Photon Counting Optical/IR Detector Arrays

1. Photon Counting Optical/IR Detector Arrays Bernard J. RauscherNASA Goddard Space Flight CenterPresented by Jonathan P. Gardner Clockwise from top: (1) DRS HgCdTe electron avalanche photodiode (e-APD) array, (2) photon-counting e2v CCD201, (3) Raytheon HgCdTe e-APD array and (4) “zero read noise” detector ROIC from RIT/LL. Presented to the NASA Cosmic Origins Program Analysis Group (COPAG) 8 January 2011

2. Overview Photon counters, this is an exciting time! As you know, photon counting detectors are an important enabling technology for spectroscopic studies of exoplanets, space-based astronomical spectroscopy more generally, time resolved astrophysics, and missions that go beyond the Zodiacal cloud. The astrophysics community has been studying silicon based photon counters for visible wavelengths for many years. Outside astrophysics, over a decade of defense investment, non-astrophysics NASA investment, and investments by the vendors themselves (totaling many tens of millions of dollars) have matured linear mode HgCdTe electron avalanche photodiodes (e-APDs) to the point where they are now beginning to enter astrophysics as wavefront sensors for ground-based observatories. In this talk, I will… • Briefly summarize the science case for photon counting detector arrays • Briefly review what the 2011 COR annual report had to say • Provide a summary of where I see the field sitting today, identify some of the major players, and comment on where other agencies are putting their development dollars. • Explain the difference between Geiger mode and linear mode photon detection. • Turning to linear-mode HgCdTe e-APDs, this is an important military technology, and we can expect ongoing large investment by the DoD to mature it. • I summarize a few of the parameters that the DoD is likely to help to mature for us in their efforts to build better detectors for themselves • And, I summarize where more targeted NASA astrophysics investment is likely to be needed to get what we need. • Finally, I provide a recap, along with my main recommendation. I believe that the time is right to form a NASA working group to guide NASA in maturing photon counting technology in concert with DoD efforts and drawing on all of NASA’s expertise (not just astrophysics)

3. Science Case for Photon Counting • After the diameter of the primary mirror, no component affects the performance of an observatory more than the detectors! 2 2 1 1 2 1 photon countingWFC3 CCDJWST H2RG photon countingWFC3 CCDJWST H2RG 1) L2 orbit2) Extra-zodiacal orbit For R=1000 spectroscopy, both orbits are read noise limited when using conventional detectors. The dramatic improvement using photon counters is even greater for emission lines until they are spectrally resolved as the line flux in the pixel remains constant while the shot noise due to zodiacal background photons scales proportional to the spectral resolution element. Broadband point source sensitivity of a JWST aperture cryogenic telescope at: (1) L2 and (2) an extra-zodiacal orbit. Photon counting detector arrays are clearly needed to take full advantage of the supremely low background afforded by an extra-zodiacal orbit.

4. Photon Counting Detectors:A Key Technology for the COR Program (1) This table was extracted from the 2011 Cosmic Origins Program Annual Technology Report

5. Photon Counting Detectors:A Key Technology for the COR Program (2) Continued from previous slide…

6. Bernie Rauscher’s perspective (1) • Optical/IR photon counting detector arrays exist today, have multiple customers, and are beginning to enter astronomy. The following are a few of the astronomy efforts/approaches that I am aware of (there are undoubtedly many more…) • Silicon detectors, including e2v electron multiplying CCDs (EMCCDs), have counted individual photons. Other silicon technologies, including scientific CMOS (sCMOS), may also be capable of photon counting. • Don Figer leads an RIT/Lincoln Lab team who are developing Geiger-mode arrays using silicon and InGaAs detector arrays bonded to a “zero read noise” readout integrated circuit (ROIC) • Don Halla has worked with Raytheon to extend operation of their GHz bandwidth photon counting HgCdTe e-APD LADAR arrays down to the KHz range with reduced dark count rates. With NSF funding the partners are now developing modest (32x32) format arrays for AO wavefront sensing. • Gert Finger tested a 320x256 pixel SELEX-Galileo HgCdTe e-APD array and found that it met requirements for astronomical wavefront sensing (Finger, G. et al., Proc SPIE, 2011, 7742 77421K-1). SELEX-Galileo is now delivering a second generation part that incorporates an astronomy ROIC that was designed to Gert’s specification • Outside astronomy, linear mode HgCdTe electron e-APD arrays have benefitted from over a decade of massive investment by the Department of Defense (DoD) and U.S. defense community, foreign defense agencies, and others including NASA for planetary and earth sensing applications An e2v CCD201 demonstrates photon counting in the Goddard Detector Characterization Laboratory: (left) classical CCD readout, (middle) intensified imaging CCD readout, and (right) photon counting aHall was also supported by a 2007 NASA ROSES/APRA award entitled, "Characterization of HgCdTEAValanche Photo Diode Arrays: A Path to an Infrared Photon Counting Array."

