1. 1 Astronomical Observational Techniques and Instrumentation RIT Course Number 1060-771 Professor Don Figer Quantum-Limited Detectors

2. 2 Aims for this lecture Motivate the need for future detectors Describe physical principles of future detectors Review some promising technologies for future detectors

3. 3 Motivation for Future Detectors

4. 4 Improving Detectors Detector properties limit sensitivity in most applications. For instance, dark current and read noise are important in low flux applications. Detectivity is a measure of system effectiveness.

5. 5 Detectivity in Broadband Applications Figure 3. Detectivity as a function of quantum efficiency and read noise for broadband astrophysics applications.

6. 6 Detectivity in Low Flux Broadband Applications Figure 4. Same parameters as used to generate Figure 3, except the exposure time is only 5 seconds, instead of 10 minutes. It is apparent that read noise becomes a dominant factor in detectivity for this case.

7. 7 Detectivity in Narrowband Applications Figure 5. Detectivity as a function of quantum efficiency and read noise for narrowband astrophysics applications.

8. 8 Detectivity in Narrowband Applications with Low Dark Current Figure 6. Same parameters as used to generate Figure 5, except the dark current is 0.0001 electrons/second/pixel, instead of 0.1 electrons/second/pixel. It is apparent that read noise becomes a dominant factor in detectivity for this case. Also, note that the detectivity is comparable to that for the broadband imaging case modeled in Figure 3.

9. 9 Detectivity in Spectroscopic Applications Figure 7. Detectivity as a function of quantum efficiency and read noise for high resolution spectroscopy astrophysics applications.

10. 10 Detectivity in Spectroscopic Applications with Low Dark Current Figure 8. Same parameters as used to generate Figure 7, except the dark current is 0.001 electrons/second/pixel, instead of 0.1 electrons/second/pixel. It is apparent that read noise becomes a dominant factor in detectivity for this case.

11. 11 Read Noise

12. 12 Aperture vs. Read Noise

13. 13 Very Low Light Level - ExoPlanet Imaging The exposure time required to achieve SNR=1 is dramatically reduced for a zero read noise detector, as compared to detectors with state of the art read noise.

14. 14 Principles of Quantum Limited Detectors

15. 15 Key Capabilities for Future Improvement photon-counting (zero read noise) wavelength-resolving polarization-measuring low power large area in-pixel processing high dynamic range high speed time resolution

16. 16 QLID Technology Contenders Table 1. Quantum-limited Detector Technologies. SuperconductorsSemiconductors Transition Edge Sensor (TES) energy resolution operating temperature of tens of mK Electron Multiplying CCD (EMCCD) commercially available excess noise factor Superconducting Tunnel Junction (STJ) energy resolution operating temperature of mK, leakage current Linear Mode Avalance Photodiode (LM-APD) ns time constant excess noise factor (although MCT has ~no excess noise) Kinetic Inductance Detector (KID) energy resolution ms time constant Geiger Mode Avalance Photodiode (GM-APD) large pulse per photon afterpulsing Superconducting Single Photon Detectors (SSPD) ns time constant low fill-factor, polarized, few K

17. 17 Key to Single-Photon Counting A photon-counting system requires that the ratio of signal from a single photon to the noise of the system be big enough to detect. This can be achieved by: –increasing numerator (e.g., charge gain) –decreasing denominator (e.g., cooling, better circuits) –decreasing what is “big enough” (e.g., better processing) –combination of all

18. 18 Superconductors Most metals have descreased resistance with lower temperature, but they still have finite resistance at T=0 K. Superconductors lose all resistance to electrical current at some temperature, T c. Examples include: Pb, Al, Sn, and Nb. Electrons in superconductors bond as “Cooper pairs” that do not interact with the ion lattice below T c because the required interaction energy exceeds the thermal energy in the crystal. In general, T c <4.2 K. Recent developments have produced “high” temperature superconductors, for which T c >77 K (temperature of liquid nitrogen).

