Pulse generation and detection

One of the first known demonstrations of broadband IR probe pulse generation and detection used a 30 ps Nd+3 YAG laser system with two... 

We now discuss and review examples of studies conducted using the broadband probe pulse generation and detection methods described above. The examples presented here were chosen to provide the reader with a synopsis of the techniques and state-of-the-art performance of the tested array technologies. It is anticipated that these examples will generate an appreciation... 

Figure Bl.4.6. Left an experimental optical THz pump-probe set-up using sub-picosecond THz pulse generation and detection by the electro-optic effect. Right the application of such pulses to the relaxation of optically excited TBNC in toluene. The THz electric field used for these experiments is shown in the upper-right inset. Three exponential decay terms, of order 2, 50 and 700 ps, are required to fit the observed temporal relaxation of the solvent [51].

If pulses can be generated and detected whose length is short compared with the time difference between reflections from the top and the bottom surfaces of a layer, then the elastic properties of the layer can be deduced from the amplitude and timing of the two echoes. The return pulses from such a situation are illustrated in Fig. 8.10. Figure 8.10(a) is an oscilloscope trace of the reference echo from the substrate at defocus z0 and with nothing on it except the coupling fluid. We can choose to write the reference signal as... 

A solid or liquid sample can be introduced to the analyzer by thermal desorption. The resultant vapors are swept through the inlet by the carrier gas and ionized by a radioactive 63Ni source. Discreet packets of ions are then pulsed down the flight tube under a controlled potential. The arrival of the ions at the detector is inversely proportional to the mass of the molecule. Thus, the smaller ions arrive at the detector first, and the larger ions arrive later. At that point the signal is amplified and read out via an appropriate computer interface. Both commercially available instruments are capable of generation and detection of both positive and negative ions. 

Terahertz imaging approaches have typically used either short-pulsed laser or continuous wave (CW) THz generation and detection. The short-pulsed method usually involves the generation and detection of sub-picosecond THz pulses using either photoconductive antenna structures or optical rectification in a non-linear crystal. Pulsed sources seem to be more favorable (in particular for close proximity applications) because they can be used for acquiring depth information. Spectral information is retrieved by a Fourier transform of the time-domain data to the frequency domain. 

The applications of lasers in kinetic studies are essentially twofold. Firstly, they can be used to produce a particular species. This might be a vibration—rotationally defined quantum state of a molecule [21], or it could be an ion [22—24] or fragment [25—28] produced by photoionization or photodissociation [29, 30] of some parent. The combination of specific frequency, short pulse duration and high powers makes selective control of chemical reactions possible. Secondly, they can be used as detectors of specific species and quantum states [31, 32]. There are a number of different methods of using lasers to detect small concentrations of a species in a chemical reaction. Lin and McDonald [33] have broadly reviewed the generation and detection of reactive species in static systems with particular emphasis on the use of lasers for this purpose. 

A state-of-the-art description of broadband ultrafast infrared pulse generation and multichannel CCD and IR focal plane detection methods has been given in this chapter. A few poignant examples of how these techniques can be used to extract molecular vibrational energy transfer rates, photochemical reaction and electron transfer mechanisms, and to control vibrational excitation in complex systems were also described. The author hopes that more advanced measurements of chemical, material, and biochemical systems will be made with higher time and spectral resolution using multichannel infrared detectors as they become available to the scientific research community. 

Thomsen C, Grahn HT, Maris HI, Tauc I. Surface generation and detection of phonons by picosecond light pulses. Phys Rev B 1986 34(6) 4129-4138. 

Transition radiation is considerably weaker than Cerenkov radiation, however since it is a surface phenomenon it avoids problems with radiator thickness and reflections inherent to Cerenkov-generating silica plates. Optical TR can be measured using a streak camera. An optical TR system has been used to time-resolve the energy spread of an electron macropulse in a free-electron laser facility [10]. Interferometry of coherent, far-infrared TR has been used to measure picosecond electron pulse widths and detect satellite pulses at the UCLA Satumus photoinjector, using charges on the order of 100 pC [11],... 

TOF analyzers offer advantages of compactness (they can be made very small— cm-length analyzers are possible), simple construction (the TOF is basically a tube), and their wide mass range (they obtain the full mass spectrum, usually 1-300 amu for elemental and isotopic analysis) for every measurement. Drawbacks include the difficulty of efficiently coupling to various ion sources, limited mass resolution, relatively low precision (1-10 % are typical) and the need to use expensive high precision and high speed electronics for pulse generation, ion detection, and data acquisition. 

The pulse velocity approach is best done in a transmission mode with the pulse created on one side of a concrete member and detected on the other side. It can be used at corners or in a reflective mode if necessary, but it loses effectiveness and interpretation gets harder. The impact-echo technique can be used with pulse generator and detector side by side as the echo is reflected back from defects. 

We will at first discuss techniques for the generation and detection of short laser pulses before their importance for different applications is demonstrated by some examples. Methods for measuring lifetimes of excited atoms or molecules and of fast relaxation phenomena are presented. These applications illustrate the relevance... 

A whole chapter is devoted to time-resolved spectroscopy including the generation and detection of ultrashort light pulses. The principles of coherent spectroscopy, which have found widespread applications, are covered in a separate chapter. The combination of laser spectroscopy and collision physics, which has given new impetus to the study and control of chemical reactions, has deserved an extra chapter. In addition, more space has been given to optical cooling and trapping of atoms and ions. 

The flow measurement principle for the time-of-flight sensor is illustrated in Fig. 4. In this method, the time taken for a heat pulse to pass over a known distance is recorded. The heat pulse generated by an electrical pulse applied to the heater microfilm travels due to the fluid flow and is detected by a sensor microfilm [8, 10]. The time between the heat pulse generation and its detection can then be used for flow estimation (Fig. 4b). 

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