Picosecond Photon Correlation

The benefit of TCSPC wide-field imaging is that it can be easily adapted to almost any microscope or other optical system. It may also be a solution for samples that preclude, for whatever reason, scanning by a laser spot of high power density. 

Classic light sources emit photons randomly, independent of each other. Typical examples are thermal sources or fluorescence from a large number of molecules. The distribution of the time intervals between successive photons drops exponentially for increasing time intervals. 

For light generated by nonlinear optical effects and fluorescence of single molecules [22, 351], quantum dots [352, 499, 552, 557] or other semiconductor nanostructures this is not necessarily the case. Nonlinear optical effects can split one photon into two, which are then highly correlated. Single molecules cannot 

Optically driven photon correlation experiments normally require confining the detection or the excitation to an extremely small sample volume. This is achieved either by confocal detection or two-photon excitation in a microscope. The optical principles are the same as in confocal and two-photon laser scanning microscopes (see Sect. 5.7, page 129). However, most correlation experiments do not require scanning and can be performed in relatively simple microscopes. 

Picosecond photon correlation experiments have some similarities to fluorescence correlation spectroscopy (PCS). PCS investigates the fluctuations of the fluorescence intensity of a small number of molecules confined in a small sample volume (see Sect. 5.10, page 176). The intensity fluctuations are correlated on a time scale from microseconds to milliseconds. Therefore, PCS differs from picosecond correlation in the way the photons are correlated. Moreover, PCS effects are driven by diffusion, conformational changes, or other sample-internal effects, while antibunching is driven by the absorption of the photons of the excitation light. 

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The techniques described under Photon Correlation exploit the correlation between the photons emitted by single molecules and by a small number of molecules. Picosecond photon correlation techniques investigate effects driven by the absorption of a single photon of the excitation light. The effects investigated by FCS are driven by Brownian motion, rotation, diffusion effects, intersystem crossing, or conformational changes. Because of these random and essentially sample-internal stimulation mechanisms, correlation techniques do not necessarily depend on a pulsed laser. 

Sample preparation was given elsewhere [2]. Femtosecond fluorescence upconversion and picosecond time-correlated single-photon-counting set-ups were employed for the measurement of the fluorescence transients. The system response (FWHM) of the femtosecond fluorescence up-conversion and time-correlated single-photon-counting setups are 280 fs and 16 ps, respectively [3] The measured transients were fitted to multiexponential functions convoluted with the system response function. After deconvolution the time resolution was 100 fs. In the upconversion experiments, excitation was at 350 nm, the transients were measured from 420 nm upto 680 nm. Experiments were performed under magic angle conditions (to remove the fluorescence intensity effects of rotational motions of the probed molecules), as well as under polarization conditions in order to obtain the time evolution of the fluorescence anisotropy. 

Until recently we were unable to determine k for 1(4) and 1(6) via this method, since this not only requires a time resolution better than 10 ps, but especially since the quenching of the donor fluorescence, that accompanies the electron transfer, makes the measurements extremely sensitive to the presence of minor, fluorescent impurities. After careful recrystallization a sample of 1(6) was now obtained for which the level of impurity fluorescence is sufficiently low to detect the very short lived ( 3-4 ps) residual donor fluorescence. A typical fluorescence decay as observed for this sample in ethylacetate via picosecond time correlated single photon counting (Bebelaar, 1986) is shown in Fig. 3. Via a biexponential reconvolution procedure the lifetime of the short component was determined to be 3.5 0.5 ps, while that of the impurity background is comparable to the lifetime of the isolated donor (-4500 ps) and thus probably stems from one or more species lacking the acceptor chromophore. Similar results were obtained in tetrahydrofuran (3 1 ps) and in acetonitrile (4 lps). 

The correlation techniques described above use different approaches for photon correlation on the picosecond scale and FCS experiments. Although the same... 

From the standpoint of time domain (e.g., time-correlated single photon counting) experiments the method of modelocking is not too crucial as long as the pulse jitter is modest (some picoseconds), and the pulse intensity doesn t vary too much if the time-to-amplitude converter is being started instead of stopped by the excitation pulse, it may be immaterial. From the standpoint of the frequency domain, however, the... 

A little work has been reported on time-correlated measurements with germanium APDs/104 105) showing the potential for extending single-photon APD fluorescence lifetime measurements up to 1.7 mwith picosecond resolution. 

The time-resolved techniques that are usually used for FLIM are based on electronic-basis detection methods such as the time-correlated single photon counting or streak camera. Therefore, the time resolution of the FLIM system has been limited by several tens of picoseconds. However, fluorescence microscopy has the potential to provide much more information if we can observe the fluorescence dynamics in a microscopic region with higher time resolution. Given this background, we developed two types of ultrafast time-resolved fluorescence microscopes, i.e., the femtosecond fluorescence up-conversion microscope and the... 

Time-Correlated Single-Photon Counting. For the application of TCSPC in the picosecond time domain, lasers with pulses whose half-widths are 20 ps or less are used. For better time resolution, the combination of a microchan-nel plate photomultiplier tube (MCP-PMT) and a fast constant fraction discriminator (CFD) are used instead of a conventional photomultiplier tube (PMT). A TCSPC system with a time response as short as 40 ps has at its core a Nd YLF (neodymium yttrium lithium fluoride) laser generating 70-ps, 1053-nm pulses at... 

Figure 2.16 shows an example for such a biexponential decay measured with time correlated single-photon counting.78,75 Several picosecond laser experiments have explored this early time behavior of the equilibration... 

The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time-resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere. For the time-correlated single photon counting measurement of which the response time is a ut 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm 10 ps FWHM) was used to raesisure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. All the solvents of spectro-grade were used without further purification. 

The present limitatkins in time resolution for the time-correlated photon counting technique are due to the time jitter in the detection electronics and the transit time spread in the photomultqjlier tube ( 500 ps). Mth future improvements in these components and using cw mode-locked lasers as an excitation source, deconvolution of fluorescence lifetimes to a few tens of picoseconds oidd be achieved. Alter-... 

Unlike Klyshko, who said that the advanced wave is a -function pulse, we consider it to be a continuous, partially incoherent wave. The duration of the advanced wave is in fact determined by the uncertainty of the photon arrival time measurement. With modern detectors, it amounts to at least tens of picoseconds. If the down-conversion experiment is performed in an ultrashort pulsed setting, this uncertainty substantially exceeds the pump pulse width, so the advanced wave can be considered continuous. On the other hand, if the pump laser is continuous, the situation is more complicated and the timing uncertainty must be taken into account more rigorously in order to determine the correct correlation function of the DFG wave and the density matrix of the conditional single photon. 

The great sensitivity of fluorescence spectral, intensity, decay and anisotropy measurements has led to their widespread use in synthetic polymer systems, where interpretations of results are based upon order, molecular motion, and electronic energy migration (1). Time-resolved methods down to picosecond time-resolution using a variety of detection methods but principally that of time-correlated single photon counting, can in principle, probe these processes in much finer detail than steady-state techniques, but the complexity of most synthetic polymers poses severe problems in interpretation of results. 

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