In 1988 physicists and chemists commemorated the centenary of the
discovery of the first liquid crystals. Fora long period after this
discovery, although many significant results were found, liquid crystal
research remained a marginal topic of condensed matter physics. The
situation changed in the sixties. At that time the remarkable
electro-optical properties of liquid crystals were recognized and found
soon widespread application in numeric displays. From a more fundamental
point of view, the interest in disordered systems. increased in general
at the same time. Liquid crystals represented an important dass of such
systems. Among others, phase transitions, hydrodynamics and topological
defects occurring in them attracted considerable attention. The
connection between the liquid-crystalline state and the structure of
biological membranes stimulated a Iot of works also. In the present
volume we discuss a relatively new and rapidly developing branch of the
fi. eld, namely nonlinear optical effects in liquid crystals. Optical
studies have always played a signifi. cant role in liquid crystal
science. Research of optical nonlinearities in liquid crystals began at
the end of the sixties. Since then it became a powerful tool in the
investigation of symmetry properties, interfacial phenomena or dynamic
behaviour. Furthermore, several new aspects of nonlinear processes were
demonstrated and studied extensively in liquid crystals. The subject
covered in this book is therefore of importance both for liquid crystal
research and for nonlinear optics itself. The term "nonlinear optics" is
used here in a broad sense.
