SinglePhotonDetectors單光子探測器學(xué)習(xí)課程

1、SemiconductorsCompounds第1頁/共47頁第一頁，編輯于星期六：二十點 三十八分。Semiconductors electrons and “holes”: negative and positive charge carries Energy-momentum relation of free particles, with different effective mass第2頁/共47頁第二頁，編輯于星期六：二十點 三十八分。Semiconductors Thermal excitations make the electrons “jump” to higher en

2、ergy levels, according to Fermi-Dirac distribution:1( )exp(/)exp()/ 1EkTff EE kTEEkT第3頁/共47頁第三頁，編輯于星期六：二十點 三十八分。Semiconductors Excitations 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(

3、)()E eVnmEnergy conservationMomentum conservationphoton momentum is negligible k2k1useful to remember:第4頁/共47頁第四頁，編輯于星期六：二十點 三十八分。Intrinsic Semiconductors Charge carriers concentration in a semiconductor without impurities:第5頁/共47頁第五頁，編輯于星期六：二十點 三十八分。N-type Semiconductor Some impurity atoms (donors)

4、 with more valence electrons are introduced into the crystal:第6頁/共47頁第六頁，編輯于星期六：二十點 三十八分。P-type Semiconductor Some impurity atoms (acceptors) with less valence electrons are introduced into the crystal:第7頁/共47頁第七頁，編輯于星期六：二十點 三十八分。The P-N Junction Electrons and holes diffuse to area of lower concentr

5、ation Electric field is built up in the depletion layer Drift of minority carriers Capacitance第8頁/共47頁第八頁，編輯于星期六：二十點 三十八分。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:1. larger electric field in the junction2.extend

6、ed space charge region第9頁/共47頁第九頁，編輯于星期六：二十點 三十八分。The P-N photodiode Electrons and holes generated in the depletion area due to photon absorption are drifted outwards by the electric field第10頁/共47頁第十頁，編輯于星期六：二十點 三十八分。The P-N photodiode The i-V curve in the reverse-biased P-N junction is changed by t

7、he photocurrentReverse biasing:Electric field in the junction increases quantum efficiencyLarger depletion layerBetter signal 第11頁/共47頁第十一頁，編輯于星期六：二十點 三十八分。The P-I-N junction Larger depletion layer allows improved efficiency Smaller junction capacitance means fast response第12頁/共47頁第十二頁，編輯于星期六：二十點 三十

8、八分。Detectors: Quantum Efficiency The 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 junction第13頁/共47頁第十三頁，編輯于星期六：二十點 三十八分。Detectors: Quantum Efficiency Wavelength d

9、ependence of :第14頁/共47頁第十四頁，編輯于星期六：二十點 三十八分。Summary: P-N photodiode Simple and cheap solid state device No internal gain, linear response Noise (“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頁/共47頁

10、第十五頁，編輯于星期六：二十點 三十八分。Avalanche photodiode High reverse-bias voltage enhances the field in the depletion layer Electrons 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 contributi

11、ng to the gain of the junction.第16頁/共47頁第十六頁，編輯于星期六：二十點 三十八分。Avalanche photodiodeP-N photodiodeAvalanche photodiode第17頁/共47頁第十七頁，編輯于星期六：二十點 三十八分。Summary: APD High reverse-bias voltage, but below the breakdown voltage. High gain (100), weak signal detection (20 photons) Average photocurrent is propor

12、tional to the incident photon flux (linear mode)第18頁/共47頁第十八頁，編輯于星期六：二十點 三十八分。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 exponentia

13、l growth in the current Individual photon counting第19頁/共47頁第十九頁，編輯于星期六：二十點 三十八分。Geiger mode quenching Shutting off an avalanche current is called quenching Passive quenching (slower, 10ns dead time) Active quenching (faster)第20頁/共47頁第二十頁，編輯于星期六：二十點 三十八分。Summary: Geiger mode High detection efficiency

14、 (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頁/共47頁第二十一頁，編輯于星期六：二十點 三十八分。Silicon Photomultipliers SiPM is an array of microcell aval

15、anche 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頁/共47頁第二十二頁，編輯于星期六：二十點 三十八分。SiPM: Examples第23頁/共47頁第二十三頁，編輯于星期六：二十點 三十八分。Summary: SiPM Very high ga

16、in (106) Dark counts: 1MHz/mm2 (20C) to 200Hz/mm2 (100K) Correction factor (other than G-APD)第24頁/共47頁第二十四頁，編輯于星期六：二十點 三十八分。Photomultiplier Photoelectric effect causes photoelectron emission (external photoelectric effect)For metals the work function W 2eV, useful for detection in the visible and UV

17、. For semiconductors can be 1eV, useful for IR detection第25頁/共47頁第二十五頁，編輯于星期六：二十點 三十八分。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第26頁/共47頁第二十六

18、頁，編輯于星期六：二十點 三十八分。Summary: Photomultiplier First to be invented (1936) Single photon detection Sensitive to magnetic fields Expensive and complicated structure第27頁/共47頁第二十七頁，編輯于星期六：二十點 三十八分。A remark image intensifiers A microchannel plate is an array consists of millions of capillaries (10 um diamet

19、er) 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28頁/共47頁第二十八頁，編輯于星期六：二十點 三十八分。Superconducting nano-wire Ultra thin, very narrow NbN strip, kept at 4.2K and current-bi

20、ased 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 pulse. Healing time 30ps第29頁/共47頁第二十九頁，編輯于星期六：二十點 三十八分。SSPD meander configuration Meander structure increases the active area and thus the

21、 quantum efficiency第30頁/共47頁第三十頁，編輯于星期六：二十點 三十八分。End of 1st part !第31頁/共47頁第三十一頁，編輯于星期六：二十點 三十八分。Hanbury Brown-Twiss Experiment (1) Back in the 1950s, two astronomers wanted to measure the diameters of stars第32頁/共47頁第三十二頁，編輯于星期六：二十點 三十八分。Hanbury Brown-Twiss Experiment (2)第33頁/共47頁第三十三頁，編輯于星期六：二十點 三十

22、八分。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頁/共47頁第三十四頁，編輯于星期六：二十點 三十八分。Coherence time The coherence time c is originat

23、ed from atomic processes Intensity fluctuations of a beam of light are related to its coherence第35頁/共47頁第三十五頁，編輯于星期六：二十點 三十八分。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 li

24、ght is defined by:(Why second order?)第36頁/共47頁第三十六頁，編輯于星期六：二十點 三十八分。Correlations (2) For much greater than the coherence time:第37頁/共47頁第三十七頁，編輯于星期六：二十點 三十八分。Correlations (3) On the other and, for much smaller than the coherence time, there will be correlations between the fluctuations at the two tim

25、es. In particular, if =0 :第38頁/共47頁第三十八頁，編輯于星期六：二十點 三十八分。Correlations: exampleIf the spectral line is Doppler broadened with a Gaussian lineshape, the second order correlation functions is given by: 第39頁/共47頁第三十九頁，編輯于星期六：二十點 三十八分。Summary: correlations in classical light第40頁/共47頁第四十頁，編輯于星期六：二十點 三十八分。

26、HBT experiments with photons The number of counts registered on a photon counting detector is proportional to the intensity第41頁/共47頁第四十一頁，編輯于星期六：二十點 三十八分。Photon bunching and antibunchingPerfectly coherent light has Poissonian photon statisticsBunched light consists of photons clumped together第42頁/共47頁第四十二頁，編輯于星期六：二十點 三十八分。Photon bunching and