Photon Counting Optical/IR Detector Arrays

Bernard J. RauscherNASA Goddard Space Flight Center

Presented by Jonathan P. GardnerPresented to the NASA Cosmic Origins Program Analysis Group (COPAG)8 January 2011

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.

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OverviewPhoton 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…

1. Briefly summarize the science case for photon counting detector arrays2. Briefly review what the 2011 COR annual report had to say3. 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.4. Explain the difference between Geiger mode and linear mode photon detection.5. 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.1. 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 themselves2. And, I summarize where more targeted NASA astrophysics investment is likely to be

needed to get what we need.6. 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)

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Science Case for Photon Counting• After the diameter of the primary mirror, no component affects the

performance of an observatory more than the detectors!

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1) L2 orbit2) Extra-zodiacal orbit

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.

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.

photon countingWFC3 CCDJWST H2RG

photon countingWFC3 CCDJWST H2RG

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Photon Counting Detectors:A Key Technology for the COR Program (1)

COR request Response

Brief Description Future NASA Optical/ near-IR missions require high QE, fast response time photon counting detector arrays to cover the optical and near-infrared.

Goals and Objectives Develop high QE photon counting detectors for wavelengths of around 400nm-1.7μm.

TRL APDs for the near-IR are under development in industry, but are low TRL (~2).

Tipping point TRL6 will be achieved via substantial military investments, but optimization for low- background purposes could be a modest NASA effort.

NASA Capability NASA will likely have to partner with industry to produce these detectors.

Benefit Future missions with spectroscopic drivers operate ~100 times faster than present. Distant missions (beyond the Zodiacal dust cloud) will observe significantly (>10x) faster even in imaging applications.

NASA Needs This technology is a key technology of benefit for NASA’s next large UV/ Optical/IR mission.

This table was extracted from the 2011 Cosmic Origins Program Annual Technology Report

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COR Request ResponseNon-NASA but aerospace needs High performance optical/IR photon counting detectors

have numerous aerospace applications, remote- sensing, situational awareness, etc.

Non-aerospace needs High performance optical/IR photon counting detectors may have applications in bio and medical imaging.

Technical risk Technical risk is moderate, as basic technology has significant prior investment.

Sequence/timing Should come early since mission definition and capabilities are built around detector performance.

Time and effort 5 year collaboration between NASA, industry, and other government agencies.

Continued from previous slide…

Photon Counting Detectors:A Key Technology for the COR Program (2)

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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 countingaHall was also supported by a 2007 NASA ROSES/APRA award entitled, "Characterization of HgCdTE AValanche Photo Diode Arrays: A Path to an Infrared Photon Counting Array."

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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’s bandgap 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

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There are many potential vendors with applicable technologies

• 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 Linear HgCdTe e-APD ~ 5 µm

< 400 nm Silicon detectors ~ 1050 nm• e2v Electron Multiplying CCDs (EMCCD)• Geiger Si APD array• LBNL charge multiplying CCD (CMCCD)• scientific CMOS (sCMOS)

Specified COR wavelength range400 nm 1.7 µm(although no solid state technology is mature for low-background astronomy today)

~800 nm InGaAs ~ 1.7 µm• Geiger InGaAs APD array

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Difference between Geiger and Linear Modes

Voltage

Vbr

on

off

GeigerMode

Voltage

Vbr

on

avalanche

off

quench

armVdc + DV

Current

Based on a figure by Don Figer, RIT

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

Large dispersion in avalanche size. Operation consists of: (1) arming the APD, (2) read an avalanche, and (3) quenching it.

Geiger Mode Linear Mode

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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

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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

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Concluding Thoughts & Recommendations• For linear mode HgCdTe e-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.