Coherent active Raman spectroscopy CARS

An interesting variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS) (99). If two laser beams, with angular frequencies CO and CO2 are combined in a material, and if cjj — is close to a Raman active frequency of the material, then radiation at a new frequency CJ3 = 2cJ2 — may be produced. Detection of this radiation can be used to characterize the material. Often one input frequency is fixed and the other frequency, from a tunable laser, varied until matches the Raman frequency. CARS has the capabiHty for measurements in flames, plasmas, and... 

The laser-based techniques currently being used and developed include vibrational Raman scattering, coherent anti-Stokes Raman scattering (CARS), Rayleigh scattering, laser-induced fluorescence, and planar laser-induced fluorescence (PLIF). This is an active research field at various research establishments. Laser-induced fluorescence spectroscopy has been used to measure several combustion intermediates, for example, CH, C2, HCH, OH, NO, NO2, HNO, CO, halogenated hydrocarbons, and polycyclic aromatic hydrocarbons. 

Resonances betwen Raman active molecular vibrations and rotations and the difference frequency combination 0)1 —0)2 can also occur. When the four-wave mixing process 2o)i — 0)2 or o)i+o)2 — 0)2 is used with these resonances it is termed coherent anti-Stokes Raman scattering (CARS). The resonant enhancement that occurs in the generated intensity as the pump frequencies are tuned through the two-photon difference-frequency resonance forms the basis of CARS spectroscopy (see Section IV.A). 

The introduction of lasers therefore has indeed revolutionized this classical field of spectroscopy. Lasers have not only greatly enhanced the sensitivity of spontaneous Raman spectroscopy but they have furthermore initiated new spectroscopic techniques, based on the stimulated Raman effect, such as coherent anti-Stokes Raman scattering (CARS) or hyper-Raman spectroscopy. The research activities in laser Raman spectroscopy have recently shown an impressive expansion and a vast literature on this field is available. In this chapter we summarize only briefly the basic background of the Raman effect and. present some experimental techniques which have been developed. For more thorough studies of this interesting field the textbooks and reviews given in [9.1-4] are recommended. 

Plenary 7(5. N I Koroteev et al, e-mail address Koroteev nik.phys.iusu.su (CARS/CSRS, CAHRS, BioCARS). A survey of the many applications of what we call the Class II spectroscopies from third order and beyond. 2D and 3D Raman imaging. Coherence as stored infonuation, quantum infonuation (the qubit ). Uses tenus CARS/CSRS regardless of order. BioCARS is fourtli order in optically active solutions. 

In the case of non-degenerate frequencies, the nonlocal third-order effects may give rise to chiral pump-probe spectroscopies. The only observation of a coherent Raman optical activity process to date is also due to a third-order pseudoscalar. Spiegel and Schneider have observed Raman optical activity in coherent anti-Stokes Raman scattering in a liquid of (-l-)-trans-pinane and report chiral signals that are 10 of the conventional electric-dipolar CARS intensity [23],... 

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