Nonlinear third-order phenomena

Most of the studies devoted to the nonlinear optical properties of metal nanoparticles use the notation x (<>>) to refer to the susceptibility for the optical Kerr effect. Unless otherwise specified, we will also adopt this simplified designation in the following. Let us just recall that it corresponds, in fact, to an experimental situation where a unique plane wave, linearly polarized (or three plane waves with same polarization and frequency), generates the third-order nonlinear optical phenomenon in an isotropic medium at the same frequency, and that the susceptibility is a priori a complex quantity. 

The optical Kerr effect is a phenomenon of the optical field-induced birefringence and refers to the linear birefiingence induced by a linearly polarized optical field. The pump and probe beams are polarized 45 to each other in optical Kerr effect experiments. The probe beam may be composed of two orthogonal linearly polarized beams, and the polarization of one beam is parallel to that of the pump beam. According to third-order optical nonlinearity, the pump beam-induced linear birefringence is given by [46]. 

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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