Streak cameras

The basic principle of a streak camera is schematically depicted in Fig. 6.62. The optical pulse with the time profile I t) is focused onto a photocathode, where it produces a pulse of photoelectrons The photoelectrons are extracted into 

In Fig. 6.63 the design and operation principle of a modem streak camera is illustrated for the example of 4 short light pulses which are monitored on the phosphor screen. The trigger signal for the spatial deflection which starts the sweep voltage is 

During recent years the development of fast photodetectors has made impressive progress. Meanwhile PIN photodiodes (Sect. 4.5) are available with a rise time of 20 ps [11.83]. However, up to now the only detector which reaches a time resolution slightly below 1 ps is the streak-camera [11.86]. Femtosecond pulses can be measured with optical correlation techniques, even if the detector itself is much slower. Since such correlation methods meanwhile represent the standard technique for measuring of ultrashort pulses, we will discuss them in more detail. 

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At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

Plenary 10. Hiro-o Hamaguchi, e-mail address lilrama ,chem.s.u-tokvo.ac.ip (time and polarization resolved multiplex 2D-CARS). Two-dimensional (tune and frequency) CARS using broadband dye source and streak camera timing. Studies dynamic behaviour of excited (pumped) electronic states. Follows energy flow within excited molecules. Polarization control of phase of signal (NR background suppression). 

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. 

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. 

The propagation of premixed flames in closed vessels has been a subject of combustion research since its inception as a defined field of study in the late 1800s, when Mallard and LeChatelier [1] explored the behavior of explosions in the tunnels of coal mines. In the early decades of the twentieth century, experimenters used streak cameras to monitor the progress of premixed flame fronts propagating in tubes and channels without... 

Figure 6. Tempcraiure dependence of the fluorescence lifetime of BMPC in 1 1 ethanol-mcihanol. Measurements were carried out at the LENS laboratory of Florence by a picosecond apparatus using as an excitation source (at 380 nm) a dye laser pumped by a frequency-doubled cw Nd-YAG laser and recording the fluorescence time jirofiles by a streak camera. Since the overall insuumental response time was 75-80 ps, decays with t>200 ps, observed at T<130 K, were analyzed without deconvolution. At 177, 178 and 193 K, the lifetimes were roughly estimated as i=(FWHM -77 ), where FWHM was the width at half maximum of the decay. Because of the rather high sample absorbances (An,x=2), self absorption may have reduced the lifetimes to some extent.

In the present work, we have examined poly(N-vinylcarbazole) (abbreviated hereafter as PVCz) and pyrene-doped poly(aethyl methacrylate) (PMMA) films by using a tine-resolved fluorescence spectroscopic aethod. Fluorescence spectra and their dynanic behavior of the forner fila were elucidated with a high intensity laser pulse and a streak camera, which nakes it possible to neasure dynaaics just upon laser ablation. This aethod reveals aolecular and electronic aspects of laser ablation phenomena (17). For the latter fila a laser pulse with weak intensity was used for characterizing the ablated and Basked areas. On the basis of these results, we demonstrate a high potential of fluorescence spectroscopy in aolecular studies on laser ablation and consider its mechanism. Experimental... 

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

The commercially available laser source is a mode-locked argon-ion laser synchronously pumping a cavity-dumped dye laser. This laser system produces tunable light pulses, each pulse with a time duration of about 10 picoseconds, and with pulse repetition rates up to 80 million laser pulses/second. The laser pulses are used to excite the sample under study and the resulting sample fluorescence is spectrally dispersed through a monochromator and detected by a fast photomultiplier tube (or in some cases a streak camera (h.)) ... 

Figure 3. Schematic diagram of the laser/streak-camera/OMA apparatus.

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... 

Chapter 6 described the current techniques employed in time-resolved fluorescence spectrocopy. The time resolution of these techniques ranges from a few picoseconds (streak cameras) to a few hundreds of picoseconds (single-photon timing with flash lamp excitation). The time resolution can be greatly improved by using the fluorescence up-conversion technique. 

Other near-IR techniques that have been used to measure lifetimes, though not to the same extent as the aforementioned methods, include fluorescence up-conversion,(19 21) parametric amplification, 22 streak camera detection,(23) and two-photon excitation,1(24) The latter technique is particularly useful as it enables the greater penetration depth of near-IR radiation in organic matter to be used to obtain a well-defined region of excitation, e.g., in single cells or mammalian tissue. 

Streak cameras and multianode microchannel plate photomultipliers (MCP-PMs) interfaced to a polychromator also permit multiwavelength fluorescence decay measurements, the spectral response of both being determined by the photocathode composition. 

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. 

Similar photocathode types such as are used in photomultipliers are available with streak cameras. For example, a streak camera with an S 1 photocathode suitable for the detection of near-IR fluorescence has been developed.(23) S20 photocathodes are also readily available. 

Figure 12.23. Photon counting streak camera measurement of the fluorescence decay of DODCI (I0 fi Afl in water, (a) counts (b) weighted residues. Lifetime = 0.66 0.01 ns, (From Ref. 95.) Note the similarities with single-photon timing data, e.g., sec Figure 12,14. Reduced/2 = 1.37.

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