteria for measuring
PD that requires accurate knowledge of TC.
Ultimately, we fabricated superconducting thermistors from Ir
(TC≥135mK) and Mo/Cu proximitized bilayers, where
TC is the thermistor transition temperature. We measured the
Ir TES arrays in our 45mK base temperature adiabatic demagnetization
refrigerator test system, which can measure up to eight 1x32 arrays
simultaneously using a time-division multiplexer, as well as our
single-pixel test system which can measure down to 15mK. In our previous
Ir array measurements our best reported performance was
NEP=2.5×10-19 W/Hz1/2 and τ~5ms for
straight-beam TESs. In fact, we expected NEP
1.5×10-19W/Hz1/2 for meander beam TESs, but
did not achieve this previously due to 1/f noise. Here, we detail
improvements toward measuring the expected NEP and demonstrate
NEP=(1.3+/-0.2)×10-19W/Hz1/2 in our
single-pixel test system and
NEP=(1.6+/-0.3)×10-19W/Hz1/2 in our array
test system.
... SAFARI employs the latest generation of ultra-sensitive TES to detect the incoming photons Audley et al. 2016;Suzuki et al. 2016;Goldie et al. 2012;Beyer et al. 2012). TES bolometers fabricated at SRON ) have already demonstrated the required NEP , as well as the required optical efficiency (Audley et al. 2016). ...
Measurements in the infrared wavelength domain allow direct assessment of the physical state and energy balance of cool matter in space, enabling the detailed study of the processes that govern the formation and evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions revealed a great deal about the obscured Universe, but were hampered by limited sensitivity.
SPICA takes the next step in infrared observational capability by combining a large 2.5-meter diameter telescope, cooled to below 8 K, with instruments employing ultra-sensitive detectors. A combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With mechanical coolers the mission lifetime is not limited by the supply of cryogen. With the combination of low telescope background and instruments with state-of-the-art detectors SPICA provides a huge advance on the capabilities of previous missions.
SPICA instruments offer spectral resolving power ranging from R ~50 through 11 000 in the 17–230 μm domain and R ~28.000 spectroscopy between 12 and 18 μm. SPICA will provide efficient 30–37 μm broad band mapping, and small field spectroscopic and polarimetric imaging at 100, 200 and 350 μm. SPICA will provide infrared spectroscopy with an unprecedented sensitivity of ~5 × 10 ⁻²⁰ W m ⁻² (5σ/1 h)—over two orders of magnitude improvement over what earlier missions. This exceptional performance leap, will open entirely new domains in infrared astronomy; galaxy evolution and metal production over cosmic time, dust formation and evolution from very early epochs onwards, the formation history of planetary systems.
... The Very Long Wavelength Grating module is shown in the right panel of Figure 11 as an example of the GM design. SAFARI employs the latest generation of ultrasensitive TES to detect the incoming photons Audley et al., 2016;Suzuki et al., 2016;Goldie et al., 2012;Beyer et al., 2012). TES bolometers fabricated at SRON have already demonstrated the required NEP , as well as the required optical efficiency (Audley et al., 2016). ...
Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultra-sensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. With cooling provided by mechanical coolers instead of depending on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. SPICA offers instrumentation with spectral resolving powers ranging from R ~50 through 11000 in the 17-230 $\mu$m domain as well as R~28.000 spectroscopy between 12 and 18 $\mu$m. Additionally SPICA will provide efficient 30-37 $\mu$m broad band mapping, and polarimetric imaging in the 100-350 $\mu$m range. SPICA will provide unprecedented spectroscopic sensitivity of ~5 x $10^{-20}$ W/m$^2$ (5$\sigma$/1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.
Originating in the late 1950's, far-infrared astronomy has advanced rapidly over the last three decades, driven by a rapidly maturing technology base and an expanding community of astronomers. This advancement has shown that observations at far-infrared wavelengths are of fundamental importance to all areas of astrophysics, from the search for habitable planets and the origin of life, to the very earliest stages of galaxy assembly in the first few million years of cosmic history. The combination of a still developing portfolio of technologies, particularly in the field of detectors, and a widening ensemble of platforms within which these technologies can be implemented, means that far-infrared astronomy currently holds even greater potential for paradigm-shifting advances. In this review, we examine current and potential far-infrared observing platforms, including ground-based, sub-orbital, and space-based facilities, and discuss the technology development pathways that will enable and enhance these platforms to best address the forefront challenges facing far-infrared astronomy in the 21st century.
