TCSPC multidimensional

Technically, the photons of all detectors are combined into a common timing pulse line. Simultaneously, a detector number signal is generated that indicates in which of the detectors a particular photon was detected. The photon pulses are sent through the normal time measurement procedure of the TCSPC device. The detector numbers are used as a channel (or routing) signal for multidimensional TCSPC, routing the photons from the individual detectors into different waveform memory sections. The principle is illustrated in Fig. 3.2. 

The architecture shown in Fig. 3.12 is similar, but not identieal with the general architecture of multidimensional TCSPC deviees. However, the arehiteeture shown in Fig. 3.12 can be used for a whole family of deteetors see also Sect. 6.1.3, page 215. 

The structure in the time-tag mode is shown in Fig. 3.15. It contains the channel register, the time-measurement block, a macro time" clock, and the FIFO buffer for a large number of photons. It has some similarity to the multidimensional TCSPC described in the paragraphs above. In fact, many advanced TCSPC modules have both the photon distribution and the time-tag mode implemented, and the configuration can be changed by a software command [25]. The sequencer then turns into the macrotime clock, and the memory turns into the FIFO buffer. 

This difficulty is easily avoided by fluorescence lifetime detection. By using the sequential recording capability of multidimensional TCSPC, the fluorescence transients can be directly observed. A simple setup for recording the nonphotochemical quenching is shown in Fig. 5.31. 

Applications of single-wavelength TCSPC imaging to autofluorescence of tissue are described in [281, 282, 283, 428]. A commercial instrument for skin inspection has been designed by Jenlab, Jena, Germany. The Dermainspect is based on a Ti Sapphire laser, a fast optical scanner, and multidimensional TCSPC. The instrument is shown in Fig. 5.66. 

This is exactly what advanced multidimensional TCSPC is capable of. The scanning technique of advaneed TCSPC (see page 37) therefore almost perfectly fits the laser seanning mieroseope. 

It is commonly known that the proximity of the SNOM tip changes the fluorescence lifetime in the seanned point of the sample. Whether this effect makes lifetime imaging in a SNOM useless or particularly interesting is hard to say as tong as only a few results exist. However, multidimensional TCSPC may be one way to make use of the dependenee of the lifetime on the tip distance. At a t)q)ical vibration frequency of the tip of a few hundred kHz, the photons for different tip dis-tanee eould be routed into different memory blocks. The result would be several images for different tip distance. 

W. Becker, A. Bergmann, E. Haustein, Z. Petrasek, P. Schwille, C. Biskup, T. Anhut, I. Riemann, K. Koenig, Fluorescence lifetime images and correlation spectra obtained by multidimensional TCSPC, Proc. SPIE 5,700 (2005)... 

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