Solid State Photon-Counters for H igh T ime R esolution A strophysics (HTRA )

1. Solid State Photon-Counters for High Time Resolution Astrophysics (HTRA) Giovanni Bonanno, Sergio Billotta, Massimiliano Belluso, M. Cristina Timpanaro, Alessandro Grillo, G. Occhipinti INAF Astronomical Observatory of Catania, Italy Giampiero Naletto Department of Information Engineering, University of Padova, Italy Tommaso Occhipinti, Enrico Verroi, Cesare Barbieri Department of Astronomy, University of Padova, Italy

2. SCIENTIFIC MOTIVATIONS FOR HTRA - 1 - Current astronomical instrumentation essentially only exploits the properties of first order coherence of light, usually for imaging (spatial coherence) or spectroscopy (temporal coherence). The second order coherence functions (Glauber (1963)), can give additional information about source emission mechanisms, i.e. thermal or laser source. The present main limitation to carry out this analysis is the sizeof today’s telescopesthat are not large enough to collect the very high number of photons necessary to discriminate the presence of coherence at the second order correlation from the poissonian photon counting statistics. Quantum Photometer More details about the possible scientific return of a quantum photometer on an ELT, and on smaller size telescopes can be found in papers of Dravins et al. (2006); and Barbieri et al. (2006, 2007).

3. SCIENTIFIC MOTIVATIONS FOR HTRA - 2 - From Andy Shearer, on behalf of the OpticonHTRA network • Quantum-photometers applied to ELTs • thanks to: • much higher photon flux • extremely high timing accuracy and very long term stability could study stellar objects with time resolution better than one or two order of magnitude of the current. Hanbury-Brown–Twiss intensity interferometry over two or more apertures.

4. OPTICON HTRA Network DETECTORS Req. from Andy Shearer, CentreforAstronomy NUI, Galway, Ireland on behalf of the OpticonHTRA network

5. IMAGING ARRAY?? The Best Solution for HTRA A photon counting imaging array detector placed at the telescope focal plane would have been the simplest and easiest way to operate, but …… • A detector system capable to be sensitive to the single photon and push the time tagging capabilities of each incoming photon toward the 100 ps – 1 ns region, sustaining up to 1GHz count rates continuously for hours of uninterrupted acquisition. • A front-end electronics able to satisfy these requirements. MCPs + CCD or CMOS timing resolution (MCP) < 100 ps few kHzand relatively low PDEin the visible range. Fast time tagging not allowed EMCCD multi-port + fast readout few kHzBinning mode? More output? Optimization. Future work. See C. MacKay, S. Tulloc this meeting. Fast time tagging difficult SPAD array extremely good timing accuracy, can sustain fairly high count rates, good quantum efficiency and low dark count. SIGLE ELEMENT OR ARRAY COUPLED TO OPTICS Fast time tagging OK!Small sensitive area and low pixel density  Fill Factor!!!

6. The Best Solution for HTRA Furthermore the choice of a single element SPAD hastwoadverse consequences: finding an optical solution coping with very small sensitive areas fixed aperture photometer without imaging capability, sets limitations to a very good scientific utilization. The baseline solution for QUANTEYE is a non-imaging photometer made by a focal reducer plus a 32x32 lenslet array 1024 pixel detector.

7. CURRENTLY AVAILABLE MOST PERFORMING DETECTOR 1. Time Accuracy After a detailed analysis of pro’s and con’s on presently available technology (and resources), we decided to adopt as detector for the first HTRA applications a single element SPAD, due to good parameters like: Time tagging accuracy PDE Dynamic range A pulsed laser system equipped with a multiple grey filter illuminates a SPAD, at the same time the trigger from the laser sends the START to a time-to-amplitude converter (TAC). When the SPAD detects the photon from the laser it sends the STOP to the TAC. The TAC is connected to a PC to build the distribution of the time intervals between the laser trigger and the photon detection on the SPAD. Time accuracy of the order of 35 ps

8. CURRENTLY AVAILABLE MOST PERFORMING DETECTOR 2. Photon Detection Efficiency PDE From right to left you can find: a Xenon lamp used as the radiation source, a wavelength selection system constituted essentially by a set of filters and a Czerny–Turner monochromator a beam splitter to direct the monochromatic radiation towards an integrating sphere that hosts a NIST traced reference detector and the SPAD. The use of an integrating sphere is crucial because the very small size of the SPAD with respect to the optical beam. PDE approaches 60% at 550 nm

9. CURRENTLY AVAILABLE MOST PERFORMING DETECTOR 3. Dynamic Range and after-pulse Active Quenching Circuit By means of several neutral filters or changing the aperture of the entrance or exit slits of the monochromator we can vary the photon flux intensity coming into the integrating sphere. AQC While the measurement of the afterpulse effects, is achieved by measuring, at different over-voltages the dark count rate varying the hold off time by means of the AQC. • - a dead time of 75 ns after each detected event • - a typical after-pulsing probability around 1% • a linear count rate up to 2 MHz (NIM output, timing accurate) or 12 MHz (TTL output, timing less accurate) • a dark count rate less than 50 cnts/s From our measurements we essentially found the characteristics reported on the manufacturer’s data sheet:

10. IQUEYE Schematic view of IQUEYE optical design A PRECURSOR FOR THE NEXT GENERATION HTRA Iqueye, is a conceptually simple fixed aperture photometer which collects the light within a FoV of just a few arcseconds (selectable from 1” to 6”), splits the telescope light beam in 4 equal parts, and focuses each sub-beam on an independent SPAD. Two wheels allow to insert along the optical path suitable filters and polarizers. Since there is no imaging capability, a field camera visualizes the portion of the sky under investigation.

