Liquid Crystals

Chapter 5 - Light Scattering

5.1.   INTRODUCTION

In earlier chapters we discussed some specific light-scattering processes in the
mesophases of liquid crystals. In particular, we found that the ability of the molecules
to scatter light is very much dependent on the orientations and fluctuations of
the director axis and their reconfiguration under applied fields. There are, however,
light-scattering processes that occur on the molecular level that involve the electronic
responses of the molecules. In this chapter we discuss the general approaches and
techniques used to analyze light-scattering processes in liquid crystals that are applicable
in many respects to other media as well.

Approaches to the problems of light scattering in liquid crystals may be classified
into two categories. In one category, such as Brillouin and Raman scatterings, knowledge
of the actual molecular physical properties, such as resonances and energy level
structures, is needed. On the other hand, in the electromagnetic formalism for light-
scattering phenomena, one needs to invoke only the optical dielectric constants and
their fluctuations. This latter approach is generally used to analyze orientational fluctuations
in liquid crystals.

The process of light scattering can also be divided into linear and nonlinear
regimes. In linear optics the properties of liquid crystals are not affected by the incident
light, which may be regarded as a probe or signal field. The resulting scattered
or transmitted light, in terms of its spatial or temporal frequency spectrum and intensity,
reflects the physical properties of the material. On the other hand, in the nonlinear
optical regime the incident light interacts strongly with and modifies the
properties of the liquid crystals. The resulting scattered or transmitted light will
reflect these strong interactions.

In this and the preceding chapters, our attention is focused on linear optical scattering
processes, which are nevertheless quite important in nonlinear optical phenomena.
Nonlinear optics and the nonlinear optical properties of liquid crystals will be presented
in Chapters 8–12. We begin with a review of the electromagnetic theory of light-
scattering terms associated with fluctuations of optical dielectric constants associated
with temperature effects. Raman and Brillouin scatterings, which involve molecular
energy levels and rovibrational excitations, will be given later in this chapter.