Single Photon Detectors

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Single Photon Detectors

• By Kobi Cohen
• Quantum Optics Seminar
• 25/11/09

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Outline

• A brief review of semiconductors
• P-type, N-type
• Excitations
• Photodiode
• Avalanche photodiode
• Geiger Mode
• Silicon Photomultipliers (SiPM)
• Photomultiplier
• Superconducting Wire
• Characterization of single photon sources
• HBT Experiment
• Second order correlation function

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Semiconductors
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Semiconductors

• electrons and holes negative and positive
  charge carries
• Energy-momentum relation of free particles, with
  different effective mass

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Semiconductors

• Thermal excitations make the electrons jump to
  higher energy levels, according to Fermi-Dirac
  distribution

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Semiconductors

• Excitations can also occur by the absorption of a
  photon, which makes semiconductors suitable for
  light detection

(T300K) Egap(eV) ?gap(nm)
Ge 0.66 1880
Si 1.11 1150
GaAs 1.42 870
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Intrinsic Semiconductors

• Charge carriers concentration in a semiconductor
  without impurities

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N-type Semiconductor

• Some impurity atoms (donors) with more valence
  electrons are introduced into the crystal

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P-type Semiconductor

• Some impurity atoms (acceptors) with less valence
  electrons are introduced into the crystal

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The P-N Junction

• Electrons and holes diffuse to area of lower
  concentration
• Electric field is built up in the depletion layer
• Drift of minority carriers
• Capacitance

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Biased P-N junction

• When connected to a voltage source, the i-V curve
  of a P-N junction is given by

• Well focus on reverse biasing
• larger electric field in the junction
• extended space charge region

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The P-N photodiode

• Electrons and holes generated in the depletion
  area due to photon absorption are drifted
  outwards by the electric field

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The P-N photodiode

• The i-V curve in the reverse-biased P-N junction
  is changed by the photocurrent

• Reverse biasing
• Electric field in the junction increases quantum
  efficiency
• Larger depletion layer
• Better signal

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The P-I-N junction

• Larger depletion layer allows improved efficiency
• Smaller junction capacitance means fast response

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Detectors Quantum Efficiency

• The probability that a single photon incident on
  the detector generates a signal

• Losses
• reflection
• nature of absorption
• a fraction of the electron hole pairs recombine
  in the junction

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Detectors Quantum Efficiency

• Wavelength dependence of a

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Summary P-N photodiode

• Simple and cheap solid state device
• No internal gain, linear response
• Noise (dark current) is at the level of several
  hundred electrons, and consequently the smallest
  detectable light needs to consist of even more
  photons

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Avalanche photodiode

• High reverse-bias voltage enhances the field in
  the depletion layer
• Electrons and holes excited by the photons are
  accelerated in the strong field generated by the
  reverse bias.
• Collisions causing impact-ionization of more
  electron-hole pairs, thus contributing to the
  gain of the junction.

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Avalanche photodiode
P-N photodiode
Avalanche photodiode
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Summary APD

• High reverse-bias voltage, but below the
  breakdown voltage.
• High gain (100), weak signal detection (20
  photons)
• Average photocurrent is proportional to the
  incident photon flux (linear mode)

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Geiger mode

• In the Geiger mode, the APD is biased above its
  breakdown voltage for operation in very high
  gain.
• Electrons and holes multiply by impact ionization
  faster than they can be collected, resulting in
  an exponential growth in the current
• Individual photon counting

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Geiger mode quenching

• Shutting off an avalanche current is called
  quenching
• Passive quenching (slower, 10ns dead time)
• Active quenching (faster)

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Summary Geiger mode

• High detection efficiency (80).
• Dark counts rate (at room temperature) below
  1000/sec. Cooling reduces it exponentially.
• After-pulsing caused by carrier trapping and
  delayed release.
• Correction factor for intensity (due to dead
  time).