SRON is developing ultra-low-noise transition edge sensors (TESs) based on a superconducting Ti/Au bilayer on a suspended SiN island with SiN legs for SAFARI aboard SPICA. We have two major concerns about realizing TESs with an ultra-low NEP of (Formula presented.): achieving lower thermal conductance and no excess noise with respect to the phonon noise. To realize TESs with phonon-noise-limited NEPs, we need to make thinner ((Formula presented.)) and narrower ((Formula presented.)) SiN legs. With deep reactive-ion etching, three types of TESs were fabricated in combination with different SiN island sizes and the presence or absence of an optical absorber. Those TESs have a thin (Formula presented.)), narrow (0.5–0.7 (Formula presented.)), and long (340–460 (Formula presented.)) SiN legs and show (Formula presented.) of (Formula presented.) and (Formula presented.) of (Formula presented.). These TESs were characterized under AC bias using our frequency-division multiplexing readout (1–3 MHz) system. TESs without the absorber show NEPs as low as (Formula presented.) with a reasonable response speed ((Formula presented.)), which achieved the phonon noise limit. For TESs with the absorber, we confirmed a higher (Formula presented.)) than that of TESs without the absorber likely due to stray light. The lowest NEP can make the new version of SAFARI with a grating spectrometer feasible.
We built membrane-isolated transition-edge sensors (TESs) for the background-limited infrared/sub-mm spectrograph using Mo/Cu superconducting bilayer thermistors of varying geometry and found that undesired proximity effects, including the so-called longitudinal proximity effect (LoPE) and the latitudinal inverse proximity effect, affect both the superconducting transition temperature TC and the sharpness of the transition α = dlogR/dlogT. The LoPE and latitudinal inverse proximity effect arise because of unintentional proximity effects between the bilayer thermistors, the superconducting wiring of the TES circuitry, and normal metal decorations added to mitigate the LoPE. We examined Mo/Cu bilayer films with widths of 120 μm and lengths of 5, 10, 20, 40, and 120 μm, and studied the variation of TC, α, and approximate 80% resistance per square (R0.8) with Ti (TC ~ 500 mK) and TiN (TC ~ 3.8 K) wiring to the devices. We found larger α values in general for the Ti wiring, where α was as high as 90 for 20-120 μm devices and decreased to 20 for 5-μm-wide devices. We then built arrays of TESs with bilayer thermistor lengths of 10 μm, Ti contacts, TiN wiring, and Au borders. The devices were expected to demonstrate a noise equivalent power less than or equal to 10-19 W/Hz1/2. We report a measured noise equivalent power at 87 mK of (0.95 ±0.2) × 10-19 W/Hz1/2 and a response time τ of (360 ± 30)ms on our best device with a thermal conductance x1/2 = (15 ±5)fW/K, TC = (120.5 ±3.5)mK, and stray power PD = (135 ±85)aW. The thermistor had a value of RN = 6 mΩ and value of α = dlogR/dlogTbetween 10 and 60 in the transition. We compare our measured performance with the performance specifications needed for ultrasensitive TESs on the Background-Limited Infrared/Sub-mm Spectrograph (BLISS) and discuss paths forward.
The Infrared Spectrograph (IRS) is one of three science instruments on the Spitzer Space Telescope. The IRS comprises four separate spectrograph modules covering the wavelength range from 5.3 to 38 μm with spectral resolutions, R = λ/Δλ ≈ 90 and 600, and it was optimized to take full advantage of the very low background in the space environment. The IRS is performing at or better than the prelaunch predictions. An autonomous target acquisition capability enables the IRS to locate the mid-infrared centroid of a source, providing the information so that the spacecraft can accurately offset that centroid to a selected slit. This feature is particularly useful when taking spectra of sources with poorly known coordinates. An automated data-reduction pipeline has been developed at the Spitzer Science Center.
Time-division SQUID multiplexers are used in many applications that require
exquisite control of systematic error. One potential source of systematic error
is the pickup of external magnetic fields in the multiplexer. We present
measurements of the field sensitivity figure of merit, effective area, for both
the first stage and second stage SQUID amplifiers in three NIST SQUID
multiplexer designs. These designs include a new variety with improved
gradiometry that significantly reduces the effective area of both the first and
second stage SQUID amplifiers.
