Gated photon counting

In time-gated photon counting, comparatively high photon count rates can be employed count rates as high as 10 MHz are often used. TG has the advantage of virtually no dead-time of the detection electronics ( 1 ns), whereas the dead-time of the TCSPC electronics is usually on the order of 125-350 ns. This causes loss of detected photons, and a reduced actual photon economy of TCSPC at high count rates. 

Fig. 16.14 (a) Setup for TERS with time-gated detection and its (b) schematic of the dual-gating photon-counting scheme (Reprinted from [66], Copyright 2007, with permission from American Institute of Physics), (c) Setup for TERS with time-gated illumination and its (d) schematic of the gating relative to the tip oscillation (Reprinted from [67], Copyright 2009, with permission from American Physical Society)... 

In practice the gate is often combined with the first counter stage. The principle is shown in Fig. 2.3. Although this figure is somewhat technical, it is essential to fully understand gated photon counting. 

The maximum count rate of a gated photon counting system can be very high. The discriminators, the gating circuitry, and the eounter ean be made as fast as 1 GHz. In practice the count rate is limited by the deteetor. With fast PMTs, a peak count rate of several hundred MHz ean be achieved. Of eourse, rates this... 

Fig. 2.6 Gated photon counting with several parallel gates...

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. 

A.J. Alfano, F.K. Fong, F.E. Lytle, High repetition rate subnanosecond gated photon counting, Rev. Sci. Instrum. 54, 967-972 (1983)... 

The experimental apparatus is depicted in Fig. 3 and is described in detail elsewhere [14]. It consists of a molecular beam scattering chamber, pulsed frequency-doubled dye laser, double fresnel rhomb polarization rotator, fibre bundles to coUect fluorescence from excited states, and time-gated photon counting and boxcar averaging equipment. 

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