Flexoelectric polarisation effects in nematic liquid crystal phase gratings

Nematic phase gratings have been studied in which a planar nematic layer of thickness 17.2 μm is sandwiched between two glass substrates coated with an alignment polymer. The upper substrate is a continuous earth plane and the lower substrate has a patterned electrode of interdigitated stripes (electrodes and gaps are both 40 μm wide). Reorientation of the nematic liquid crystal occurs in response to d.c. electric fields applied between the interdigitated electrodes. These nematic reorientation regions have been used to investigate the influence of the flexoelectric polarisation in the nematic liquid crystal by observing the resultant (i) movement of tilt fringes in a Mach-Zehnder interferometer, and (ii) optical diffraction patterns. In the Mach-Zehnder interferometer the periodic variation of the refractive index resulting from the periodic distortion profile is measured directly from the displacement of the tilt fringes. The asymmetry in the response to positive and negative polarities of the d.c. voltage for both measurement techniques is directly related to the sum of the flexoelectric coefficients, e1 + e3

Interferometric method for the measurement of the flexoelectric polarisation using nematic liquid crystal phase gratings

An interferometric method using a nematic liquid crystal phase grating has recently been developed for determining the sum of the flexoelectric coefficients, (e1 + e3) [C. Trabi, C. V. Brown, A. A. T. Smith and N. J. Mottram, Appl. Phys. Lett. 92, 223509 (2008)]. The switching response of the device can be simulated if the applied 0.2 Hz squarewave voltage in the nematic layer is modified so that it is enhanced by a reverse ion-field after each polarity reversal and then decays with time. This process has been investigated using measurements of the transient capacitance of a planar nematic layer to the same waveform