Ultrashort pulse techniques

Nevertheless, these ultrashort pulse techniques do find applications in areas of spectroscopy where one is dealing with broad bands in terms of frequency distribution, and spectral bandwidth is not the limiting factor. 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

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

D. Heiman, Spectroscopy of Semiconductors at Low Temperatures and High Magnetic Fields A. V. Nurmikko, Transient Spectroscopy by Ultrashort Laser Pulse Techniques A. K. Ramdas and S. Rodriguez, Piezospectroscopy of Semiconductors O. J. Glembocki and B. V. Shanabrook, Photoreflectance Spectroscopy of Microstructures D. G. Seiler, C. L. Littler, and M. H. Wiler, One- and Two-Photon Magneto-Optical Spectroscopy of InSb and Hgj Cd Te... 

Abstract Optical techniques for three-dimensional micro- and nanostructuring of transparent and photo-sensitive materials are reviewed with emphasis on methods of manipulation of the optical field, such as beam focusing, the use of ultrashort pulses, and plasmonic and near-field effects. The linear and nonlinear optical response of materials to classical optical fields as well as exploitation of the advantages of quantum lithography are discussed. 

Little is known about the "molecular scale of time". Chemical research in the past has had its accent on highly varied spectroscopic methods, but these have been mainly for the study of spatial and frequency resolution. This spectrum of spectroscopies is incomplete without the inclusion of time. The availability of ultrashort pulses of energy and their application to molecular problems are therefore expected to form an important extension to the field of molecular spectroscopy. During the next decade the creation of new frontiers in chemistry through studies based on such techniques is inevitable. 

In CARS two ultrashort pulses of laser light (from femtoseconds to picoseconds in duration) arrive simultaneously at the sample of interest (Mukamel, 2000 Fourkas, 2001 and references herein). The difference between the frequencies (W) - w2) matches the frequency of a Raman active vibrational mode in the sample. A probe pulse (w3) emits a signal pulse of frequency Wj - w2 + w3 in a unique special direction. By scanning the delay time between the pump and probe pulses, the delay of the vibrational coherence can be measured. The distinct advantage of CARS is that it is a background free technique, since the signal propagates in a unique direction. 

Techniques and equipment used to obtain data in this type of experiment are discussed and described in quite a number of papers published during the year. The generation and properties of ultrashort pulses has been clearly presented in an article published in American Scientist . An apparatus for carrying out pump-probe broad band spectroscopy by transient absorption in the subpicosecond region has been described in detail. This equipment has been used to observe the photodissociadon of bis-(p-aminophenyl) disulphide and also to show a biexponential frequency shift arising from solvent relaxation of the photo-generated p-aminophenyl thiyl radicals. Another picosecond time resolved absorption spectrometer system using a streak camera has been reported by Japanese workers . Okamoto and Yoshihara ... 

Recently, Zewail and co-workers [54] applied the degenerate four-wave mixing (DFWM) technique with an ultrashort pulsed laser to a study of chemical reactions (unimolecular dissociation reaction of Nal, bimolecular reaction Na + H2) in the gas phase. Comparing this technique with other means, they showed that the wave packet motion extracted from the DFWM is in excellent agreement with results obtained from the laser induced fluorescence technique. 

They can deliver large quantities of energy at very precise wavelengths, and the power can be delivered in very short bursts—on the order of only 10 second (one femtosecond) in duration. This allows the excitation of molecules with an ultrashort pulse of energy, after which the molecule can be observed as it decays back to the ground state by various pathways. These techniques are now being used to elucidate the mechanisms of chemical reactions, as we will discuss in Chapter 15. 

Micro- and Nano-Machining by Ultrashort Voltage Pulsing Technique... 

There are two different techniques that are used to measure the time profiles and optical oscillations of ultrashort pulses noncoUinear intensity correlation and interferometric autocorrelation. While the former measures the envelope of the pulse, the latter can even measure the optical oscillations within the pulse envelope. Combined with the spectral resolution, the time profiles of the different spectral components within the optical pulse spectrum can be simultaneously measured by the FROG technique. The relative phases of these spectral components are observable using the SPIDER technique (see Sect. 6.2.4). 

Hybrid mode locking techniques combine the best features of other mode locking methods. For example, an active mode locking element may be inserted into a passively mode locked fiber laser cavity to produce an ultrashort pulse laser with well-defined repetition rate. Similarly, semiconductor saturable absorbers may be inserted into passively mode locked fiber lasers to initiate soliton generation, resulting in an ultrashort pulse laser with low selfstarting threshold. 

Ultrashort pulses may be also used for vibrational spectroscopy with high frequency resolution. As a first example we have demonstrated FT-CARS of a supersonic expansion. Several advantages of the technique should be noted. The effect of transit time broadening can be eliminated. Artifacts via the nonresonant part of the third order susceptibility are negligible. A possible dynamic Stark effect during the excitation process does not influence the ns signal transient. Precise spectroscopic information is provided without narrow-band laser sources. 

During recent years the development of fast photodetectors has made impressive progress. For example, PIN photodiodes (Sect. 4.5) are available with a rise time of 20 ps [11.100]. However, until now the only detector that reaches a time resolution slightly below Ips is the streak camera [11.101]. Femtosecond pulses can be measured with optical correlation techniques, even if the detector itself is much slower. Since such correlation methods represent the standard technique for measuring of ultrashort pulses, we will discuss them in more detail. 

Recently, measurements of ultrashort pulses have attracted much attention. In all cases, except the case of Treacy using the compression technique, the experiments measure only the pulse-width. Both the second-harmonic-generation technique and the two-phonon-fluorescence technique are inherently symmetric. They measure the auto cor-... 

The availability of powerful lasers providing ultrashort pulses in a wide range of frequencies has stimulated - besides rapid development of experimental techniques - the theorists to develop sophisticated methods to treat photodissociation processes, at least for small molecules, in an essentially exact quantum mechanical way. The marvellous book by Schinke [122] gives an excellent overview of the state-of-the-art. However, for clusters only very simple models have been used up to now to analyze the real-time photodissociation. 

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