High frequency hysteresis-free switching in thin layers of smectic-C*ferroelectric liquid crystals

Abstract

The insulating layers used for the alignment of ferroelectric liquid crystals (FLC) in electro-optical cells usually have non-negligible thickness and their capacitance determines the type of the director switching caused by a triangular-form external voltage U(tr) . With decreasing frequency of U(tr) , the hysteresis in a switching direction changes from the normal to the abnormal one at a characteristic hysteresis inversion frequency f(i) . In the vicinity of f(i) , the electro-optical response is thresholdless and the optical transmission manifests the V -shape field dependence. The V -shape regime is very interesting for certain applications, in particular to microdisplays due to a possibility of the gray scale realization. However, f(i) has to be enhanced from the usually observed frequency of a few Hz up to the range of hundreds of Hz. To this effect, a special FLC material has been designed and its basic properties (tilt angle, spontaneous polarization, rotational viscosity, and electric conductivity) have been measured over the entire range of the smectic-C* phase. Upon variation of cell parameters (thickness of both the FLC and alignment layers), temperature, and external voltage, the frequency of the V -shape effect as high as 150-1000 Hz (in the temperature range 30-75 degrees C) has been found experimentally. The operating voltage remains lower than 8 V. A quantitative interpretation of these results has been done using the modeling procedure developed earlier [S.P. Palto, Cryst. Rep. 48, 124 (2003)]. The modeling has been performed with the experimental values of the FLC material and the cell parameters and has shown very good agreement with experiment. The key point of this approach is consideration of the internal voltage on the FLC layer, the sign, amplitude, and form of which differ from U(tr) . The results of the modeling allow further improvement of the performance of electro-optical FLC cells for high frequency V-shape effect.