Photon detectors photo-diode

Figure 8. Photon-pair correlation analysis of single molecule emission, (a) The temporal separation of photon pairs can be analyzed by a Hanbury-Brown and Twiss experiment, where the fluorescence photon flux is divided by a 50/50 beamsplitter and detected by two avalanche photo diodes (APDs). By delaying the arrival time of signals from one detector, simultaneous photon events can be detected if the delay time is known, (b) Photon-pair correlation analysis of - 1000 molecules of Rhodamine 6G probed individually by the setup shown in (a). Single fluorescent molecules can only emit one molecule at a time (photon antibunching), which results in an anti-correlation of photon events for times shorter than the fluorescence lifetime. By fitting such a histogram, the fluorescence lifetime and the number of molecules probed in the excitation spot can be extracted. For an increasing number of molecules, the dip at time zero begins to become less well expressed, because the probability for simultaneous photon emission increases.

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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