11. ACQUISITION AND TIMING SYSTEM OF IQUEYE The peculiar and innovative part of Iqueye is the data acquisition system that, utilizing a rubidium oscillator and a GPS receiver, allows to time tag the detected photons with a final absolute UTC referenced rms time accuracy better than 0.5 nsover one hour of observation. IQUEYE at NTT Nasmyth B ACQUISITION AND TIMING SYSTEM SPAD DETECTORS

12. MODULAR SYSTEM EASILY EXPANDED OPTICAL FIBER BRIDGE 60 Mb/s The data acquisition system itself can be easily expanded to a large number of pixels by simply add/replacing VME boards. The rest will remain the same with slightly changes in software. 16 pixels data are long stringsoftimetags 128 pixels Demonstrator for high density SPAD detector of the HTRA or HBT II instrument on ELT. 1024 pixels Can give useful information to design SPAD arrays architectures,that can arrangeTDCs directly on chip by using the CMOS technology implement FPGA-ASIC specialized circuits

13. Observations at the NTT On 14-19 January 2009 IQUEYE + NTT MAIN CHARACTERISTICS PRELIMINARY RESULTS Crab Nebula V =16.5 Individual counts with 0.33 ms bin size Crab Nebula light curves obtained with 1 sec of acquisition Crab Nebula T bins 0.07 ms Crab Nebula T bins 0.6 ms Great sensitivity !!! Ability to resolve with good statistics the 33 ms single periods More details in A&A G. Naletto et al.: Iqueye, a single photon counting photometer applied to the ESO NTT

14. NEAR FUTURE DEVELOPMENTS FOR HTRA --1— SINGLE PIXEL Significant amount of scientific data now under analysis • Solid-State photon-counters • Good quality of the La Silla site • Performance of NTT allowed us to obtain • The detectors shown good performances in terms of: • TIME TAGGING ACCURACY • PDE • HIGH DYNAMIC RANGE • MODEST DARK COUNT • RELIABILITY AND EASY OPERATION • But, we have to quote: • LONG DEAD TIME (preventing to reach a count rate claimed by each SPAD itself) • AFTERPULSING NOT NEGLIGIBLE • SINGLE PIXEL SMALL AREA CONFIGURATION see next slide MPD (the SPAD manufacturer) is already working to find the best compromise between deadtime, timing jitter and afterpulsing. CAEN (the VMECrate manufacturer) is working on a dedicated TDC board with the acquisition computer directly inside the VME crate to increase the communication bandwith.

15. NEAR FUTURE DEVELOPMENTS FOR HTRA --2— SPAD ARRAY POLIMI SINGLE-PHOTON ARRAY SINGLE PIXEL ARRAY LAYOUT D.E.I. F. Zappa • 1024 Single-Photon pixels • Up to 100,000 frame/s for the whole imager • Pixels work completely in parallel • Negligible interframe dead-time (20 ns) • Global shutter • Multiphoton detection capability 32 X 32 pixels 100µm 100µm PIXEL LAYOUT CMOS SPAD (20µm) INTEGRATED QUENCHING CIRCUIT GLOBAL SIGNALS 8 BIT COUNTER 1 ms 1 ms 100 ms 10 ms FIRST TESTS INTERNAL MEMORY BUFFER NO AR COATING, NO MICROLENSES

16. NEAR FUTURE DEVELOPMENTS FOR HTRA --3— SPAD ARRAY 32 x 32 pixels SINGLE-PHOTON ARRAY OPERATING MODE D.E.I. 2nd STEP 3rd STEP 1st STEP F. Zappa ROW SELECTOR ROW SELECTOR ROW SELECTOR COLUMN SELECTOR COLUMN SELECTOR COLUMN SELECTOR Integration time During this time the detected photons are counted Stop integration During this phase the counts are stored in memory Array readout During this phase each memory contents is readout while the detected photons are counted No sacrifice of multi-photon detection

17. DETECTORS FOR HTRA NEXT STEP 1024 Pixels Replace the 8-bit COUNTER withan8-bitTDC and live the samememory buffer and bus Build a 32x32 SPAD array in epi-technologywith appropriate AQC boards and use multiple TDCs VME boards Build a microlensesarray and placeit on the detector surfacetoimprove the fillfactor Build a fiber’s holdertobeplaceddirectly on the detector surface and use a bundle offibrestoreach the telescopefocalplane Fiber’s holder Extreme Large Telescope focal plane

18. Photon Detection Efficiency at different over-voltages of a CMOS-SPAD Cross-section of a CMOS-SPAD integrated into one cell 0.35 μm high-voltage CMOS technology

19. SUMMARY AND CONCLUSIONS • HTRA can rely on extremely fast photon counting photometers utilizing new solid state detectors and appropriate electronics capable of accurate time tagging better than 1 ns • These detectors are characterized by a fast time response of 100 ps, a PDE that approaches 60 % in the visible, a very low dark count rate and a good linearity. • Time-resolved measurements have been demonstrated by Iqueye, a fast photometer based on these detectors, mounted at the ESO-NTT focal plane • HTRA needs for sure two dmensional arrays. • -- Some work have been done to have SINGLE-PHOTON ARRAYS for very-fast IMAGING. • -- Some other improvements are in progress. Thank you very much for your attention