Single photon counting avalanche

T. A. Louis, G. Ripamonti and A. Lacaita, Photoluminescence lifetime microscope spectrometer based on time-correlated single-photon counting with an avalanche diode detector, Rev. Sci Instrum. 61, 11-22(1990). 

The ability to obtain single-photon counting using methods such as avalanche photon detectors and negative electron affinity photocathode photomultiphers has thus far been limited to the visible and infrared regions. The vertical QCD which utihzes a triple-well quantum (Q) dot system of the type illustrated in Fig. 9 offers a novel approach to sense THz radiation. Here, the detector is first primed into active-mode by tunnel injection into the top Q-dot (QDl) of the SES followed by an IR pulse that puts the electron into the middle Q-dot (QD2) of the THz-RDC. This electron will remain in the QD2 until a THz photon induces the electron s transition to QD3. Finally, an IR photon ejects the electron from QD3 thus resetting the detector. Since the electron injection into the QCD system and ejection from the detector are quick transitions, only the middle Q-dot (QD2) will be occupied for a significant period of time. Consequently, to successfully read-out the state of our triple Q-dot system one must be able to differentiate between the two possible states (1) if THz photons are present, the electron will quickly be ejected from the entire QCD system and no electrons will be present in QD2 or (2) if no THz photon is present, QD2 will remain occupied by an electron. 

T. Louis, G.H. Schatz, P. Klein-Bolting, A.R. Holzwarth, G. Ripamonti, S. Cova, Performance comparison of a single-photon avalanche diode with a microchannel-plate photomultiplier in time-correlated single-photon counting, Rev. Sci. Instram. 59, 1148-1152 (1988)... 

Fig. 14 Typical time course of the fluorescence intensity of single Cy3 molecules. The fluorescence intensity was measured with an avalanche photodiode single photon counting module (SPCM-AQR-16, Perkin Elmer, Canada). Excitation wavelength, 514.5 nm. Sampling interval, 30 ms...

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... 

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. 

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. 

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. 

The single photon detector - typically an avalanche photo diode (APD) driven in counting mode- detects the arrival of fluorescence photons. The recorded photon trace is finally evaluated according to the chosen FFS method, i.e. the sequence of detection events is numerically processed for yielding information about the investigated sample. 

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