Specifications

Anti-Bunching 29
Anti-Bunching
Anti-bunching information is contained in picosecond fluorescence correlation data, see Fig.
41. However, anti-bunching curves can also be recorded via the classic Hanbury-Brown-Twiss
start-stop experiment [14]. The light is split in two or more components and fed into separate
detectors. The photons of one component are used as start pulses, the photons of another de-
tector as stop pulses. The photon pulses in the stop channels must be delayed in order to ob-
tain positive start-stop times. A delay can be introduced by increasing the optical path length,
by increasing the cable length, or, in the DPC-230, by defining a TDC offset in the system
parameters. Except for the delay, the optical and the electrical setup are the same as for fluo-
rescence correlation. The difference is only in the interpretation of the data. Picosecond FCS
calculates cross-correlation between different detectors, the classic start-stop experiment
builds up histograms of the times between the photons recorded in different detectors.
DPC system parameters for classic anti-bunching are shown in Fig. 42. In the simplest case,
the DPC builds up the start-stop histograms online, without saving any data of the individual
photons Fig. 42, left. However, the DPC can also build up FCS curves simultaneously with
the start-stop histograms (Fig. 42 middle and right). The single-photon data can be discarded
(Fig. 42, middle), or saved for later analysis (Fig. 42, middle).
Fig. 42: System parameters for classic start-stop experiments. Left: Histograms built up online, single-photon
data discarded. Middle: Histograms and FCS curves built up online, single photon data discarded. Right: Histo-
grams and FCS curves built up online, single-photon data saved.
The input configuration panel is shown in Fig. 43. In the example shown two detectors are
used. The start detector is connected to channel 4 (LVTTL Input 2), and declared as ‘Refer-
ence’. The second detector is connected to channel 6 (LVTTL channel 4) and used as a stop
detector. More stop detectors can be used, and declared as ‘Input’ channels.