Demonstration of a Far-IR Detector for Space Imaging

1. National Aeronautics and Space Administration Demonstration of a Far-IR Detector for Space Imaging Principal Investigators: C. Darren Dowell (326), Jonas Zmuidzinas (Caltech) Co-Investigators: Peter Day (385), David Goodstein (Caltech), Rick Leduc (385) • Project Objectives: • Demonstrate detection of infrared radiation with the distributed antenna-coupled far-IR transition-edge sensor. • Show that the sensitivity, response time, and polarization response meet requirements for SAFIR/CALISTO. • Address scalability of the detector in fielding large arrays. • FY07-FY08 Results: • First demonstration of response to far-IR radiation of this detector, through a broadband 200 mm filter. • Measured Noise Equivalent Power at fundamental limits, meeting the 10-18 W Hz-1/2 requirement. • Measured radiation response times of 0.1 to 1 msec (meets requirements). • Cross-polar leakage was unacceptably large (50%); could be the test setup or the antenna design. • Benefits to NASA and JPL: • Future far-IR space astrophysics missions such as SAFIR/CALISTO will need large arrays of ultrasensitive detectors. • Noise Equivalent Power (NEP) of 10-18 W Hz-1/2 is required for broadband space far-IR imaging and polarimetry, which has been demonstrated under this task. • Straightforward wafer processing permits fabrication of large detector arrays with high yield. Figure 1: Architecture of a single pixel. (Left) A one-dimensional array of slots in the niobium groundplane concentrates the IR radiation electric field, which is tapped by a two-dimensional array of TiN hot-electron transition-edge bolometers which are wired in parallel and read as a single signal. (Center) Photo of a single pixel fabricated in MDL. Wire bond pads at the wafer edge are used in this prototype. (Right) The detector is back-side illuminated. A quartz anti-reflection layer can be used to maximize absorption efficiency, theoretically >90%. Figure 2: (Left) The detector was tested in a dilution refrigerator at Caltech with an optical window and a commercial SQUID amplifier. To simulate the small radiation power in space, the far-IR signals from the room were coupled to the detector through f/200 optics at 4 K. (Right) Detector IV curves with a 25 mK base temperature. The superconducting transition occurs at 77 mK. The detector was exposed to 200 mm radiation from external loads through the optical window; sensitivity to the temperature of the load demonstrates a basic radiation response. Figure 3: Demonstration of photon-noise-limited sensitivity of the distributed FIR detector. For the two curves, the bolometers were heated and biased in exactly the same way. A clear photon noise excess is seen when the detector is exposed to a 300 K radiation load. The dark NEP of 2×10-18 W Hz-1/2 agrees with the theoretical expectation for phonon noise. Other detectors with lower thermal conductance have measured NEP below 1×10-18 W Hz-1/2. Figure 4: Demonstration of sub-msec radiation response time in the detector. A monochromatic THz radiation source at room temperature illuminated the detector with a square wave time profile. Future Work: Under our follow-on NASA/APRA grant, we intend to: 1) test a new antenna design with dual-polarization response per pixel, much better cross-polar leakage, and improved bandwidth; 2) integrate the detector array with a multiplexer using through-wafer vias and bump bonding. Publication: “Distributed Antenna-Coupled TES for FIR Detector Arrays”, P. Day, H. G. Leduc, C. D. Dowell, R. A. Lee, A. Turner, and J. Zmuidzinas, Journal of Low Temperature Physics, Vol. 151, pp 477-482 NationalAeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology Pasadena, California www.nasa.gov Poster No. 07-D-14