Detectors streak camera

Lifetimes shorter than a nanosecond can be measured using picosecond lasers with suitable detectors (streak camera) [3], bearing in mind that, as a rule of thumb, the cost of the equipment is inversely proportional to its time resolution. However, the measurement of lifetimes shorter than a nanosecond is most commonly performed with a single photon apparatus (see Sect. 7.2.3). Lasers with pulse duration shorter than 100 femtoseconds (1 fs = 1 x 10 s) are also available, but with such equipment the sample emission eannot be monitored for technical reasons, and transient absorption must be measured instead (see Chap. 8). 

An optical detector with appropriate electronics and readout. Photomultiplier tubes supply good sensitivity for wavelengths in the visible range, and Ge, Si, or other photodiodes can be used in the near infrared range. Multichannel detectors like CCD or photodiode arrays can reduce measurement times, and a streak camera or nonlinear optical techniques can be used to record ps or sub-ps transients. 

Lifetime instruments using a streak camera as a detector provide a better time resolution than those based on the single-photon timing technique. However, streak cameras are quite expensive. In a streak camera, the photoelectrons emitted... 

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. 

In the nanosecond (ns) time-scale the use of kinetic detection (one absorption or emission wavelength at all times) is much more convenient than spectrographic detection, but the opposite is true for ps flash photolysis because of the response time of electronic detectors. Luminescence kinetics can however be measured by means of a special device known as the streak camera (Figure 8.2). This is somewhat similar to the cathode ray tube of an oscilloscope, but the electron gun is replaced by a transparent photocathode. The electron beam emitted by this photocathode depends on the incident light intensity I(hv). It is accelerated and deflected by the plates d which provide the time-base. The electron beam falls on the phosphor screen where the trace appears like an oscillogram in one dimension, since there is no jy deflection. The thickness of the trace is the measurement of light intensity. 

Sampling techniques are not as fast as the streak camera because the response time of the detectors is a limiting factor. The interpretation of the data is however much simpler and does not require complex computer programs. 

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. 

The development of the ultrafast streak camera (8) in the early 1970 s provided a continuous time base for the detection of transient photon signals within the picosecond timescale. Almost immediately the usefulness of image detectors became apparent. Instead of recording streak camera events on film, coupling of the streak camera through an image intensifier to an optical... 

When the experimental system emits light after the initial pumping pulse, quite different techniques can be used to obtain a time-resolved spectrum of the sample emission. The simplest of these is time-correlated single photon counting. The time resolution of this technique is limited by the design of the photon detectors. Two other methods used in emission spectroscopy are the streak camera and... 

A few reports have appeared on combining the streak camera temporal dispersion with polychromators and three dimensional optical multichannel detection. This approach yields three dimensional fluorescence data for each laser pulse. With the present technological limitations of three dimensional detectors and streak cameras, however, data of this type suffer from low wavelength resolution. As detector and streak camera technology improve, this technique may become the method of choice for time and wavelength resolved emission spectra. 

Linearity. A common source of error for picosecond time-resolved emission spectrometers is a non-linear response of the detector to emission intensity. Streak camera temporal dispersors, for example, exhibit a limited dynamic range, which, in unfavorable cases, can lead to severe experimental artifacts (20-21). 

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. 

Fic. 8. Schematic diagram showing a lifetime apparatus using a synchronously pumped laser as excitation source and a synchroscan streak camera as detector. 

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