ABSTRACT
The main principles of contemporary time domain measurements using time-correlated single-photon counting have not changed since the early 1960s. However, the advances in instrumentation and measurement techniques have now allowed for a great improvement in time resolution and measurement of time. From what were once measurements involving substantial equipment performed in specialized research laboratories, time domain fluorescence techniques are becoming commonplace tools to study a variety of areas, ranging from biology to materials science. Advances in software have also allowed accessibility and ease of use of the technique. Time-resolved fluorescence has the advantage over steady-state measurements that the luminescence lifetime is independent of the sample concentration. Using single-photon counting (only available in the time domain) it is even possible to detect single-
molecule fluorescence. These factors make fluorescence well suited to the study of nanomaterials. Although a major effort has been to improve time resolution, it should not be forgotten that there is still an important area of investigation using “longer” timescales (µs and ms), for example, using lanthanides, with potential applications such as biological labels, nonlinear optical materials, and optical devices. The instrumentation has evolved over the years from using “simple” spark sources to femtosecond laser systems, producing an improvement in time resolution of several orders of magnitude. The advent of semiconductor light-emitting diodes and laser diodes, with wavelengths toward the ultraviolet region of the spectrum, means that relatively inexpensive and compact instruments are available to study diverse areas (e.g., protein fluorescence, semiconductor properties) at picosecond time resolution. Furthermore, application of time-tag techniques means that dynamic events can be followed. 8.1 IntroductionThe main principles of contemporary time domain measurements making use of time-correlated single-photon counting (TCSPC) have not significantly changed since the first use of a time-to-amplitude converter in the early 1960s by Bollinger and Thomas.1 Their system was employed to measure the scintillation response of crystals and glasses upon excitation with different forms of radiation. This approach was consolidated during the following decade and is now more usually associated with the measurement of fluorescence decay parameters after optical excitation. The principles of TCSPC have remained largely unaltered, but major advances have been made in improved instrumentation. Several comprehensive texts provide details concerning the main concepts behind this technique2-4, so only a brief overview will be given here. The major reason for the popularity of TCSPC is the great sensitivity that it affords depending on the detection of the arrival times of individual photons after an optical excitation pulse. At low signal levels the probability of detecting more than one photon in the detection window is small, and upon repeated excitation-collection cycles a histogram can be accumulated, which is indicative of the decay process. This is shown schematically in Fig. 8.1.
