Single-photon APD

Nevertheless, commercially available single photon APDs (RCA C30921S and variants and EG G SPCM 100/200PQ modules) with a more workable active diameter of 500 gm (0.2 mm square area) have already been proved useful in fluorescence measurements/14,32) in spite of having a slower impulse response of ca. 400 psec... 

A little work has been reported on time-correlated measurements with germanium APDs/104 105) showing the potential for extending single-photon APD fluorescence lifetime measurements up to 1.7 mwith picosecond resolution. 

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. 

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. 

Most FCS setups therefore use single photon avalanehe photodiodes [323,424], usually SPCM-AQR detectors from Perkin Elmer [408]. These deteetors have a quantum efficiency that reaches 80% at 800 nm. However, single photon APDs often have a timing delay and transit time jitter dependent on wavelength and count rate. The changes can be of the order of 1 ns. Recording a fluorescence decay curve under this condition delivers questionable results. 

The optical setup is similar to that shown in Fig. 5.129, page 198. A frequeney-doubled YAG laser is used for excitation. The wavelength is 632 nm, the pulse width 150 ps, the repetition rate 76 MHz. An NA =1.3 oil immersion lens foeuses the laser into the sample. The sample is mounted on a piezo-driven sean stage. The fluoreseence is collected back through the microscope lens and separated from the exeitation by a dichroic mirror. A second dichroic mirror splits the fluoreseenee into two wavelength intervals, which are detected by two single-photon APD... 

APDs suitable for single photon detection must be free of premature breakdown at the edge of the junction or at local lattice defects. So far, only selected silicon APDs can be operated in the passive or active quenching mode, and only a few single photon APD detectors are commercially available [245, 354, 408]. 

Passively and actively quenched single-photon APDs must normally be cooled. Except for diodes of extremely small area the thermal carrier generation rate at... 

Fig. 6.55 TCSPC response of an actively quenched single photon APD (SPCM-AQR). Left IRF measured at 650 nm, 50 kHz, and 500 kHz count rate, 1 ns/div. Right Autocorrelation of the photon pulses measured at a count rate of 10 kHz...

Detector modules with internal diseriminators, sueh as the Hamamatsu H7421 PMT modules or the Perkin Elmer SPCM-AQR single photon APD modules, deliver stable output pulses without amplitude jitter. The timing performance is defined by the internal diseriminator of the deteetor module, not by the CFD of the TCSPC device. Thus changing the CFD eonfiguration does not improve the time resolution of these detectors. 

An additional push can be expected from new technical developments in TCSPC itself. The largest potential is probably in the development of new detectors. The introduction of direct (wide-field) imaging techniques is clearly hampered by the limited availability of position-sensitive detectors. In addition the selection of multianode PMTs is still very limited, especially for NIR-sensitive versions. Large-area detectors with 64 or more channels may result in considerable improvements in DOT techniques. Single photon APDs with improved timing stability are urgently required for single-molecule spectroscopy and time-resolved microscopy. 

Single-photon silicon APDs possess a quantum efficiency ofca. 20-40% between 700 and 900 nm which compares very favorably with ca. 3% at best expected from an S20R or SI photocathode over this range. The lack of late-pulsing in an APD response as compared with a linear focused photomultiplier also has some virtues in the reconvolution analysis of fluorescence decay curves. 

Phase-modulation fluorometry has been performed with APDs to a lesser extent than has single-photon timing, but nevertheless there are some reports of this combination006, 107)... 

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

APD Multi-photon diffusion FCS/PCH [73] Single-photon diffusion FRET [49] 3 colour FRET [65] Lifetime measurements [69]... 

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