Summary
In the present work we discuss the possibility to measure fluorescence lifetimes by coupling pulsed excitation with phase-shift and amplitude-modulation detection techniques. We show that, because of the high harmonic content of narrow light pulses, the sample may be considered as simultaneously excited with a set of modulation frequencies up to the GHz region with noticeable power. By measuring then the phase-shift and modulation ratio of fluorescence with respect to the exciting light in the high-frequency region, impressive time resolutions may be achieved. On the other hand, since we dispose of a wide range of modulation frequencies, the problem of the multiexponential decay may be easily handled and has an exact analytical solution, provided the signal-to-noise ratio is good enough. We anticipate the possibility to introduce cross-correlation methods in order to perform the experiment in the very-low-frequency region. A short discussion about the notion of time resolution for this kind of measurement is also included.
Riassunto
Nel presente lavoro si discute la possibilità di misurare tempi di vita di stati fluorescenti eccitati da sorgenti periodiche impulsate usando misure di fase ed ampiezza nello spazio delle frequenze. Poiché un impulso molto stretto come quello che si ottiene da lampade impulsate o dalla luce emessa da un sincrotrone contiene armoniche di frequenza molto alta nella regione del gigahertz, si può pensare che il campione sia eccitato contemporaneamente da un insieme discreto di frequenze. Misurando ampiezza e fase di ciascuna armonica si può ottenere una risoluzione temporale dell'ordine del picosecondo. Inoltre il problema dell'eterogeneità del campione, cioè l'esistenza di diversi tempi di vita di fluorescenza, è facilmente risolto.
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Gratton, E., Lopez-Delgado, R. Measuring fluorescence decay times by phase-shift and modulation techniques using the high harmonic content of pulsed light sources. Nuov Cim B 56, 110–124 (1980). https://doi.org/10.1007/BF02738361
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DOI: https://doi.org/10.1007/BF02738361