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1、SemiconductorsCompounds第1页/共47页Semiconductorselectrons and“holes”:negative and positive charge carriesEnergy-momentum relation of free particles,with different effective mass第2页/共47页SemiconductorsThermal excitations make the electrons“jump”to higher energy levels,according to Fermi-Dirac distributio
2、n:第3页/共47页SemiconductorsExcitations can also occur by the absorption of a photon,which makes semiconductors suitable for light detection:(T=300K)Egap(eV)gap(nm)Ge0.661880Si1.111150GaAs1.42870Energy conservationMomentum conservationphoton momentum is negligible k2k1useful to remember:第4页/共47页Intrinsi
3、c SemiconductorsCharge carriers concentration in a semiconductor without impurities:第5页/共47页N-type SemiconductorSome impurity atoms(donors)with more valence electrons are introduced into the crystal:第6页/共47页P-type SemiconductorSome impurity atoms(acceptors)with less valence electrons are introduced
4、into the crystal:第7页/共47页The P-N JunctionElectrons and holes diffuse to area of lower concentrationElectric field is built up in the depletion layerDrift of minority carriersCapacitance第8页/共47页Biased P-N junctionWhen connected to a voltage source,the i-V curve of a P-N junction is given by:Well focu
5、s on reverse biasing:1.larger electric field in the junction2.extended space charge region第9页/共47页The P-N photodiodeElectrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric field第10页/共47页The P-N photodiodeThe i-V curve in the reverse-biased P-
6、N junction is changed by the photocurrentReverse biasing:Electric field in the junction increases quantum efficiencyLarger depletion layerBetter signal 第11页/共47页The P-I-N junctionLarger depletion layer allows improved efficiencySmaller junction capacitance means fast response第12页/共47页Detectors:Quant
7、um EfficiencyThe probability that a single photon incident on the detector generates a signalLosses:reflectionnature of absorption a fraction of the electron hole pairs recombine in the junction第13页/共47页Detectors:Quantum EfficiencyWavelength dependence of:第14页/共47页Summary:P-N photodiodeSimple and ch
8、eap solid state deviceNo internal gain,linear responseNoise(“dark”current)is at the level of several hundred electrons,and consequently the smallest detectable light needs to consist of even more photons第15页/共47页Avalanche photodiodeHigh reverse-bias voltage enhances the field in the depletion layerE
9、lectrons 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.第16页/共47页Avalanche photodiodeP-N photodiodeAvalanche photodiode第17页/共47页Summary:A
10、PDHigh 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)第18页/共47页Geiger modeIn the Geiger mode,the APD is biased above its breakdown voltage for operation in very high gai
11、n.Electrons and holes multiply by impact ionization faster than they can be collected,resulting in an exponential growth in the currentIndividual photon counting第19页/共47页Geiger mode quenchingShutting off an avalanche current is called quenchingPassive quenching(slower,10ns dead time)Active quenching
12、(faster)第20页/共47页Summary:Geiger modeHigh 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).第21页/共47页Silicon PhotomultipliersSiPM
13、 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第22页/共47页SiPM:Examples第23页/共47页Summary:SiPMVery high gain(106)Dark counts:1MHz
14、/mm2(20C)to 200Hz/mm2(100K)Correction factor(other than G-APD)第24页/共47页PhotomultiplierPhotoelectric 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第2
15、5页/共47页PhotomultiplierLight 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第26页/共47页Summary:PhotomultiplierFirst to be invented(1936)Single photon detectionSensitive to m
16、agnetic fieldsExpensive and complicated structure第27页/共47页A remark image intensifiersA 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
17、is coated with electron-emissive material.第28页/共47页Superconducting nano-wireUltra 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 puls
18、e.Healing time 30ps第29页/共47页SSPD meander configurationMeander structure increases the active area and thus the quantum efficiency第30页/共47页End of 1st part!第31页/共47页Hanbury Brown-Twiss Experiment(1)Back in the 1950s,two astronomers wanted to measure the diameters of stars第32页/共47页Hanbury Brown-Twiss E
19、xperiment(2)第33页/共47页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.第34页/共47页Coherence timeThe coherence time c is originated from ato
20、mic processesIntensity fluctuations of a beam of light are related to its coherence第35页/共47页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?)
21、第36页/共47页Correlations(2)For much greater than the coherence time:第37页/共47页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:第38页/共47页Correlations:exampleIf the spectral line is Doppler broa
22、dened with a Gaussian lineshape,the second order correlation functions is given by:第39页/共47页Summary:correlations in classical light第40页/共47页HBT experiments with photonsThe number of counts registered on a photon counting detector is proportional to the intensity第41页/共47页Photon bunching and antibunch
23、ingPerfectly coherent light has Poissonian photon statisticsBunched light consists of photons clumped together第42页/共47页Photon bunching and antibunchingIn antibunched light,photons come out with regular gaps between them第43页/共47页Experimental demonstration of photon antibunchingAntibunching effects ar
24、e only observed if we look at light from a single atom第44页/共47页Antibunching has been observed from many other types of light emittersExperimental demonstration of photon antibunching第45页/共47页BibliographyFundamentals 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)第46页/共47页感谢您的观看！第47页/共47页