Streak camera applications

In this final section, we summarize the operation and characteristics of the principal vacuum tube and solid state detectors that are available for red/near-IR fluorescence studies. These include conventional photomultipliers, microchannel plate versions, streak cameras, and various types of photodiodes. Detector applicability to both steady-state and time-resolved studies will be considered. However, emphasis will be placed on photon counting capabilities as this provides the ultimate sensitivity in steady-state fluorescence measurements as well as permitting lifetime studies. 

The operation and application of streak cameras in fluorescence lifetime spectroscopy has been reviewed previously (see, e.g., Refs. 91 and 92). Streak cameras are useful in 2-D time-resolved imaging applications such as microscopy or multiwavelength array fluorometry. The operating principle is based on converting an optical pulse into a photoelectron pulse and spatially dispersing the electron image on a phosphor by means of a synchronized deflection voltage across two plates. 

By separating the coating from the substrate after deposition, the unique coating features of parylenes, especially continuity and thickness control and uniformity, can be imparted to a freestanding film. Applications include optical beam splitters, a window for a micrometeoroid detector, a detector cathode for an x-ray streak camera, and windows for x-ray proportional counters. 

With a possible resolution of less than 1 ps [65, 66], streak cameras offer the highest temporal resolution of any device currently available. The study of picosecond phenomena is a rapidly expanding field and the state-of-the-art in picosecond techniques was recently reviewed [lc]. Here we are concerned only with the general principles of streak cameras and their application to measuring fluorescence lifetimes. 

An excellent alternative to the streak camera approach is fluorescence time resolution by the "up-conversion" method, which we describe in detail below. In the simplest form of this technique, non-linear optical methods are employed to essentially construct a picosecond "shutter" or "gate". In most applications a single emission datum is acquired for each laser pulse, i.e. I(t=constant, X=constant). By repeating the experiment at different delay times, kinetic traces can be acquired that are of comparable quality to those obtained by streak camera methods (16-17). Alternatively, if delay time is held constant but X is scanned, high quality emission spectra can be obtained (3). 

Typically, in measurements of time-resolved luminescence in the time regime of tens of picoseconds, data obtained from 10 to 20 laser shots are averaged to improve the signal-to-noise ratio and to minimize the effects of shot-to-shot variations in the laser pulse energy and shape. Once the reliability of the data has been ensured by application of the corrections described above and made necessary by detector-induced distortions, the time-resolved fluorescence data is analyzed in terms of a kinetic model which assumes that the emitting state is formed with a risetime, xR, and a decay time, Tp. Deconvolution of the excitation pulse from the observed molecular fluorescence is performed numerically. The shape of the excitation pulse to be removed from the streak camera data is assumed to be the same as the prepulse shape, and therefore the prepulse is generally used for the deconvolution procedure. Figure 6 illustrates the quality of the fit of the time-dependent fluorescence data which can be achieved. 

In this article we discuss the implementation of an optical multichannel analyzer on a jitter-free streak camera which allows signal averaging without operator intervention at a repetition rate of 1 Hz and provides for acquisition routines which are specifically suited to this application. Several experimental examples are discussed which illustrate the features of the system. 

This technique can be directly employed at higher repetition rates, and recently operation at up to 500 Hz has been demonstrated. It should be noted that this deflection scheme is applicable to any streak camera. The switch requires 100 pJ of incident optical energy. 

A ps absorption-emission spectrometer design which uses both pump-probe and streak camera measurement with a single-mode locked Nd-YAG laser has been described in detail.The theory of non-stationary time-dependent emission measurement and its application to ultrafast processes has been exemplified by analysis of data on the fs time-resolved emission from dye molecules in water.The power of this experimental technique is exemplified by the... 

Some applications of time-resolved fluorimetry using streak cameras will now be considered in order to illustrate the various possibilities available to the photochemist. The ps decay of the acridine singlet state has been determined as a function of temperature and solvent using the third harmonic of a mode-... 

For many applications in the picosecond range the streak camera [11.21] can be used which may reach time resolutions of a few picoseconds. It consists essentially of a fast image intensifier (see Sect.4.5.5) where the... 

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