Nonlinear second-order phenomena

If a material possesses second-order optical nonlinearity and is photocon-ductive, it may be photorefractive [136], Photorefractivity is a x<3> phenomenon. The phenomenon occurs as photogenerated carriers redistribute and... 

Due to possible utilization of photoinduced orientation in polymeric films in optical data storage, this phenomenon and the quadratic nonlinear optical effects were extensively investigated in the last few years. It was reported, for instance, that to study photoisomerization in a polymeric environment, a series of polymers containing azo dyes with large differences in the second order transition temperature were compared. Particular emphasis was placed on the relationship between photoisomerization, Tgof the polymers, and their molecular structure. As a result, it was shown that light-induced nonpolar orientation in very high Tg polyimides (Tg up to 350 °C) can take place even at room temperature. The polymers used in one of these studies can be illustrated as follows... 

The interfaces in general, and particularly with solid substrates break the head-to-tail symmetry of a liquid crystal phase and induce polar orientational order. The symmetry is reduced to the conical group Coov The latter allows a finite value of the second-order nonlinear susceptibility X2 responsible for the second optical harmonic generation [11]. This phenomenon has been observed in experiments on generation of the second harmonic in a ultrathin nematic layers on a solid substrate as shown in Fig. 10.9. 

The electro-optic phenomenon, as the name implies, involves the interaction of electrical (DC-200 GHz frequency) and optical (2-4 X lO Hz frequency) fields within a material characterized by large hyperpolarizability (molecular second-order optical nonlinearity). Applications of this phenomenon focus on either the transduction of electrical signals into optical signals (e.g., as in the transduction of television signals onto fiber optic transmissions as in the community antenna television (CATV) industry) or the switching of an optical signal (e.g., as effected in local nodes of a local area network, LAN) between different transmission lines. 

Kramers and Heisenberg [2], who predicted the phenomenon of Raman scattering several years before Raman discovered it experimentally, advanced a semiclas-sical theory in which they treated the scattering molecule quantum mechanically and the radiation field classically. Dirac [3] soon extended the theory to include quantization of the radiatiOTi field, and Placzec, Albrecht and others explored the selection rules for molecules with various symmetries [4, 5]. A theory of the resonance Raman effect based on vibratiOTial wavepackets was developed by Heller, Mathies, Meyers and their colleagues [6-11]. Mukamel [1, 12] presented a comprehensive theory that considered the nonlinear response functions for pathways in LiouvUle space. Having briefly described the pertinent pathways in Liouville space above, we will first develop the Kramers-Heisenberg-Dirac theory by a second-order perturbation approach, and then turn to the wavepacket picture. 

Ferroelectricity is an electrical phenomenon and also an important property in solids. It arises in certain crystals in terms of spontaneous dipole moment below Curie temperature [1], The direction of this moment can be switched between the equivalent states by the application of an external electric field [2-4], It is observed in some crystal systems that undergo second-order structural changes below the Curie temperature, which results in the development of spontaneous polarization. This can be explained by Landau-Ginzburg free energy functional [3, 4, 9]. The ferroelectric behavior is commonly explained by the presence of domains with uniform polarization. This behavior is nonlinear in terms of hysteresis of polarization (P) and electric field (E) vectors. Phenomenological models of ferroelectrics have been developed for engineering computation and for various applications. 

Beyond the linear regime, there is also growing interest in second- and third-order response " in all these fields. In particular the field of nonlinear optics has been investigated heavily, especially the phenomenon... 

AH the properties dealt with up until now involve linear interactions between light and polymer. Interaction of li t with polymers in the nonlinear region involves second- and third-order effects as well as the phenomenon of photo refrac-tivity (56,57). An optical nonlinear optical (NIX)) polymer is one that, in response to an externally applied electric field, can either vary the speed of incoming light or alter its fi uency. Var3dng the speed of light involves a change in the reflective index of the material. An optically nonlinear polymer has two components the polymer itself and an optically nonlinear molecule (chromophore), which is either chemically attached to the polymer or dissolved in it. 

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