Electro-optical switching of antiferroelectric liquid crystals

Several different aspects regarding electro-optical switching of antiferroelectric and ferroelectric liquid crystals are experimentally investigated in this thesis.; The pre-transitional fluctuation behavior of a pitch-compensated antiferroelectric liquid crystal driven by a weak electric field is analyzed, and the two Goldstone azimuthal fluctuation modes, optic-like and acoustic like, are found coupling with each other under a non-zero weak electric field. The mode coupling was confirmed with quasi-elastic light scattering experiments. This part of work is detailed in Chapter 2.; Chapter 3 details our experimental investigations of the Freedericksz transition in an antiferroelectric liquid crystal. It is shown that a very-long-pitch, surface-stabilized, antiferroelectric liquid crystal undergoes a two-step electric-field-induced transition to the ferroelectric phase. The liquid crystal remains undistorted below a threshold field E_th. For E > E _th a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal. At higher fields a transition to the ferroelectric phase occurs via solitary waves.; Chapter 4 deals with finger-like solitary wave behavior of the phase transition from antiferroelectric to ferroelectric under higher electric fields. The velocities of propagating fingers of ferroelectric liquid crystalline phase invade the antiferroelectric phase were measured at different applied electric fields E larger than a characteristic threshold field E_th. The front velocity was found to be highly nonlinear in E, with enormous velocities reaching 400 cm s−1 for the maximum applied field. These are by far the largest velocities ever observed for a liquid crystal. The results suggest that the transition from an antiferroelectric to ferroelectric phase can be faster than the current theoretical prediction. Possible mechanisms, including the field dependent molecular interaction coefficients and shear thinning are discussed.; Two important application problems, charge migration and fringing field effects in switching of liquid crystal displays, are studied in Chapter 5. Residual current measurement and quasi-static ramp I-V measurement are introduced to investigate the charge migration processes in LCDs. Compared with the earlier proposed residual voltage measurement, these two methods are less environmentally sensitive, and thus, are more related to the internal charge migration process. Quasi-static ramp I-V measurement yields self-consistent parameter extraction compared to direct residual current measurements. The dynamic evolution process of fringing-field induced disclinations in vertically aligned LC light valves is studied in the second part of Chapter 5. Experimental results show that, due to the elastic coupling, the transformation speed of LC directors at the location of disclination between two topologically nonequivalent states depends strongly on display patterns. Both the charge migration and the fringing field experimental results revealed the corresponding mechanisms and clarified the directions for future improvement.