Liquid Crystals

Chapter 12 - Nonlinear Optical Phenomena Observed in Liquid Crystals

Liquid crystals possess wonderful light-scattering abilities, linear or nonlinear. As a
result, studies of their nonlinear optical responses have been vigorously pursued in
various contexts. As in the case of electro-optics, it would require a treatise to summarize
all the work done to date, as almost all conceivable nonlinear optical phenomena
have been observed in liquid crystals in all their mesophases. Some of these
phenomena were studied for their novelty; others have been developed into diagnostic
tools or practical devices. In this chapter, we limit our attention here to only exemplary
studies which are fundamentally interesting and/or practically important.


12.1.1.   Self-Focusing and Self-Phase Modulation and
cw Optical Limiting with Nematic Liquid Crystals

In earlier studies of self-focusing effects in liquid crystals, the main emphasis was on
the understanding of fundamental phenomena in nonlinear optics or liquid crystals
such as laser-induced ordering and self-focusing and the associated beam breakups
in an extended interaction region.1 In the nematic phase, the fact that the liquid crystal
film is thin but highly nonlinear allows one to employ the so-called nonlinear diffraction
theory and the external self-focusing and self-phase modulation effects
discussed in Chapter 11.

The discoveries of photorefractivity2 and supraoptical nonlinearity3 in nematic
liquid crystals have ushered in the era of external self-focusing with laser power as
low as nanowatts in thin optical media. Figure 12.1 shows a typical optical limiting
setup using external self-defocusing effect. A linearly polarized laser beam is
focused by a 15 cm focal length input lens to a spot diameter of 0.1 mm onto a 25 μm
thick liquid crystal film placed just behind the focal plane of the input lens. The
nematic film is tilted to enhance the nonlinear refractive index change experienced

by the extraordinary incident ray.4 An aperture of 5 mm diameter is placed at 40 cm
behind the sample to monitor the central region of the transmitted beam. Above an
input power of ~70, it is observed that the central region of the transmitted beam
becomes progressively darkened, and the beam divergence increases dramatically, as
depicted by the two photographs in Figure 12.1. This change in the central beam
power as a function of the input power is plotted in Figure 12.2, which exhibits a typical
limiting behavior.


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