We have found experimentally that the critical current of a square thin-film superconducting transition-edge sensor (TES) depends exponentially upon the side length L and the square root of the temperature T, a behavior that has a natural theoretical explanation in terms of longitudinal proximity effects if the TES is regarded as a weak link between superconducting leads. As a consequence, the effective transition temperature T{c} of the TES is current dependent and at fixed current scales as 1/L{2}. We have also found that the critical current can show clear Fraunhofer-like oscillations in an applied magnetic field, similar to those found in Josephson junctions. We have observed the longitudinal proximity effect in these devices over extraordinarily long lengths up to 290 microm, 1450 times the mean-free path.
We report progress in fabricating ultra-sensitive superconducting transition-edge sensors (TESs) for BLISS. BLISS is a suite of grating spectrometers covering 35–433 μm with R∼700 cooled to 50 mK that is proposed to fly on the Japanese space telescope SPICA. The detector arrays for BLISS are TES bolometers readout with a time domain SQUID multiplexer. The required noise equivalent power (NEP) for BLISS is NEP=10−19 W/Hz1/2 with an ultimate goal of NEP=5×10−20 W/Hz1/2 to achieve background limited noise performance. The required and goal response times are τ=150 ms and τ=50 ms respectively to achieve the NEP at the required and goal optical chop frequency 1–5 Hz. We measured prototype BLISS arrays and have achieved NEP=6×10−18 W/Hz1/2 and τ=1.4 ms with a Ti TES (T
C=565 mK) and NEP∼2.5×10−19 W/Hz1/2 and τ∼4.5 ms with an Ir TES (T
C=130 mK). Dark power for these tests is estimated at 1–5 fW.
The discovery of galaxies beyond z~1 which emit the bulk of their luminosity at long wavelengths has demonstrated the need for high-sensitivity, broad-band spectroscopy in the far-IR/submm/mm bands. Because many of these sources are not detectable in the optical, long-wavelength spectroscopy is key to measuring their redshifts and ISM conditions. The continuum source list will increase in the coming decade with new ground-based instruments (SCUBA2, Bolocam, MAMBO), and the surveys of HSO and SIRTF. Yet the planned spectroscopic capabilities lag behind, in part due to the difficulty in scaling existing IR spectrograph designs to longer wavelengths. To overcome these limitations, we are developing WaFIRS, a novel concept for long-wavelength spectroscopy which utilizes a parallel-plate waveguide and a curved diffraction grating. WaFIRS provides the large (~60%) instantaneous bandwidth and high throughput of a conventional grating system, but offers a dramatic reduction in volume and mass. WaFIRS requires no space overheads for extra optical elements beyond the diffraction grating itself, and is two-dimensional because the propagation is confined between two parallel plates. Thus several modules could be stacked to multiplex either spatially or in different frequency bands. The size and mass savings provide opportunities for spectroscopy from space-borne observatories which would be impractical with traditional spectrographs. With background-limited detectors and a cooled 3.5 m telescope, the line sensitivity would be comparable to that of ALMA, with instantaneous broad-band coverage. We present the spectrometer concept, performance verification with a mm-wave prototype, and our progress toward a cryogenic astronomical instrument
QUIET is an experimental program to measure the polarization of the Cosmic Microwave Background (CMB) radiation from the ground. Previous CMB polarization data have been used to constrain the cosmological parameters that model the history of our universe. The exciting target for current and future experiments is detecting and measuring the faint polarization signals caused by gravity waves from the inflationary epoch which occurred < 10-30 s after the Big Bang. QUIET has finished an observing season at 44 GHz (Q-Band); observing at 95 GHz (W-Band) is ongoing. The instrument incorporates several technologies and approaches novel to CMB experiments. We describe the observing strategy, optics design, detector technology, and data acquisition. These systems combine to produce a polarization sensitivity of 64 (57) muK for a 1 s exposure of the Phase I Q (W) Band array. We describe the QUIET Phase I instrument and explain how systematic errors are reduced and quantified.
