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Single Photon Detectors By: Kobi Cohen Quantum Optics Seminar 25/11/09

Single Photon Detectors

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Single Photon Detectors. By: Kobi Cohen Quantum Optics Seminar 25/11/09. Outline. A brief review of semiconductors P-type, N-type Excitations Photodiode Avalanche photodiode Geiger Mode Silicon Photomultipliers (SiPM) Photomultiplier Superconducting Wire - PowerPoint PPT Presentation

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Page 1: Single Photon Detectors

Single Photon Detectors

By: Kobi CohenQuantum Optics Seminar25/11/09

Page 2: Single Photon Detectors

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

Page 3: Single Photon Detectors

Semiconductors

Compounds

Page 4: Single Photon Detectors

Semiconductors electrons and “holes”:

negative and positive charge carries

Energy-momentum relation of free particles, with different effective mass

Page 5: Single Photon Detectors

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

Page 6: Single Photon Detectors

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:

Page 7: Single Photon Detectors

Intrinsic Semiconductors Charge carriers concentration in a

semiconductor without impurities:

Page 8: Single Photon Detectors

N-type Semiconductor Some impurity atoms (donors) with

more valence electrons are introduced into the crystal:

Page 9: Single Photon Detectors

P-type Semiconductor Some impurity atoms (acceptors) with

less valence electrons are introduced into the crystal:

Page 10: Single Photon Detectors

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

Page 11: Single Photon Detectors

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

Page 12: Single Photon Detectors

The P-N photodiode Electrons and holes generated in the depletion area due

to photon absorption are drifted outwards by the electric field

Page 13: Single Photon Detectors

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

Page 14: Single Photon Detectors

The P-I-N junction Larger depletion layer allows improved efficiency Smaller junction capacitance means fast response

Page 15: Single Photon Detectors

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

Page 16: Single Photon Detectors

Detectors: Quantum Efficiency Wavelength dependence of α:

Page 17: Single Photon Detectors

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

Page 18: Single Photon Detectors

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.

Page 19: Single Photon Detectors

Avalanche photodiode

P-N photodiode Avalanche photodiode

Page 20: Single Photon Detectors

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)

Page 21: Single Photon Detectors

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

Page 22: Single Photon Detectors

Geiger mode – quenching Shutting off an avalanche

current is called quenching Passive quenching (slower,

~10ns dead time) Active quenching (faster)

Page 23: Single Photon Detectors

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).

Page 24: Single Photon Detectors

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

Page 25: Single Photon Detectors

SiPM: Examples

Page 26: Single Photon Detectors

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

Page 27: Single Photon Detectors

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

Page 28: Single Photon Detectors

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

Page 29: Single Photon Detectors

Summary: Photomultiplier First to be invented (1936) Single photon detection Sensitive to magnetic fields Expensive and complicated

structure

Page 30: Single Photon Detectors

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.

Page 31: Single Photon Detectors

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

Page 32: Single Photon Detectors

SSPD – meander configuration Meander structure increases the active

area and thus the quantum efficiency

Page 33: Single Photon Detectors

End of 1st part !

Page 34: Single Photon Detectors

Hanbury Brown-Twiss Experiment (1)

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

Page 35: Single Photon Detectors

Hanbury Brown-Twiss Experiment (2)

Page 36: Single Photon Detectors

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.

Page 37: Single Photon Detectors

Coherence time

The coherence time τc is originated from atomic processes

Intensity fluctuations of a beam of light are related to its coherence

Page 38: Single Photon Detectors

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?)

Page 39: Single Photon Detectors

Correlations (2) For τ much greater than the coherence time:

Page 40: Single Photon Detectors

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 :

Page 41: Single Photon Detectors

Correlations: example

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

Page 42: Single Photon Detectors

Summary: correlations in classical light

Page 43: Single Photon Detectors

HBT experiments with photons The number of counts registered on a photon counting

detector is proportional to the intensity

Page 44: Single Photon Detectors

Photon bunching and antibunching Perfectly coherent light has Poissonian photon statistics Bunched light consists of photons clumped together

Page 45: Single Photon Detectors

Photon bunching and antibunching

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

Page 46: Single Photon Detectors

Experimental demonstration of photon antibunching

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

Page 47: Single Photon Detectors

Antibunching has been observed from many other types of light emitters

Experimental demonstration of photon antibunching

Page 48: Single Photon Detectors

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)