19. 19 Slide Title

20. 20 Avalanche Photodiodes (APDs)

21. 21 Geiger-Mode Imager: Photon-to-Digital Conversion Quantum-limited sensitivity Noiseless readout Photon counting or timing APD Digital timing circuit Digitally encoded photon flight time photon Lenslet array APD/CMOS array Focal-plane concept Pixel circuit

22. 22 Geiger-Mode Operation

23. 23 Gain of an APD 1 10 100 M Breakdown0 Ordinary photodiode Linear-mode APD Geiger-mode APD Response to a photon M 1 ∞ I(t)

24. 24 Current Voltage Current Linear mode Geiger mode V br on off Current Voltage Current Linear mode Geiger mode V br on avalanche off quench arm V dc +  V Operation of Avalanche Diode

25. 25 Avalanche Diode Architecture

26. 26 Performance Parameters Photon detection efficiency (PDE)  The probability that a single incident photon initiates a current pulse that registers in a digital counter Dark count Rate (DCR)/Probability (DCP)  The probability that a count is triggered by dark current instead of incident photons time Single photon input APD output Discriminator level Digital comparator output Successful single photon detection Photon absorbed but insufficient gain – missed count Dark count – from dark current

27. 27 APD Charge Gain Show animation with thumping euro-techno disco music http://techresearch.intel.com/spaw2/uploads/files/SiliconPhotonics.html

28. 28 32x32 Timing Circuit Array 0.35-  m CMOS process fabricated through MOSIS 1.2 GHz on-chip clock Two vernier bits 0.2-ns timing quantization 100-  m spacing to match the 32x32 APD array Timing image/histogram measuring propagation of electronic trigger signal Vernier bitsCounter Time bin Pixels

29. 29 32x32 APD/CMOS Array with Integrated GaP Microlenses

30. 30 Shortcomings of Conventional Imaging When the 3D world is projected into a flat intensity image, there is a huge information loss. Image processing algorithms attempt to use intensity edges to infer properties of 3D objects. Consequences of lost information for automated image segmentation and target detection/recognition: –Depth ambiguity –Sensitivity to lighting, reflectivity patterns, and point of observation –Obscuration and camouflage

31. 31 Ladar Imaging System Imaging system photon starved –Each detector must precisely time a weak optical pulse –Sub-ns timing, single photons Microchip laser Geiger-mode APD array Color-coded range image

32. 32 Laser Radar Brassboard System (Gen I) 4  4 APD array External rack-mounted timing circuits Doubled Nd:YAG passively Q-switched microchip laser (produces 30 µJ, 250 ps pulses at = 532 nm) Transmit/receive field of view scanned to generate 128  128 images Taken at noontime on a sunny day

33. 33 Conventional vs Ladar Image Conventional image 3D image

34. 34 Foliage Penetration Experiment Laser radar on tower elevator View from 100 m tower Objects under trees

35. 35 Foliage Penetration Imagery

36. 36 Transition Edge Sensors (TESs)

37. 37 Transition Edge Sensors (TES) A TES is similar to a bolometer, in that photon energy is detected when it is absorbed in a material that changes resistance with temperature. The difference is that a TES is held at a temperature just below the transition temperature at which the material becomes supconducting. The effective change in resistance when photons are absorbed is very large (and easy to detect). One of the disadvantages of using TES’s is that the transition temperature is usually very low, requiring exotic cooling techniques.

38. 38 TES Schematic

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40. 40 TES Wavelength Resolution

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42. 42 Prototype TES Device

43. 43 Superconducting Tunneling Junctions (STJs)

44. 44 Superconducting Tunneling Junctions (STJs) An STJ uses the current response of a Josephson junction (aka STJ) when struck by a photon to detect light. The junction is similar to semiconducting junction and is composed of superconductor-insulator-superconductor. The gap energy is generally much less than for silicon, so optical photons induce charge gain that depends on photon energy.

45. 45 TES vs. STJ

46. 46 Superconducting Single Photon Detectors (SSPDs)

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