Photon avalanche effect

Joubert ME, Guy S, Jacquier B, Linares C (1994) The photon-avalanche effect review model and application. Opt Mater 4 43 9... 

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

In describing this effect as an avalanche excitation mechanism, it is clear that the details of the process differ from those of the Photon Avalanche described in Sect. 9 since,being ultimately a single-ion effect, this mechanism does not involve runaway cross relaxation as an essential step, but is instead intimately related to temperature effects. Within the avalanche formalism, this mechanism is best described as a thermal avalanche, in which high excitation powers result in runaway sample heating rather than runaway cross relaxation. This mechanism is illustrated schematically in Fig. 17 a. The dashed fines in Fig. 17 a show the isothermal excitation behaviors for two internal sample temperatures, and... 

The sample is a fine cylinder usually comprised of a capillary filled with the powder we wish to study. It can also be made of a thin wire, particularly when the material we wish to study is a metal. In any case, this sample is placed in the center of a curved, position sensitive gas detector used to simultaneously detect all of the diffracted beams. We saw before that when these gas detectors are irradiated with X photons, the gas is ionized and a local avalanche effect takes place which leads to the ionization of the neighboring atoms. The size of this ionized zone depends on... 

Electron multiplier tubes are similar in design to photomultiplier tubes. They consist of a primary cathode and a series of biased dynodes that eject secondary electrons. Therefore, any incident charged particle induces a multiplied electron current. A channeltron is a hom-shaped continuous dynode stmcture that is coated on the inside with an electron-emissive material. Any charged particle, but also high-energy U Vor X-ray photons, striking the channeltron creates secondary electrons that have an avalanche effect to create the final current. 

Not only PMTs and other detectors such as avalanche photodiodes suffer from dead-time effects also the detection electronics may have significant dead-times. Typical dead-times of TCSPC electronics are in the range 125-350 ns. This may seriously impair the efficiency of detection at high count rates. The dead-time effects of the electronics in time-gated single photon detection are usually negligible. 

The most important light detector in photochemistry is the photomultiplier (PM) tube. It is based on the photoelectric effect (section 2.1), but the primary electrons released by light are accelerated over a number of dynodes to produce an avalanche of secondary electrons (Figure 7.24). A single photon can produce a pulse of some 106 electrons at the anode. Each of these pulses lasts about 5 ns, so that when the light intensity is rather high these single pulses combine to form a steady electric current. This current is amplified and displayed on a chart recorder or computer. 

Electrons, generated by the photoelectric effect, upon the absorption of an X-ray photon, are swept towards the positively charged anode (see Fig. 1). If E(r) is sufficiently high, the drifting electrons acquire enough kinetic energy to ionize other neutral gas atoms by collision. Thus, secondary charges are created in the form of an avalanche. This is the basis of gas amplification. 

A troublesome effect in photon correlation experiments is light emission from single photon APD detectors. When an avalanche is triggered in the APD, a small amount of light is emitted. The effect and its implications for photon correlation experiments and quantum key distribution are described in detail in [515] and [299]. If the detectors are not carefully optically decoupled, false coincidence peaks appear. An example is shown in Fig. 5.105. 

The second photon effect of general utility is the photovoltaic effect. Unlike the photoconductive effect, it requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. Although it is possible to have an extrinsic photovoltaic effect, see Ryvkin [2.32], almost all practical photovoltaic detectors employ the intrinsic photoeffect. Usually this occurs at a simple p — n junction. However, other structures employed include those of an avalanche, p—i — n, Schottky barrier and heterojunction photodiode. There is also a photovoltaic effect occuring in the bulk. Each will be discussed, with emphasis on the p—n junction photoeffect. 

Brown and coworkers have tested avalanche photodiodes (APD) as replacements for PMTs. Preliminary tests were encouraging. Single photon counting was possible, though dead-time effects in the range of 1-2 /as limited the maximum count rate. Special active quenching circuitry has reduced this... 

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