Optimized flexoelectric response in a chiral liquid-crystal phase device

In this paper, a device type is presented in which the conventional geometry for the flexoelectro-optic effect is rotated, utilizing planar-aligned short-pitch chiral nematic and in-plane electric fields. The cell is optically neutral at zero applied field due to having its helix axis lie in the direction of light propagation, and at optical communication wavelengths (1550 nm) polarization rotation is insignificant due to the helical pitch of the material being shorter than the illuminating wavelength. An electric field, applied in the plane of the cell, has been found to induce a birefringence via a combination of dielectric helix unwinding and flexoelectric deformation of the director helix. The magnitude of the birefringence and direction of the induced optic axis in the plane of the cell are dependent on the amplitude and direction of the applied electric field, providing potential for use in a fast endlessly rotatable polarization controller. Herein, the chiral nematic materials utilized in the cell are bimesogenic liquid crystals designed to optimize the contribution from the flexoelectro-optic effect, and eliminate dielectric helix unwinding. The materials are also polymer network stabilized to preserve the texture against degradation in the applied fields. The results presented show a progression from a combined dielectric and flexoelectrically induced birefringence of 0.016 at field strengths up to 6.8 V/mu m, to a purely flexoelectric-induced birefringence of 0.0135, sufficient for a quarter wave plate in a 29-mu m-thick cell. Response times are of the order of hundreds of microseconds for both reaction to an applied field and relaxation upon removal. (c) 2005 American Institute of Physics.