Photons, avalanche

Upconversion excitation features not present in the GSA spectrum (indicative of an ESA step in the mechanism, see Sect. 3) 

Two regimes in the upconversion power dependence, with an extremely nonlinear intensity jump when pump powers rise above a certain critical threshold 

A lengthening of the time needed to achieve steady state conditions when using powers near the critical threshold 

A loss of transparency at the excitation wavelength when using high excitation powers 

The kinetics associated with the photon avalanche have been explored for several systems using rate equations analogous to those presented in Eq. (10), now including additional terms to account for cross-relaxation events. In this context, the critical ESA pumping rate constant, E, at which the population of level 1 via CR occurs rapidly enough to lead to an avalanche, is given by Eq. (32) [53]  

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Mosconi, D., Stoppa, D., Pancheri, L., Gonzo, L. and Simoni, A. (2006). CMOS single-photon avalanche diode array for time-resolved fluorescence detection. IEEE ESSCIRC. 564-67. 

Niclass, C., Gersbach, M., Henderson, R., Grant, L. and Charbon, E. (2007). A single photon avalanche diode implemented in 130-nm CMOS technology. IEEE J. Sel. Top. Quant. Electron 13, 863-9. 

Figure 12.25. Wavelength dependence in the temporal response of a single-photon avalanche photodiode.

S. Cova, A. Longoni and G. Ripamonti, Active-quenching and gating circuits for single-photon avalanche diodes (SPADS), IEEE Trans. Nucl. Sci. NS-29, 599-601 (1982). 

S. Cova, A. Lacaita, M. Ghioni, G. Ripamonti and T. A. Louis, 20ps timing resolution with single-photon avalanche diodes, Rev. Sci. Instrum. 60, 1104-1110(1989). 

In a laser system, the wave is initiated by spontaneous emission from the excited state atoms in the active medium. The spontaneously emitted photons traveling parallel to the resonator axis are able to create new photons by stimulated emission. Above the threshold they induce a photon avalanche, which grows until the depletion of the population inversion compensates the repopulation due to pumping. 

Upconversion lanthanide-containing nanophosphors, which emit higher-energy photons when excited by lower-energy photons have stirred increasing research interest in recent years. The predominant mechanisms of upconversion in nanophosphors are excited-state absorption (ESA), energy-transfer upconversion (ETU) and photon avalanche (PA) (Prasad, 2004 Auzel, 2005). In the ESA process, two photons are sequentially absorbed by the same ion,... 

Fig.13a-c. Schematic representation of the sequence of events in a simple three-level photon avalanche excitation mechanism a nonresonant GSA to generate one ion in level 1 b resonant ESA to generate one ion in level 2 c nonradiative cross relaxation to generate two ions in level 1... 

As described in Sect. 9, photon avalanches are only observed in cases where ai > Oq. In treating this case as an avalanche, the following question arises How does an ESA cross section become substantially greater than the GSA cross section in a cooperative luminescence system in which by definition the GSA and ESA steps are the same single-ion transition The answer to this question is... 

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... 

Whereas many examples of the photon avalanche phenomenon exist in the literature, only one study has been made for elpasolite systems [347], for Cs2NaGdCl6 Tm3+, where the blue upconverted emission is due to the 1G4 3H6 transition. However, the situation is rather more complex than in Fig. 24j because several other processes can occur, which also lead to emission from D2. Three features related to the 64 emission are highlighted here. First, Fig. 27a shows that a quadratic emission intensity-excitation power dependence is obtained at low excitation intensities for samples of Cs2NaGdCl6 Tm3+ doped with between 6-15 mol% Tm3+. However, a dramatic increase of the emission intensity appears above the excitation threshold value, ca. 9 kW cm 2. In Fig. 27a, the slope increases to 6 for the 10 mol% Tm3+-doped sample. Second, the time-dependence of the upconverted emission exhibits different behaviour at different excitation powers. A notable difference from other systems is that, at the threshold excitation power, Pthres, the blue emission has an almost linear rise-time which is followed by a further slower rise over several seconds. Third, at high excitation powers, the establishment of the stationary state is quicker, and the 3F4—>3G4 ESA decreases the transmitted laser light by several percent, Fig. 27b. 

Single photon Avalanche Photodiode. An avalanche photodiode (APD) is operated above the breakdown voltage. A detected photon causes an avalanche breakdown with an easily detected current pulse. SPAD operation requires an APD with uniform break-... 

Single photon Avalanche Photodiode, see SPAD. Time-to-Amplitude Converter. Converts the time between a start and a stop pulse into a voltage. TACs can be built with a resolution down to a few picoseconds. 

Like all photon counting techniques, gated photon counting uses a fast, high-gain detector, which is usually a PMT or a single-photon avalanche photodiode. Due to the moderate time resolution of the gating technique, there are no special requirements to the transit time spread of the detector. However, the transit time distribution should be free of bumps, prepulses or afterpulses, and should remain stable up to a count rate of several tens of MHz. 

Currently available single photon avalanche photodiodes (SPADs) are not applicable to optical tomography. Although the efficiency in the NIR can be up to 80%, the detector area is only of the order of 0.01 mm. Diffusely emitted light cannot be concentrated on such a small area. A simple calculation shows that SPADs carmot compete with PMTs unless their active area is increased considerably. Another obstacle is the large IRF count-rate dependence sometimes found in single-photon APDs. 

Single photon avalanche photodiodes (SPADs) achieve the highest radiant sensitivity of all detectors in the NIR. Currently available APD detectors have ex-... 

Recently new single-photon avalanche photodiodes have been introduced, see Sect. 6.4.10 page 258. Compared with the SPCM-AQR the new devices have a considerably improved timing behaviour but lower quantum efficiency in the NIR. However, the efficiency below 600 nm is comparable or even better than for the SPCM-AQR. It is likely though not proved that these detectors are superior to the SPCM-AQR for correlation measurements in the visible spectral range. 

Fig. 6.8 Single photon avalanche photodiode (SPAD). Left Passive quenching, right active quenching...

TTS exists also in single photon avalanche photodiodes (SPADs). The source of TTS in SPADs is the different depth at which the photons are absorbed, and the nonuniformity of the avalanche multiplication efficiency. This results in differing delays in the build-up of the carrier avalanche and in different avalanche transit times. Consequently the TTS depends on the wavelength and the voltage. Moreover, if a passive quenching circuit is used, the reverse voltage may not have completely recovered from the breakdown of the previous photon. The result is an increase of the TTS width or a shift of the TTS with the count rate. 

Compared to PMTs, single photon avalanche photodiodes (SPADs) have a considerably higher efficiency in the near infrared. Figure 6.17 compares the QE of a silicon SPAD module [408] with the QE of a GaAsP PMT [214]. 

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