Photon Counting NIR LmAPD Arrays for Ultra-low Background Space Observations

Two of NASA's four large mission concepts for the 2030's, HabEx and LUVOIR, identify a requirement for large format near infrared - 0.8 to 1.8, 2.5 or 5 microns - arrays with read noise << 1 e- and dark current <0.001 e-/s/pix. Linear-mode avalanche photo-diode (LmAPD) 320 x 256 @24-micron pitch ‘SAPHIRA’ arrays manufactured by Leonardo approach this performance. In HgCdTe, the LmAPD process provides noiseless multiplication of photo-electrons within the photo-diode, reducing the relative contribution of read noise to sub-electron levels. SAPHIRA arrays have achieved dark current~ 0.001 e-/s/pix at avalanche bias up to 10 volts (an avalanche gain ~ 10) and have a been operated in photon counting mode at higher avalanche gain (and dark current). UH is now funded by NASA's ROSES APRA program to develop a 1k x 1k @15-micron pitch LmAPD array, the L1RG-15, optimized for ultra-low-background operation. The ME1070 read out integrate circuit (ROIC) has been produced and the first engineering grade LmAPD arrays will be available in 2019. Although the SAPHIRA is estimated to already be at TRL-5, these new LmAPD arrays will start at TRL-3. Description of the key, central objectives. The central objective of the proposed investigation is to advance the Technical Readiness Level of these L1RG-10 arrays from TRL-3 to TRL-6. Secondary objectives are to characterize their dark current vs avalanche bias voltage and to further investigate their photon counting properties. Methods/techniques to accomplish these. Our current cameras and test facilities are all set up for either very low background measurements in the lab or for high background observations at telescopes on Mauna Kea with SAPHIRA arrays. We propose to refurbish an existing camera to allow operation of the L1RG-10 arrays at low background both in the laboratory and at the telescope. Among several options, our preferred choice is to upgrade the United Kingdom Infra-Red Telescope (UKIRT) Fast-Track Imager (UFTI), a facility instrument at UKIRT, which is now owned and operated by UH and will be available throughout the duration of the proposed project. In order to achieve TRL-6, this new camera will utilize an ASIC controller in place of the conventional controllers used with the SAPHIRA systems. The proposal will be baselined for the SIDECAR ASIC developed for JWST but will keep open the option of upgrading to the ACADIA ASIC currently being developed for the WFIRST mission. We will develop camera controller software as part of this. The radiation properties of the ROIC at L-2 can be extrapolated from similar arrays in missions such as JWST. We will investigate the radiation properties of the metal organic vapor phase epitaxial HgCdTe used in the LmAPD arrays at the same UC Davis cyclotron. The investigation will also involve characterization the 1k x 1k arrays relative to the requirements of read noise << 1 e- and dark current <0.001 e-/s/pix along with their photon counting properties. Significance of the proposed work. Achievement of TRL-6 for a 1k x 1k @15-micron pitch LmAPD array with read noise << 1 e- and dark current <0.001 e-/s/pix will qualify a new technology meeting the very demanding NIR array requirements of the HabEx and LUVOIR missions. As it is closely based on the traditional source follower HAWAII arrays being flown on JWST, Euclid and WFIRST, there is a direct route to the scaling of the LmAPD arrays to the 4k x 4k @15-micron pitch of the WFIRST arrays and their incorporation into large mosaic focal planes. These LmAPD HgCdTe arrays also have the potential to benefit other NASA missions.

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