Detecting photons: Properties and challenges

Jessica Y. Cheung, Christopher J. Chunnilall, Nigel P. Fox Optical Radiation Measurement Team National Physical Laboratory United Kingdom 21st June 2005 - 4th International Conference on New Developments in Photodetection, Beaune, France

'© 2003 British Crown Copyright' Overview of talk

• Introduction to NPL and ORM • Cryogenic radiometry • Importance of traceability • Detector characterisation facilities • Current challenges - few photon regime – Photon counting applications and challenges – Detector calibration: current technique – Detector calibration: correlated photons • Single photon counting workshop National Physical Laboratory Vision

• NPL is the United Kingdom's national standards laboratory, an internationally respected and independent centre of excellence in research, development and knowledge transfer in measurement and materials science. For more than a century we have developed and maintained the nation's primary measurement standards - the heart of an infrastructure designed to ensure accuracy, consistency and innovation in physical measurement.

Quality of Life

Enabling Metrology

Engineering and Process Control Quality of Life: Optical Radiation Measurement

Optical region: 200 nm to 100 µm

Detectors Sources Materials -Cryogenic radiometry - L aser rad iometry - Spectrophotometry Manufacture + Supply - Filter radiometry (photometry) - Goniometric measurements - Detector characterisation - New transfer standards - Colour Facilities - LEDs and Pulsed source - Fluorescence

Focused Applications •Earth Observation •Thermodynamic temperature • Eutectics •Soft Metrology • Appearance, Vision/human factors •Low cost access to SI • Transfer standards, internet calibration Cryogenic radiometry

• Pioneered at NPL, late 1970’s • Primary standard – thermal detector • Basis of realisation of the SI unit of luminous intensity, the candela • Links optical heating to electrical effect – electrical SI units (Electrical Substitution) • Underpins optical radiation source and detector scales • Optical power 0.005% accuracy

optical power thermal resistance reflectance Poptical Pbackground Pbackground

lead heating absorbing coating ∆Τopt = ∆Τelec Popt = Pelec heater, P electric Temperature sensor, ∆T heat link heat sink Pbackground Pbackground Traceability: detector calibrations

0.002% Primary standard Fundamental Cryogenic radiometry constants

Characterisation at discrete laser 0.005% wavelengths (210 nm to 11 µm) Calibrate transfer secondary standards at different wavelengths

UV VIS IR Far - IR: 20µm

PtSi, GaN, GaAsP Si trap detectors InGaAs Cavity pyroelectric detectors

dissemination to laboratories and industry

photodiodes thermal thermal photon power detectors imagers counters meters Detector characterisation capabilities

• Spectral range - 200 nm to ~ 30 µm • Solid state and thermal • Photon counting ~ 5000 pht s-1 (upwards) • Single elements through to linear and 2D arrays Individual pixels and intra-pixel • Optical instruments – spectrometers, cameras Vis and IR • Spectral response, linearity, spatial uniformity, response time, noise Photon counting applications Photon counting challenges

• High quantum efficiency • Timing, jitter, dead-time, dark counts, after pulsing, breakdown flashes • Faster response time • Photon number resolving capabilities • Spatial uniformity • Wavelength range of detectors • High accuracy metrology techniques to quantify all of the above! Calibrating photon counting detectors

•Current method of calibrating photon e.g. trap counting detectors. reference •Compare response of reference standard detector and photon counting detector using same power source •Avoid saturation of photon counting high power (mW) detector using attenuators – reduce source by ~10 orders of magnitude photon •Attenuators can introduce further counting uncertainties detector •Past calibrations on PMT tubes uncertainties ~ 0.5% – 1% depending calibrated on system [1] attenuator

References [1] Biller, Jelley Thorman, Fox, Ward, Nucl.Instr.Meth. A432:364-373, 1999 Correlated Photon Metrology Absolute metrology! non-linear crystal

high energy photons

• Non linear process: Spontaneous parametric downconversion first predicted in 1961 by Louisell et al. [2] • First detection efficiency measurement carried out on PMT 1970. [3] • First rigorous application of technique to measure q.e. of Si detectors, 1995 Migdall et al. NIST [4] • Technique is being pursued by NIST, IEN and NPL • Technique offers an alternative and direct method of measuring quantum efficiency of photon counting detectors and realising primary radiometric scales

