Resonance enhancement third order susceptibility

Figure 27 shows spectra of third-order susceptibility x(3) of the oriented MOPPV LB film. Owing to the orientation of MOPPV chains, anisotropy of x(3) was observed. The x(3) values of the LB film, which is enhanced by the three photon resonance, are maximized at a fundamental wavelength of 1600 nm, as observed in a MOPPV cast film. The maximum values of X ( 3) and are 3.2 x 10 10 esu... 

In previous years, the major application of CARS was in the analysis of gases and gas mixtures, but recent advances have demonstrated that CARS can be applied to nongaseous systems as weU. A problem in the application of CARS to molecules in solution is interference from the solvent, which can contribute a background emission resulting from the third-order susceptibility of the solvent. To a certain extent, this problem may be overcome when the CARS experiment is carried out under resonance conditions. As the Vp beam frequency approaches an electronic transition of the solute, the solute s CARS signal is resonance enhanced whereas the background emission remains unchanged. 

Higher-order non-linearities are not so widely studied as the second- and third-order terms. The effects are usually very small, but if the process is resonantly enhanced they can easily be detected. It should be mentioned here that the susceptibility is in general complex and frequency dependent because of the finite response time of the medium. 

The nonlinear susceptibility is evaluated using third-order perturbation theory, and resonant enhancement is readily demonstrated to occur. Four-wave mixing is a useful experimental technique to extend the energy range available to tunable dye lasers [468]. It is also of interest that processes involving excitation by three photons allow transitions between even and odd parity states to be excited, as do single-photon transitions. 

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