Single Photon Detectors单光子探测器幻灯片PPT

Semiconductors
Thermal excitations make the electrons “jump〞 to higher energy levels, according to Fermi-Dirac distfr( iE b) u tie ox np [ :(E E 1 f)/k T ] 1 E k T e x p ( E /k T )
•Larger depletion layer
•Better signal
The P-I-N junction
Larger depletion layer allows improved efficiency Smaller junction capacitance means fast response
release. Correction factor for intensity (due to dead time).
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.
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
Detectors: Quantum Efficiency
Wavelength dependence of α:
Summary: P-N photodiode
Simple and cheap solid state device No internal gain, linear response Noise (“dark〞 current) is at the level of several
•photon momentum is negligible k2≈k1 •useful to remember: E(eV ) 1240
(nmm)
Ge
0.66
1880
Si
1.11
1150
GaAs
1.42
870
Intrinsic Semiconductors
Total area 1x1mm2.
The independently operating pixels are connected to the same readout line
SiPM: Examples
Summary: SiPM
Very high gain (~106) Dark counts: 1MHz/mm2 (~20C) to 200Hz/mm2 (~100K) Correction factor (other than G-APD)
Electrons and holes excited by the photons are accelerated in the strong field generated by the reverse bias.
Collisions causing impactionization of more electronhole pairs, thus contributing to the gain of the junction.
Detectors: Quantum Efficiency
The probability that a single photon incident on the detector generates a signal
( 1 R )[ 1 e x p ( d )]
Losses: • reflection •nature of absorption • a fraction of the electron hole pairs recombine in the junction
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
Electrons and holes multiply by impact ionization faster than they can be collected, resulting in an exponential growth in the current
Individual photon counting
We’ll focus on reverse biasing: 1. larger electric field in the
junction 2. extended space charge region
The P-N photodiode
Electrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric field
P-type Semiconductor
Some impurity atoms (acceptors) with less valence electrons are introduced into the crystal:
The P-N Junction
Electrons and holes diffuse to area of lower concentration
hundred electrons, and consequently the smallest detectable light needs to consist of even more photons
Avalanche photodiode
High reverse-bias voltage enhances the field in the depletion layer
Geiger Mode
Silicon Photomultipliers (SiPM) Photomultiplier Superconducting Wire Characterization of single photon sources
HBT Experiment Second order correlation function
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
Average photocurrent is proportional to the
incident photon flux (linear mode)
Geiger mode
In the Geiger mode, the APD is biased above its breakdown voltage for operation in very high gain.
A microchannel plate is an array consists of millions of capillaries (~10 um diameter) in a glass plate (~1mm thickness).
Single Photon Detectors单光子探 测器
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Outline
A brief review of semiconductors
P-type, N-type Excitations
Photodiode Avalanche photodiode
Avalanche photodiode
P-N photodiode
Avalanche photodiode
Summary: APD
High reverse-bias voltage, but below the
breakdown voltage.
High gain (~100), weak signal detection (~20 photons)
Semiconductors
Excitations can also occur by the absorption of a photon, which makes semiconductors suitable for light detection:
•Energy conservation
•Momentum conservation
Semiconductors
Compounds
Semiconductors
electrons and “holes〞: negative and positive charge carries
Energy-momentum relation of free particles, with different effective mass
Geiger mode – quenching
Shutting off an avalanche current is called quenching
Passive quenching (slower, ~10ns dead time)
Active quenching (faster)
Summary: Geiger mode
Summary: Photomultiplier
First to be invented (1936) Single photon detection Sensitive to magnetic fields Expensive and complicated structure
A remark – image intensifiers
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
Charge carriers concentration in a semiconductor without impurities:
N-type Semiconductor
Some impurity atoms (donors) with more valence electrons are introduced into the crystal:
Electric field is built up in the depletion layer
Drift of minority carriers Capacitance
Biased P-N junction
When connected to a voltage source, the i-V curve of a P-N junction is given by: