Single hoton Detectors单光子探测器

1、single photon detectorsby: kobi cohenquantum optics seminar25/11/09outlinena brief review of semiconductorsnp-type, n-typenexcitationsnphotodiodenavalanche photodiodengeiger modensilicon photomultipliers (sipm) nphotomultipliernsuperconducting wirencharacterization of single photon sourcesnhbt exper

2、imentnsecond order correlation functionsemiconductorscompoundssemiconductorsnelectrons and “holes”: negative and positive charge carriesnenergy-momentum relation of free particles, with different effective masssemiconductorsnthermal excitations make the electrons “jump” to higher energy levels, acco

3、rding to fermi-dirac distribution:1( )exp(/)exp()/ 1ektff ee kteektsemiconductorsnexcitations can also occur by the absorption of a photon, which makes semiconductors suitable for light detection:(t=300k)egap(ev)gap(nm)ge0.661880si1.111150gaas1.428701240()()e evnmenergy conservationmomentum conserva

4、tionphoton momentum is negligible k2k1useful to remember:intrinsic semiconductorsncharge carriers concentration in a semiconductor without impurities:n-type semiconductornsome impurity atoms (donors) with more valence electrons are introduced into the crystal:p-type semiconductornsome impurity atoms

5、 (acceptors) with less valence electrons are introduced into the crystal:the p-n junctionnelectrons and holes diffuse to area of lower concentrationnelectric field is built up in the depletion layerndrift of minority carriersncapacitancebiased p-n junctionnwhen connected to a voltage source, the i-v

6、 curve of a p-n junction is given by:well focus on reverse biasing:1. larger electric field in the junction2. extended space charge regionthe p-n photodiodenelectrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric fieldthe p-n photodiodenthe i

7、-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 the p-i-n junctionnlarger depletion layer allows improved efficiencynsmaller junction capacitance means fast respon

8、sedetectors: quantum efficiencynthe probability that a single photon incident on the detector generates a signal(1) 1 exp()rdlosses: reflectionnature of absorption a fraction of the electron hole pairs recombine in the junctiondetectors: quantum efficiencynwavelength dependence of :summary: p-n phot

9、odiodensimple and cheap solid state devicenno internal gain, linear responsennoise (“dark” current) is at the level of several hundred electrons, and consequently the smallest detectable light needs to consist of even more photonsavalanche photodiodenhigh reverse-bias voltage enhances the field in t

10、he depletion layernelectrons and holes excited by the photons are accelerated in the strong field generated by the reverse bias.ncollisions causing impact-ionization of more electron-hole pairs, thus contributing to the gain of the junction.avalanche photodiodep-n photodiodeavalanche photodiodesumma

11、ry: apdnhigh reverse-bias voltage, but below the breakdown voltage.nhigh gain (100), weak signal detection (20 photons)naverage photocurrent is proportional to the incident photon flux (linear mode)geiger modenin the geiger mode, the apd is biased above its breakdown voltage for operation in very hi

12、gh gain.nelectrons and holes multiply by impact ionization faster than they can be collected, resulting in an exponential growth in the currentnindividual photon countinggeiger mode quenchingnshutting off an avalanche current is called quenchingnpassive quenching (slower, 10ns dead time)nactive quen

13、ching (faster)summary: geiger modenhigh detection efficiency (80%).ndark counts rate (at room temperature) below 1000/sec. cooling reduces it exponentially.nafter-pulsing caused by carrier trapping and delayed release.ncorrection factor for intensity (due to dead time).silicon photomultipliersnsipm

14、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.nthe independently operating pixels are connected to the same readout linesipm: examplessummary: sipmnvery high gain (106)ndark counts: 1mhz/mm2 (2

15、0c) to 200hz/mm2 (100k)ncorrection factor (other than g-apd)photomultipliernphotoelectric 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 detectionphotomul

16、tipliernlight excites the electrons in the photocathode so that photoelectrons are emitted into the vacuum nphotoelectrons are accelerated due to between the dynodes, causing secondary emissionsummary: photomultipliernfirst to be invented (1936)nsingle photon detectionnsensitive to magnetic fieldsne

17、xpensive and complicated structurea remark image intensifierssuperconducting nano-wirenultra thin, very narrow nbn strip, kept at 4.2k and current-biased close to the critical current.na photon-induced hotspot leads to the formation of a resistive barrier across the sensor, and results in a measurab

18、le voltage pulse.nhealing time 30pssspd meander configurationnmeander structure increases the active area and thus the quantum efficiencyend of 1st part !hanbury brown-twiss experiment (1)nback in the 1950s, two astronomers wanted to measure the diameters of starshanbury brown-twiss experiment (2)ha

19、nbury brown-twiss experiment (3)nin their original experiments, hbt set =0 and varied d.nas d increased, the spatial coherence of the light on the two detectors decreased, and eventually vanished for large values of d.coherence timenthe coherence time c is originated from atomic processesnintensity

20、fluctuations of a beam of light are related to its coherencecorrelations (1)nwe shall assume from now on that we are testing the spatially-coherent light from a small area of the source.nthe second order correlation function of the light is defined by:(why second order?)correlations (2)nfor much gre

21、ater than the coherence time:correlations (3)non 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: examplenif the spectral line is doppler broadened with a gaussian lineshape, the second

22、order correlation functions is given by: summary: correlations in classical lighthbt experiments with photonsnthe number of counts registered on a photon counting detector is proportional to the intensityphoton bunching and antibunchingnperfectly coherent light has poissonian photon statisticsnbunched light consists of photons clumped