Single Photon Detectors By: Kobi Cohen Quantum Optics Seminar 25/11/09.
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Transcript of Single Photon Detectors By: Kobi Cohen Quantum Optics Seminar 25/11/09.
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Single Photon Detectors
By: Kobi CohenQuantum Optics Seminar25/11/09
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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
Compounds
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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:
1( ) exp( / )
exp[( ) / ] 1E kT
ff E E kT
E E kT
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Semiconductors Excitations can also occur by the absorption
of a photon, which makes semiconductors suitable for light detection:
(T=300K)
Egap(eV)
λgap(nm)
Ge 0.66 1880
Si 1.11 1150
GaAs 1.42 870
1240( )
( )E eV
nm
•Energy conservation
•Momentum conservation
•photon momentum is negligible k2≈k1
•useful to remember:
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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:
We’ll focus on reverse biasing:
1. larger electric field in the junction
2. 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
(1 ) [1 exp( )]R d
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 α:
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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 1950’s, 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 τ=0 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 τc 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 τ much greater than the coherence time:
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Correlations (3) On the other and, for τ much smaller than the coherence
time, there will be correlations between the fluctuations at the two times. In particular, if τ=0 :
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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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Antibunching has been observed from many other types of light emitters
Experimental demonstration of photon antibunching
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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) Golts’man 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)