SinglePhotonDetectors單光子探測(cè)器課件

1、Single Photon DetectorsBy: Kobi CohenQuantum Optics Seminar25/11/09第1頁，共48頁。OutlineA brief review of semiconductorsP-type, N-typeExcitationsPhotodiodeAvalanche photodiodeGeiger ModeSilicon Photomultipliers (SiPM) PhotomultiplierSuperconducting WireCharacterization of single photon sourcesHBT Experim

2、entSecond order correlation function第2頁，共48頁。SemiconductorsCompounds第3頁，共48頁。Semiconductorselectrons and “holes”: negative and positive charge carriesEnergy-momentum relation of free particles, with different effective mass第4頁，共48頁。SemiconductorsThermal excitations make the electrons “jump” to highe

3、r energy levels, according to Fermi-Dirac distribution:第5頁，共48頁。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 momentu

4、m is negligible k2k1useful to remember:第6頁，共48頁。Intrinsic SemiconductorsCharge carriers concentration in a semiconductor without impurities:第7頁，共48頁。N-type SemiconductorSome impurity atoms (donors) with more valence electrons are introduced into the crystal:第8頁，共48頁。P-type SemiconductorSome impurity

5、 atoms (acceptors) with less valence electrons are introduced into the crystal:第9頁，共48頁。The P-N JunctionElectrons and holes diffuse to area of lower concentrationElectric field is built up in the depletion layerDrift of minority carriersCapacitance第10頁，共48頁。Biased P-N junctionWhen connected to a vol

6、tage 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 region第11頁，共48頁。The P-N photodiodeElectrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric field第

7、12頁，共48頁。The P-N photodiodeThe i-V curve in the reverse-biased P-N junction is changed by the photocurrentReverse biasing:Electric field in the junction increases quantum efficiencyLarger depletion layerBetter signal 第13頁，共48頁。The P-I-N junctionLarger depletion layer allows improved efficiencySmalle

8、r junction capacitance means fast response第14頁，共48頁。Detectors: Quantum 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15頁，共48頁。Detectors: Quantum Efficien

9、cyWavelength dependence of :第16頁，共48頁。Summary: P-N photodiodeSimple and cheap 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17頁，共48頁。Avalanch

10、e photodiodeHigh 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-ionization of more electron-hole pairs, thus contributing to the gain of the junction.

11、第18頁，共48頁。Avalanche photodiodeP-N photodiodeAvalanche photodiode第19頁，共48頁。Summary: APDHigh 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20頁，共48頁。Geiger modeIn th

12、e 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 counting第21頁，共48頁。Geiger mode quenchingShutting off an

13、avalanche current is called quenchingPassive quenching (slower, 10ns dead time)Active quenching (faster)第22頁，共48頁。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 d

14、elayed release.Correction factor for intensity (due to dead time).第23頁，共48頁。Silicon PhotomultipliersSiPM 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 con

15、nected to the same readout line第24頁，共48頁。SiPM: Examples第25頁，共48頁。Summary: SiPMVery high gain (106)Dark counts: 1MHz/mm2 (20C) to 200Hz/mm2 (100K)Correction factor (other than G-APD)第26頁，共48頁。PhotomultiplierPhotoelectric effect causes photoelectron emission (external photoelectric effect)For metals t

16、he work function W 2eV, useful for detection in the visible and UV. For semiconductors can be 1eV, useful for IR detection第27頁，共48頁。PhotomultiplierLight excites the electrons in the photocathode so that photoelectrons are emitted into the vacuum Photoelectrons are accelerated due to between the dyno

17、des, causing secondary emission第28頁，共48頁。Summary: PhotomultiplierFirst to be invented (1936)Single photon detectionSensitive to magnetic fieldsExpensive and complicated structure第29頁，共48頁。A remark image intensifiersA microchannel plate is an array consists of millions of capillaries (10 um diameter)

18、 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.第30頁，共48頁。Superconducting nano-wireUltra thin, very narrow NbN strip, kept at 4.2K and current-biased close to the critical

19、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第31頁，共48頁。SSPD meander configurationMeander structure increases the active area and thus the quantum efficiency第32頁，共48頁。End of 1st part !第33頁，

20、共48頁。Hanbury Brown-Twiss Experiment (1)Back in the 1950s, two astronomers wanted to measure the diameters of stars第34頁，共48頁。Hanbury Brown-Twiss Experiment (2)第35頁，共48頁。Hanbury Brown-Twiss Experiment (3)In their original experiments, HBT set =0 and varied d.As d increased, the spatial coherence of th

21、e light on the two detectors decreased, and eventually vanished for large values of d.第36頁，共48頁。Coherence timeThe coherence time c is originated from atomic processesIntensity fluctuations of a beam of light are related to its coherence第37頁，共48頁。Correlations (1)We shall assume from now on that we ar

22、e 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?)第38頁，共48頁。Correlations (2)For much greater than the coherence time:第39頁，共48頁。Correlations (3)On the other and, for much smaller than the coherenc

23、e time, there will be correlations between the fluctuations at the two times. In particular, if =0 :第40頁，共48頁。Correlations: exampleIf the spectral line is Doppler broadened with a Gaussian lineshape, the second order correlation functions is given by: 第41頁，共48頁。Summary: correlations in classical light第42頁，共48頁。HBT experiments with photonsThe number of counts registered on a photon counting detector is proportional to the intensity第43頁，共48頁。Photon bunching and antibunchingPerfectly coherent light has Poissonian photon statisticsBunched light consist