References [2] Louisell, Yariv and Siegman, 1961, Phys. Rev., 124, 1646. [3] Burnham and Weinberg, 1970, Phys. Rev. Lett., 25, 84. [4] Migdall, Datla, Sergienko, Orsak, Shih, 1995/96, Metrologia, 32, 479 Detector calibration using correlated photons

N: number of photon pairs

NC: number of coincidences

ηA = NC / NB A is DUT channel, B is trigger channel

Key sources of uncertainty

• Measurement of NC

• Accidental coincidences AC

• Measurement of NB

• False triggers due to shot noise, background: DB

• Losses in channel A: tA • Optical elements can be placed in the trigger path B • Alignment – optimisation, stability, reproducibility

ηA = (NC –AC) / [tA (NB –DB)] Detector calibration facility NPL detector calibration facility

Counting electronics

View of facility towards crystal

BBO type I crystal Characterising optical components

• National Laser Radiometry facility (NLRF) – Lenses and crystals - 0.02% – Ti-sapphire tuneable stabilised laser – Mode-locking techniques to measure transmittance: avoids NLRF interference effects caused by coherence effects of laser in samples with parallel sides, NRS • National Reference Spectrophotometer (NRS) – Filter transmittance – (0.03 – 0.05)% Mode locked laser techniques

• Mode-locked laser has pulses of width approx. 0.3 mm. – No interference for sample thicker than 0.15 mm • transmittance of glass plate at 800 nm with cw and with mode-locked laser

variation of transmittance of glass sample with angle of incidence of laser

1.06 o t

. 1.04

e 1.02

ce rel cw operation

n 1 a t

t modelocked operation i

averag 0.98

sm 0.96 an r t 0.94 00.511.5 Angle of incidence

[4] Hartree, Theocharous. and Fox, 2003, Proc. SPIE, 4826 104-112 Limited by quality of crystal and spatial uniformity of detectors

Map scan of 3mm thick BBO crystal Map scan of DUT (silicon APD) (with AR coatings – 632nm) each contour = 0.0025 units, 543nm

Uniformity of BBO crystal

0.998-1

0.996-0.998 0.994-0.996

0.992-0.994

0.99-0.992

0.988-0.99 0.986-0.988 Measurement of coincidence counts

ηA = (NC –AC) / [tA (NB –DB)]

Nc area under curve in region B between P and Q Ac accidental count rate in region B between P and Q NB trigger count rate during downconversion DB trigger count rate downconversion OFF

Region C detectors and counting electronics deadtime Region D occurs after dead-time

Small peak in region D, after pulse events (0.4% of coincidences in region B)

Cheung, Chunnilall and Wang, 2004, Proceedings of SPIE, 5551 220-230 After-pulsing and linearity

autocorrelated DUT autocorrelated Trigger

coincidence peak

after-pulse Current and future plans at NPL

• Validate technique via • In-house comparisons (cryogenic radiometer) – aim 0.1% agreement • Inter-comparisons with other NMIs (e.g. NIST, IEN) • Extend into telecommunications region • Meet future requirements of technology for detector calibration • Primary radiometric scales: independent and direct – 1 to 2 orders of magnitude required improvement:

Current accuracies Cryogenic Radiometer (%) Correlated Photons (%) Responsivity 0.01 → 0.001 ~0.1

Roadblocks? Uncertainties in characterising loss mechanisms Acknowledgements

John Mountford Collaborators Jian Wang Alan Migdall (NIST) Bill Hartree Stefania Castelletto (IEN) Martin Vaughan (Essex University) Michael Ware (Brigham Young University) Peter Thomas (St. Andrews University) ORM Team Glazebrook fellowship funding and NIST www.npl.co.uk/optical_radiation guest researcher grant for 4 month secondment to NIST. Funding National Measurement Services Directorate, Department of Trade and Industry, UK Single Photon Workshop 2005

Single Photon Detector Workshop 2005

24 – 26 October 2005 SPW2005

National Physical Laboratory Teddington Middlesex UK

www.spw2005.npl.co.uk email. [email protected] www.npl.co.uk/optical_radiation