Liquid Crystals

Chapter 8 - Laser-Induced Orientational Optical Nonlinearities in Liquid Crystals

8.1.   GENERAL OVERVIEW OF LIQUID CRYSTAL NONLINEARITIES

In linear optical processes the physical properties of the liquid crystal, 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 crystal). 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-optical
effects which are widely used in many electro-optical display and image-processing
applications as discussed in previous chapters.

Liquid crystals are also optically highly nonlinear materials in that their physical
properties (temperature, molecular orientation, density, electronic structure, etc.) are
easily perturbed by an applied optical field.1-5 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 light 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.

Other commonly occurring mechanisms that give rise to refractive index changes
are laser-induced changes in the temperature, ΔT, and the density, Δρ. These changes
could arise from several mechanisms. A rise in temperature is a natural consequence
of photoabsorptions and the subsequent inter- and intramolecular thermalization or
nonradiative energy relaxation processes. In the isotropic phase the change in the
refractive index is due to the density change following a rise in temperature. In the
nematic phase the refractive indices are highly dependent on the temperature through
their dependence on the order parameters, as well as on the density, as discussed in
Chapter 3.