We report on the characterization of SixNy (Si-N) optical absorbers and support beams for transition-edge sensors (TESs). The absorbers and support beams measured are suitable to meet ultra-sensitive noise equivalent power (NEP
The discovery of galaxies beyond z~1 which emit the bulk of their luminosity at long wavelengths has demonstrated the need for high-sensitivity, broad-band spectroscopy in the far-IR/submm/mm bands. Because many of these sources are not detectable in the optical,
long-wavelength spectroscopy is key to measuring their redshifts and ISM conditions. The continuum source list will increase in the coming decade with new ground-based instruments (SCUBA2, Bolocam, MAMBO), and the surveys of HSO and SIRTF. Yet the planned spectroscopic capabilities lag behind, in part due to the difficulty in scaling existing IR spectrograph designs to longer wavelengths. To overcome these limitations, we are developing WaFIRS, a novel concept for long-wavelength spectroscopy which utilizes a parallel-plate waveguide and a curved diffraction grating. WaFIRS provides the large (~60%) instantaneous bandwidth and high throughput of a conventional grating system, but offers a dramatic reduction in volume and mass. WaFIRS requires no space overheads for extra optical
elements beyond the diffraction grating itself, and is two-dimensional because the propagation is confined between two parallel plates. Thus several modules could be stacked to multiplex either spatially or in different frequency bands. The size and mass savings provide opportunities for spectroscopy from space-borne observatories which would be impractical with traditional spectrographs. With background-limited detectors and a cooled 3.5 m telescope, the line sensitivity would be comparable to that of ALMA, with instantaneous broad-band coverage. We present the spectrometer concept, performance verification with a mm-wave prototype, and our progress toward a cryogenic astronomical instrument© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The Usadel equations give a theory of superconductivity, valid in the diffusive limit, that is a generalization of the microscopic equations of the BCS theory. Because the theory is expressed in a tractable and physical form, even experimentalists can analytically and numerically calculate detailed properties of superconductors in physically relevant geometries. Here, we describe the Usadel equations and review their solution in the case of predicting the transition eeo temperature T-C of a thin normal-superconductor bilayer. We also extend this calculation for thicker bilayers to show the dependence on the resistivity of the films. These results, which show a dependence on both the interface resistance and heat capacity of the films, provide important guidance on fabricating bilayers with reproducible transition temperatures. Published by Elsevier Science B.V.
In recent years, superconducting transition-edge sensors (TES) have emerged as powerful, energy-resolving detectors of single photons from the near infrared through gamma rays and sensitive detectors of photon fluxes out to millimeter wavelengths. The TES is a thermal sensor that measures an energy deposition by the increase of resistance of a superconducting film biased within the superconducting-to-normal transition. Small arrays of TES sensors have been demonstrated, and kilopixel arrays are under development. In this Chapter, we describe the theory of the superconducting phase transition, derive the TES calorimeter response and noise theory, discuss the state of understanding of excess noise, and describe practical implementation issues including materials choice, pixel design, array fabrication, and cryogenic SQUID multiplexing.
A novel type of superconducting transition edge sensor is proposed. In this sensor, the temperature of a superconducting film is held constant by feeding back to its position on the resistive transition edge. Energy deposited in the film is measured by a reduction in the feedback Joule heating. This mode of operation should lead to substantial improvements in resolution, linearity, dynamic range, and count rate. Fundamental resolution limits are below ΔE=√kT<sup>2</sup>C, which is sometimes incorrectly referred to as the thermodynamic limit. This performance is better than any existing technology operating at the same temperature, count rate, and absorber heat capacity. Applications include high resolution x‐ray spectrometry, dark matter searches, and neutrino detection. © 1995 American Institute of Physics.
We measured the noise equivalent power (NEP) of an ultra-sensitive membrane-isolated transition-edge sensor (TES) designed for the SpicA FAR Infrared instrument (SAFARI) along with several Johnson noise Thermometry Devices (JTDs) simultaneously in a “dark” experimental setup. Our goal was to characterize the dark power P<sub>D</sub> present in our experimental setup and its effect on the measured NEP. By employing resistive attenuators, filters, lossy coaxial connections, and a light-tight box, we reduced P<sub>D</sub> to 0.25 fW in our setup. For the TES, G was measured to be (325±25) fW/K at T<sub>C</sub>=78 mK giving an upper-limit expected NEP of (3.5±0.25)×10<sup>-19</sup> W/Hz<sup>1/2</sup>. We measured an actual NEP on the TES equal to (6.5±1.5)×10<sup>-19</sup> W/Hz<sup>1/2</sup>, which is approximately double the expected value and likely due to residual electrical dark power in our setup.