Liquid Crystals in Guided Wave Optics

Sammanfattning: Liquid crystals are unique optical materials due to their large effective electrooptic and opto-optic coefficients. Currently, liquid crystals are primarily used in flat-panel dis-plays, but there is also an increasing number of nondisplay applications requiring new types of devices employing materials with unique optical properties. Waveguides are important as photonic components, such as integrated-optic switches. Liquid crystals should be useful as active materials in this context. Moreover, the unique optical proper-ties of liquid crystals can be investigated using optical guided-wave techniques.

The main aim of this work was to demonstrate the use of liquid crystals as active elements in optical waveguides. To this end, a nonlinear waveguide, having as active core a dye-doped nematic liquid crystal, was realized and nonlinear beam splitting was ob-tained above an input power threshold. However, most of the interest was focused on the use of the electro-optic effect in ferroelectric liquid crystals (FLCs). Two configurations were tested: FLC as guiding medium (active core); and, FLC as overlayer (active clad-ding). Various combinations of passive, thin-film planar waveguides and electrooptically active FLC-elements were constructed and tested, thus identifying the technical issues connected with FLC-integration, including the development and characterization of a new, passive polymer material for optical waveguides. Electrooptic modulation of the guided light was obtained with contrast ratios greater than 10:1 and switching times of the order of 10¹ -10² microseconds.

The second aim of this work was to use highly sensitive guided-wave techniques, in particular the fully leaky guided-mode technique, for the characterization of a novel, or-thoconic antiferroelectric liquid crystal (AFLC) in its field-free state, slightly deviating from ideal orthoconic conditions. Since it is this state that would provide the dark state in a display, it is of fundamental importance to the achievable contrast of the device. The characterization of this state is thus of great importance for materials development and de-vice manufacturing. The guided-mode study revealed the dielectric tensor orientation, in the investigated cell, of this complicated material in its field-free state, otherwise inacces-sible using standard optical techniques, and allowed determination of the tolerance against deviations from ideal conditions while keeping satisfactory high-contrast elec-trooptic characteristics.