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1、Single Photon Detectors,By:Kobi CohenQuantum Optics Seminar25/11/09,Outline,A brief review of semiconductorsP-type,N-typeExcitationsPhotodiodeAvalanche photodiodeGeiger ModeSilicon Photomultipliers(SiPM)PhotomultiplierSuperconducting WireCharacterization of single photon sourcesHBT ExperimentSecond
2、order correlation function,Semiconductors,Semiconductors,electrons and“holes”:negative and positive charge carriesEnergy-momentum relation of free particles,with different effective mass,Semiconductors,Thermal excitations make the electrons“jump”to higher energy levels,according to Fermi-Dirac distr
3、ibution:,Semiconductors,Excitations can also occur by the absorption of a photon,which makes semiconductors suitable for light detection:,Energy conservationMomentum conservationphoton momentum is negligible k2k1useful to remember:,Intrinsic Semiconductors,Charge carriers concentration in a semicond
4、uctor without impurities:,N-type Semiconductor,Some impurity atoms(donors)with more valence electrons are introduced into the crystal:,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
5、of lower concentrationElectric field is built up in the depletion layerDrift of minority carriersCapacitance,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 junctionextended space charge re
6、gion,The P-N photodiode,Electrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric field,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
7、quantum efficiencyLarger depletion layerBetter signal,The P-I-N junction,Larger depletion layer allows improved efficiencySmaller junction capacitance means fast response,Detectors:Quantum Efficiency,The probability that a single photon incident on the detector generates a signal,Losses:reflectionna
8、ture of absorption a fraction of the electron hole pairs recombine in the junction,Detectors:Quantum Efficiency,Wavelength dependence of:,Summary:P-N photodiode,Simple and cheap solid state deviceNo internal gain,linear responseNoise(“dark”current)is at the level of several hundred electrons,and con
9、sequently the smallest detectable light needs to consist of even more photons,Avalanche photodiode,High reverse-bias voltage enhances the field in the depletion layerElectrons and holes excited by the photons are accelerated in the strong field generated by the reverse bias.Collisions causing impact
10、-ionization of more electron-hole pairs,thus contributing to the gain of the junction.,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)Average photocurrent is proportional t
11、o 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.Electrons and holes multiply by impact ionization faster than they can be collected,resulting in an exponential growth in the currentIndividual photon
12、counting,Geiger mode quenching,Shutting off an avalanche current is called quenchingPassive quenching(slower,10ns dead time)Active quenching(faster),Summary:Geiger mode,High detection efficiency(80%).Dark counts rate(at room temperature)below 1000/sec.Cooling reduces it exponentially.After-pulsing c
13、aused by carrier trapping and delayed 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.Total area 1x1mm2.The independently operati
14、ng 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),Photomultiplier,Photoelectric effect causes photoelectron emission(external photoelectric effect),For metals the work function
15、 W 2eV,useful for detection in the visible and UV.For semiconductors can be 1eV,useful for IR detection,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 emiss
16、ion,Summary:Photomultiplier,First to be invented(1936)Single photon detectionSensitive to magnetic fieldsExpensive and complicated structure,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 t
17、he plate are coated by thin metal,and act as electrodes.The inner side of each tube is coated with electron-emissive material.,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
18、 resistive barrier across the sensor,and results in a measurable voltage pulse.Healing time 30ps,SSPD meander configuration,Meander structure increases the active area and thus the quantum efficiency,End of 1st part!,Hanbury Brown-Twiss Experiment(1),Back in the 1950s,two astronomers wanted to measu
19、re the diameters of stars,Hanbury Brown-Twiss Experiment(2),Hanbury Brown-Twiss Experiment(3),In their original experiments,HBT set=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.,Coherence time,The cohere
20、nce time c is originated from atomic processesIntensity fluctuations of a beam of light are related to its coherence,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 defi
21、ned by:,(Why second order?),Correlations(2),For much greater than the coherence time:,Correlations(3),On the other and,for much smaller than the coherence time,there will be correlations between the fluctuations at the two times.In particular,if=0:,Correlations:example,If the spectral line is Dopple
22、r broadened with a Gaussian lineshape,the second order correlation functions is given by:,Summary:correlations in classical light,HBT experiments with photons,The number of counts registered on a photon counting detector is proportional to the intensity,Photon bunching and antibunching,Perfectly coh
23、erent light has Poissonian photon statisticsBunched light consists of photons clumped together,Photon bunching and antibunching,In antibunched light,photons come out with regular gaps between them,Experimental demonstration of photon antibunching,Antibunching effects are only observed if we look at
24、light from a single atom,Antibunching has been observed from many other types of light emitters,Experimental demonstration of photon antibunching,Bibliography,Fundamentals of Photonics,Saleh&Teich,Wiley 1991Quantum Optics:An introduction,Mark Fox,Oxford University Press 2006Hamamatsu MMPC datasheet(online)PerkinElmer APCM datasheet(online)Goltsman G.,SSPD,APL 79(6),2001,705-707Hanbury Brown,R.,and Twiss,R.Q.,Nature,177,27(1956)Hanbury Brown,R.,and Twiss,R.Q.,Nature,178,1046(1956),