7. Bernie Rauscher’s perspective (2) • Today there is the potential for enormous synergy between the DoD and NASA in the near-infrared • For astronomy, SWIR wavelengths are important because they image older stellar populations, cut through obscuring dust, correspond to the rest frame visible at high-z, and contain important spectral diagnostics for planetary atmospheres • Important defense applications include sensors for “eye safe” lasers, 3-dimensional laser range gated imaging, and long range Light Detection and Ranging (LADAR) • For both NASA and the DoD, the ability to tune HgCdTe’sbandgap is an important plus, as is the comparative maturity of HgCdTe fabrication technology • For both NASA and DoD, HgCdTe’s unique ability to operate in linear mode with no excess noise factor is important • Regarding performance, the defense community shares our interest in high QE, zero noise, low dark current, and large formats • Better detectors can be paired with lower power lasers, smaller optics, and lighter and less expensive payloads for them • Large format (megapixel class) detectors have been used by the defense community and will continue to be used for large area surveillance • These are just a few of the considerations that are motivating ongoing large investments in linear mode HgCdTe e-APD arrays by the military • Now is a good time for NASA to make targeted investments to leverage what DoD is doing for astronomy A Raytheon HgCdTe e-APD array detects individual photons in 2008 “The focus on photon counting for astronomy has enormous synergy for other DoD activities and vice versa.” – Mike Jack, Raytheon

8. There are many potential vendors with applicable technologies (although no solid state technology is mature for low-background astronomy today) 400 nm Specified COR wavelength range 1.7 µm • Silicon and InGaAs are usually Geiger-mode photon counters • Although there are exceptions. For example, the Fig. on chart 5 (also at right) was made by thresholding an e2v EMCCD that did not undergo avalanche breakdown • HgCdTe is a linear mode photon counter < 400 nm Silicon detectors ~ 1050 nm • e2v Electron Multiplying CCDs (EMCCD) • Geiger Si APD array • LBNL charge multiplying CCD (CMCCD) • scientific CMOS (sCMOS) ~800 nm InGaAs ~ 1.7 µm • Geiger InGaAs APD array 400 nm Linear HgCdTe e-APD ~ 5 µm

9. Difference between Geiger and Linear Modes Geiger Mode Linear Mode Geiger Mode on quench on Current avalanche off off Tiny dispersion in avalanche size. These Teledyne e-APDs demonstrate that every avalanche is the same size for a specified diode and bias. There is no need to quench. For any other material, there would be a large dispersion in photocurrent gain arm Vdc + DV V V br br Based on a figure by Don Figer, RIT Voltage Voltage Large dispersion in avalanche size. Operation consists of: (1) arming the APD, (2) read an avalanche, and (3) quenching it.

10. Linear Mode HgCdTe e-APDs:Where Investment is Needed and Others are Likely to Help • HgCdTe growth • Pixel design • Understanding the trade space for tuning the cutoff wavelength • High speed, high dynamic range readout • Radiation tolerance • Vibration tolerance • Large format arrays • Shelf life

11. Linear Mode HgCdTe e-APDs:Where investment is needed and NASA is likely to lead • Dark count rate • Characterization to astronomy performance levels • Readout integrated circuits (ROIC) for ultra-low background space astrophysics • Array formats optimized for astronomy

12. Concluding Thoughts & Recommendations • For linear mode HgCdTee-APD arrays, the situation today is analogous to IR arrays in the early 1980s • Large non-astrophysics investments have been made (totaling at least in the tens of millions of dollars) and continue, spurring rapid development • There are several competing vendors and technologies (this is a good thing) • No one clear market leader has emerged • The unique advantages of linear mode HgCdTe APDs and the military investment in the basic technology make them an attractive candidate for astrophysics missions. They merit careful consideration relative to conventional APD materials. • Silicon is a mature detector material for visible wavelengths • In use by astronomers since 1970s for CCDs. • Several groups are working to develop silicon photon counting technologies • InGaAs is another material for the SWIR that is under development by the RIT/Lincoln Lab team. RECOMMENDATION: In 1983, Craig McCreight organized an Infrared Detector Workshop that marked the birth of infrared astronomy with arrays for many. We are in a similar situation today. I recommend a NASA Workshop bringing together the leaders in relevant photon counting detector technologies. The purpose would be to develop a roadmap, leveraging NASA and non-NASA investments, for further NASA development.