General Overview of Liquid Crystal Nonlinearities

In linear optical processes the physical properties of the liquid ciystal, such as its molecular structure, individual or collective molecular orientation, temperature, density, population of electronic levels, and so forth, are not affected by the optical fields. The direction, amplitude, intensity, and phase of the optical fields are affected in a unidirectional way (i.e., by the physical parameters of the liquid ciystal). The optical properties of liquid crystals may, of course, be controlled by some externally applied dc or low-frequency fields this gives rise to a variety of electro-optieal effects which are widely used in mai eleetro-optieal display and image-processing applications as discussed in previous chapters. 

Liquid ciystals are also optically highly nonlinear materials in that their physical properties (temperature, molecular orientation, density, electronic stmctuie, etc.) are easily perturbed by an applied optical field. Nonlinear optical processes associated with electronic mechanisms will be discussed in Chapter 10. In this and the next chapter, we discuss the principal nonelectronic mechanisms for the nonlinear optical responses of liquid crystals. 

Since liquid crystalline molecules are anisotropic, a polarized hght from a laser source can induce an alignment or ordering in the isotropic phase, or a realignment of the molecules in the ordered phase. These result in a change in the refractive index. 

Liquid Crystals, Second Edition By lam-Choon Khoo Copyright 2007 John Wiley Sons, Inc. 

If the electric field is spatially varying (i.e., there are regions of high and low energy densities E ), there will be a force acting on the molecule given by 

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