The Streak Camera

Most optical centers show luminescence decay times in the nanoseconds-milliseconds range. However, many other physical processes involved in optical spectroscopy are produced in the picoseconds-femtoseconds range, and mnch more complicated instrumentation becomes necessary. For instance, interband Inminescence in solids, which is of particular interest in semiconductors, can involve decay times in the range of picoseconds. Pulses generated from solid state lasers have already reached this femtosecond domain. 

In this section, we will briefly describe the two main techniques devoted to detecting ultrashort pulses the streak camera and the auto correlator. 

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The preferable excitation source is an Nd-YAG laser because of its bandwidth (which is only about 0.3 ps), its easier mode locking, and its wavelength range. The use of an ultrafast laser creates the need for time resolution in a similarly short regime. The fastest photodiodes and oscilloscopes cannot resolve times <50 ps, and so other methods have been developed. One of them is the streak camera but it is not all that fast (0.5-5 ps resolution), and it is none too sensitive to small signals. 

Figure 1. A schematic diagram of the streak camera system for laser ablation.

The streak camera is especially suitable for the analysis of signals within a timescale of the order of picoseconds. The operational scheme of a streak camera is shown in Figure 3.27. 

The jitter between the laser pulse and the electron pulse was estimated from the measurement using a streak camera (C1370, Hamamatsu Photonics Co. Ltd.), because the jitter is one of important factors that decide the time resolution of the pulse radiolysis. The jitter was several picoseconds. To avoid effects of the jitter on the time resolution, a jitter compensation system was designed [74]. The time interval between the electron pulse (Cerenkov light) and the laser pulse was measured by the streak camera at every shot. The Cerenkov radiation was induced by the electron pulse in air at the end of the beam line. The laser pulse was separated from the analyzing light by a half mirror. The precious time interval could be... 

Booster was used to initiate the expl at one end. The radial motion of the cylinder wall was measured in a plane perpendicular to the cylinder axis 7 inches from the booster end, A streak camera recorded the motion, using conventional shadowgraph technique (See under CAMERAS, HIGH-SPEED PHOTOGRAPHIC in Vol 2 of Encycl, p CI3-L). In addn, the deton velocity of the expl was measured by placing pin switches 9-i° apart on the surface of the cylinder, as described, for example, by Cook (Ref 2, p 29). The streak camera record was read on a precision comparator which punched out the data directly on IBM cards. A computer code con-... 

In order to accurately determine the speed of the flash-across phenomenon, the experiment was repeated and recorded by streak camera with color film. Also thinner SPHF plates were used. In the streak camera trace, 8.5 (isec after each initial wave entered the NM, a hot spot appeared at the surface of each plate and flashed to the center of the chge each at the phenomenal speed of 35 mm/fisec. Cook (Ref 3) considers the flash-across phenomenon to be the heat pulse predicted by M.A. Cook, R. Keyes A.S. Filler (Ref 1)... 

The streak camera viewed the chge upward thru a periscope in which the line of sight was reflected to a horizontal direction by a front surface mirror. Measurements of the peak pressures by the aquarium technique were found to be the C-J or detonation pressures of the thermohydrodynamic theory... 

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. 

The streak camera gives a time resolution of about 5 ps. It requires a rather complex calibration procedure since the incident light intensity appears as the thickness of trace on the screen. It is used mostly for luminescence kinetics measurements, one of its advantages being that it can record single events. 

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. 

Figure 8.2 Principle of the streak camera. w, window p, photocathode a, accelerating plates f focussing plates d, deflection plates i, image intensifier c, camera...

The experimental decays iB(t) of the 350 nm band have been compared with curves calculated (solid lines in Fig. 5.1) by adjusting the parameters t" and r° in Eqs. (4.218) and (4.219) the spontaneous decay rate kr has been approximated by the value kB = kf + kB measured in a nonpolar solvent. It should be noted that with the photon-counting detection method the investigation of the fast initial nonexponential decay is hindered at low viscosity by poor resolution and only the exponential part of the decay is observable. At high viscosities (i7>100cp) the deviation from an exponential law is clearly visible. For the streak camera measurements the observations are opposite to those previously mentioned at high viscosities the semilogarithmic plot of iB(f) appears linear, whereas at low viscosities the decay shows nonexponential behavior. In Fig. 5.2 are represented the actual B decays calculated with the best fit values of the two relaxation times t° and r". Their variation with the temperature has also been examined Fig. 5.3 shows that they follow well those of -q/T and 17, respectively, as expected from the expressions (4.216) and (4.220) ... 

A good fit (Fig. 5.7) was obtained between the streak camera experimental data iA(t) [collected in propanol in the same temperature range as iB(0] and curves calculated with Eq. (5.9) by using the values of and t" that fit iB(t). 

Figure 9. Picosecond photoluminescence maps of opal CT (left pannel) and opal A (right pannel). The PL intensity z is plotted in logarithmic scale and increases from blue colors to red colors. Zero-time delay corresponds to the beginning of the streak camera sweep. Opal CT "common" is from Mapimi, Mexico opal A "noble" is from Lightning Ridge, Australia.

Microseconds to nanoseconds As previously mentioned, gated intensified vidicons can provide temporal resolutions as low as 40 nsec. New intensifiers, e.g., microchannel plates, may extend this range to 1-5 nsec or less. However, by far, the most useful tool for ns to ps spectroscopy is the streak camera, i.e., an ultra-rapid temporal-to-spatial electronic image sweeper (16). Streak cameras convert an optical signal, e.g., a spectrum, into... 

The techniques that have been developed to probe the kinetics of energy transfer processes In materials on a picosecond time scale can be divided into three general categories. They are the optical Kerr gate, the excite and probe technique, and the streak camera technique. 

Figure 4. Fluorescence kinetics of erythrosin in water measured by the streak camera-OMA system. The decay is a single exponential with a decay time of 78...

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... 

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