Liquid Crystals

Chapter 10 - Electronic Optical Nonlinearities

10.1.   INTRODUCTION

In this chapter we treat those nonlinear optical processes in which the electronic
wave functions of the liquid crystal molecules are significantly perturbed by the
optical field. Unlike the nonelectronic processes discussed in the previous chapters,
these electronic processes are very fast; the active electrons of the molecules
respond almost instantaneously to the optical field in the form of an induced electronic
polarization. Transitions from the initial level to some final excited state
could also occur.

Such processes are obviously dependent on the optical frequency and the resonant
frequencies of the liquid crystal constituent molecules. They are also understandably
extremely complicated, owing to the complex electronic and energy level structure of
liquid crystal molecules. Even calculating such basic quantities as the Hamiltonian,
the starting point for quantum mechanical calculations1 of the electronic wave function
and energy levels and linear optical properties, requires very powerful numerical
computational techniques.

We shall adopt a greatly simplified approach where the liquid crystal molecule is
represented as a general multilevel system. Only dipole transitions among the levels
are considered. Using this model, we quantitatively illustrate some important basic
aspects of the various electronic nonlinear optical processes and their accompanying
nonlinearities. Special features pertaining to liquid crystalline materials are then discussed.

 

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