Structural Dependence of Nonlinear Susceptibilities

The magnitudes of the various tensorial components of the nonlinear susceptibilities of liquid crystals are determined fundamentally by their electronic stmcture. Since 

In the past two decades, an extensive volmne of research has been performed in the field of molecular nonlinear optics (for details see Refs. 1, 5, and 18). In spite of the tremendous variations in the molecular structures of the organic materials, the following conclusions seem to be well documented. 

It is important to note that although the nonlinear polarizabihties (and susceptibilities) of a particular class of molecules may be improved by molecular engineering, other physical properties (e.g., viscosity, size, elastic constant, absorption, etc.) are also likely to be affected. These physical properties play an equally (in some cases even more) important role in actual nonlinear optical processes and applications. A simple example of this necessity to consider the overall picture, rather than an optimization procedure based solely on a particular molecular parameter, is discussed in Chapter 8. 

In some absorbing (at the 5145 A line of an Ar laser) liquid crystals [4-4 -bis(heptyloxy )azoxybertzene], a study ° has shown peculiar polarization dependences on the optical fields. These effects are undoubtedly related to the electronic stmctures of these liquid crystals and their changes following photoabsorption, but the exact mechanisms remain to be ascertained. 

In some nematic liquid crystals (e.g., MBBA or azoberrzene liquid crystals), theories and experiments have shown that, when photoexcited, the molecules will undergo some stmctural or conformational changes (from trans state to cis state) and will exhibit unusually large orientational optical nonlinearities. Such electronic-oiientational nonlinearities are discussed earlier in Chapter 8. 

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