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Silicon Photomultipliers

• SiPM is an array of microcell avalanche
  photodiodes (20um) operating in Geiger mode,
  made on a silicon substrate, with 500-5000
  pixels/mm2. Total area 1x1mm2.
• The independently operating pixels are connected
  to the same readout line

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SiPM Examples
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Summary SiPM

• Very high gain (106)
• Dark counts 1MHz/mm2 (20C) to 200Hz/mm2 (100K)
• Correction factor (other than G-APD)

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Photomultiplier

• Photoelectric effect causes photoelectron
  emission (external photoelectric effect)

For metals the work function W 2eV, useful for
detection in the visible and UV. For
semiconductors can be 1eV, useful for IR
detection
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Photomultiplier

• Light excites the electrons in the photocathode
  so that photoelectrons are emitted into the
  vacuum
• Photoelectrons are accelerated due to between the
  dynodes, causing secondary emission

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Summary Photomultiplier

• First to be invented (1936)
• Single photon detection
• Sensitive to magnetic fields
• Expensive and complicated structure

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A remark image intensifiers

• A microchannel plate is an array consists of
  millions of capillaries (10 um diameter) in a
  glass plate (1mm thickness).
• Both faces of the plate are coated by thin metal,
  and act as electrodes.
• The inner side of each tube is coated with
  electron-emissive material.

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Superconducting nano-wire

• Ultra thin, very narrow NbN strip, kept at 4.2K
  and current-biased close to the critical current.
• A photon-induced hotspot leads to the formation
  of a resistive barrier across the sensor, and
  results in a measurable voltage pulse.
• Healing time 30ps

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SSPD meander configuration

• Meander structure increases the active area and
  thus the quantum efficiency

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• End of 1st part !

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Hanbury Brown-Twiss Experiment (1)

• Back in the 1950s, two astronomers wanted to
  measure the diameters of stars

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Hanbury Brown-Twiss Experiment (2)
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Hanbury Brown-Twiss Experiment (3)

• In their original experiments, HBT set t0 and
  varied d.
• As d increased, the spatial coherence of the
  light on the two detectors decreased, and
  eventually vanished for large values of d.

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Coherence time

• The coherence time tc is originated from atomic
  processes
• Intensity fluctuations of a beam of light are
  related to its coherence

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Correlations (1)

• We shall assume from now on that we are testing
  the spatially-coherent light from a small area of
  the source.
• The second order correlation function of the
  light is defined by

(Why second order?)
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Correlations (2)

• For t much greater than the coherence time

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Correlations (3)

• On the other and, for t much smaller than the
  coherence time, there will be correlations
  between the fluctuations at the two times. In
  particular, if t0

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Correlations example

• If the spectral line is Doppler broadened with a
  Gaussian lineshape, the second order correlation
  functions is given by

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Summary correlations in classical light
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HBT experiments with photons

• The number of counts registered on a photon
  counting detector is proportional to the intensity

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Photon bunching and antibunching

• Perfectly coherent light has Poissonian photon
  statistics
• Bunched light consists of photons clumped together

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Photon bunching and antibunching

• In antibunched light, photons come out with
  regular gaps between them

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Experimental demonstration of photon antibunching

• Antibunching effects are only observed if we look
  at light from a single atom

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Experimental demonstration of photon antibunching

• Antibunching has been observed from many other
  types of light emitters

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Bibliography

• Fundamentals of Photonics, Saleh Teich, Wiley
  1991
• Quantum Optics An introduction, Mark Fox, Oxford
  University Press 2006
• Hamamatsu MMPC datasheet (online)
• PerkinElmer APCM datasheet (online)
• Goltsman G., SSPD, APL 79(6),2001, 705-707
• Hanbury Brown, R. , and Twiss, R. Q. , Nature,
  177, 27 (1956)
• Hanbury Brown, R. , and Twiss, R. Q. , Nature,
  178, 1046 (1956)