Decaying memory principle

Additional restriction on G 2(t) is imposed by the decaying memory principle that has clear physical meaning the state of a medium at the present moment of time is more dependent on the stresses arising at t = tj than on that stresses arising at t = t, if t, < t2. This principle impliesfliatflie inequality / >0must be satisfied. 

Figure 11.13 Representation of fluorescence decay and of the principle of the measurement. On the graph, each point represents the contents of a memory channel (coupling time/numher of photons). The exponential curve of decay appears here in the form of a straight line. This is due to the choice of a logarithmic scale for the ordinate. On the right, a representation of the TCSPC (time-correlated single-photon counting).

The validity of the physics that adopts the point of view of decaying states depends on the characteristics of the process of excitation-preparation. Specifically, one must assume that the duration of the pulse of excitation energy is much shorter than the lifetime of the unstable state. This implies that indeed the system is prepared in a nonstationary state at f = 0, i.e., in the localized state (T o/ Eo)/ while losing memory of the excitation step. For long-lived unstable states, this is expected to be achievable easily. For shortlived unstable atomic or molecular states, say of the order of 10 s, this is also achievable, in principle, via modern pump-probe techniques with time-delays in the range of a few femtoseconds or of a couple of hundreds of attoseconds. 

Figure 4 TCSPC measurement principle. The random nature of the events recorded yields a luminescence decay histogram. The memory cells are memory addresses to record a photon at a specified time interval.

The most studied and technologically interesting band is the so-called S-band (S for slow decay times are rather long compared to those of direct bandgap semiconductors). It is electrically excitable and its properties (e.g., emission spectrum, efficiency, chemical activity) can be in principle engineered. Its main characteristics are summarized in Table 3. It originates mostly from exciton recombinations in Si nanocrystals as indicated by polarization memory of PL, PL saturation under... 

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