Superconducting Materials and Devices
MDL Detectors Enable a New Probe of the Infant Universe
Transition edge sensor (TES) bolometers sense small temperature changes that occur when photons are absorbed and converted to heat. The use of TESs enables arrays with a much larger number of pixels than is practical with spider-web bolometers. Sustaining its leading role in superconducting TES array technology, MDL developed and continues to improve a process to create arrays of thousands of TESs with high yield (>90 percent). These arrays are being employed on three major astrophysics projects, all with the same goal: generating detailed maps of the polarization of the cosmic microwave background (CMB).
On March 17, 2014, major media outlets around the world spread the amazing news that scientists had managed to see back very nearly to the beginning of time, to 10–35 seconds after the Big Bang. This discovery, made using MDL-developed superconducting detectors deployed in the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) telescope at the South Pole, was announced by Caltech professor and JPL Senior Research Scientist Jamie Bock and his university collaborators. Their measurement of a specific polarization signal in the cosmic microwave background (CMB) is an important confirmation of key aspects of the theory of cosmic inflation and is also the first experimental indication of the connection between gravity and quantum mechanics.
Keck Polarimeter Array
In 2012, MDL increased the optical efficiency of our science-grade arrays by ~1.5 times. These more efficient detector arrays were hybridized into the Keck experiment and placed into two of the five science-grade focal planes that are now deployed and observing the CMB.
Each focal plane unit (FPU) consists of four 150-GHz focal plane arrays that are fabricated at MDL. A focal plane array contains 64 dual-polarization pixels for a total of 512 pixels in each FPU. Each pixel absorbs radiation from the CMB using a phased array of slot antennas, then transfers the absorbed radiation via a niobium superconducting strip-line to a thermally isolated termination resistive element. On the thermally isolated membrane, a superconductive transition-edge sensor (TES) is employed to measure the change in temperature of the isolated membrane. The TES elements are measured using a multiplexed superconducting quantum interference device (SQUID) ammeter from NIST.
The final of the three missions hosting MDL’s CMB focal plane arrays is Spider, a high-altitude, balloon-borne CMB instrument providing a higher sensitivity to the CMB signal by eliminating much of the atmospheric interaction. In 2013, MDL completed and delivered focal planes for a successful test flight of Spider out of Palestine, Texas. MDL increased the optical efficiency of the arrays to above 50%, not only for Spider but also for BICEP and Keck, and also fabricated new arrays tuned for 96 GHz for the 2014 season.
Similar technology is being applied in a new sensor that will be used by the TIME project to map redshifted far-infrared line emission produced by star-forming regions in the first generation of galaxies. TES arrays are also being developed for imaging, spectroscopy, and polarimetry for future missions such as JPL’s proposed BLISS instrument for the Japanese SPICA mission and SAFIR (Single Aperture Far-Infrared observatory).