Optimized flexoelectric response in a chiral liquid-crystal phase device

Abstract

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 nm͒ 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 / m, to a purely flexoelectric-induced birefringence of 0.0135, sufficient for a quarter wave plate in a 29-m-thick cell. Response times are of the order of hundreds of microseconds for both reaction to an applied field and relaxation upon removal.

The limits of flexoelectricity in liquid crystals

“…The implications of approaching this limit for distorted equilibrium structures 15,16 suggest an interesting avenue of research. Further, in some device configurations-such as the flexoelectric effect in chiral nematics 3,[5][6][7][8][9][10][11][12][13] -the magnitude of the observed flexoelectric response is governed by a ratio of the form |e/K |. Generally, the higher the ratio |e/K |, the lower the electric field needed to drive the device.…”

“…1 In the ground state, n is uniform, and an elastic energy cost is associated with distortions of n. Flexoelectricity is a coupling between certain such distortions and electric polarization; an applied electric field may induce distortion, and distortion may induce polarization. 2 The effect is of interest in devices such as display panels, [3][4][5][6][7][8][9][10][11][12][13] and electro-mechanical transducers for sensing or energy-harvesting applications. 14 More fundamentally, flexoelectricity has recently been studied as a factor influencing the energetics of distorted equilibrium structures.…”

Flexoelectric coefficient measurements in the nematic liquid crystal phase of 5CB

“…3,4 The converse effect-polarization-induced distortion-may be exploited in electro-optic devices such as display panels, providing a mechanism whereby the optical properties can be manipulated by an applied voltage. [5][6][7][8][9][10][11][12][13][14][15] Values of the flexoelectric coefficients are required for inclusion in theoretical models, to test simulations, [16][17][18][19] and to inform the development of devices. Thus, for fundamental and applied reasons, it is important to be able to measure the flexoelectric coefficients of a given material.…”

Linear oscillatory dynamics of flexoelectric membranes embedded in viscoelastic media with applications to outer hair cells

“…Current potential applications of LC flexoelectricity include energy harvesting, electro-mechanical transducers, and displays [7][8][9][10][11][12][13].…”