Large Electro‐optic Kerr Effect in Polymer‐Stabilized Liquid‐Crystalline Blue Phases

Abstract

Optical shutters whose operation is based on the Kerr effect, which is a quadratic electro-optic effect generated in optically isotropic substances, have extremely fast response times. However, the magnitude of the induced birefringence in conventional Kerr materials is too small for them to be used in flat-panel displays. We show that a polymer-stabilized blue phase has a Kerr constant which is about 170 times larger than that of nitrobenzene. We also demonstrate microsecond electro-optical switching over a wide temperature range for flat Kerr cells containing the polymer-stabilized blue phase. These achievements can contribute to providing fast-response flat-panel liquid-crystal displays that need not undergo a rubbing process during manufacture.The Kerr effect is the development of birefringence in an optically isotropic substance, such as a liquid, when the substance is placed in an electric field. The magnitude of the birefringence induced via the Kerr effect, Dn, can be expressed bywhere Dn is the induced birefringence, k is the wavelength of light, and K is the Kerr constant. The Kerr cell, which is used as an optical shutter, consists of a Kerr substance (usually a polar, organic liquid) placed in an insulating container with two electrodes. This is positioned between crossed linear polarizers whose transmission axes are at ±45 to the applied electric field. In order to obtain the half-wavelength retardation at which maximum contrast can be obtained, the cell needs to have a thickness (optical path length), d k/2 , ofFor example, a Kerr cell containing nitrobenzene, which is generally known to have a very large Kerr constant (K = 2.2 10 ±12 m V ±2 ), [1] requires a cell thickness of about 45 mm for application of an electric field, E, of strength E = 5 10 6 V m ±1 . Fast optical shutters can be used in flat-panel liquid-crystal displays, as these displays consist of arrays of small light shutters. However, Kerr shutters made of nitrobenzene are too thick to be used. Blue phases are liquid-crystalline phases that appear in a very small temperature range between the cholesteric phase (Ch) and the isotropic phase (Iso).[2±5] There are three types of blue phasesÐBPI, BPII, and BPIII. The BPI and BPII phases are characterized by cubic symmetry of the director field, with lattice constants that are comparable to the wavelength of visible light. [6,7] Since blue phases are optically isotropic in zero electric field due to their structural symmetry, surface treatment to obtain a specific molecular orientation, e.g., rubbing to obtain twisted alignment, is redundant. This is a great advantage for device fabrication, because the rubbing process introduces degradation of the display quality and increases manufacturing costs. Recently, we reported that the temperature range over which BPI exists, usually a few degrees Kelvin, was successfully extended to more than 100 K in the polymer±liquid crystal (blue phase) composite system, referred to as the ªpolymer-stabilized blue phaseº. [8,9] In this study, the Kerr effe...

Switchable Two-Dimensional Liquid Crystal Grating in Blue Phase

“…The response time for the proposed BPLC gratings is in the sub-millisecond range, which is faster than that for the general LC grating. The response time can be further reduced by changing the BPLC to the PSBPLC [36]. However, the driving voltage of a PSBPLC grating will be increased.…”

MULTIPLE TIME SCALES OPTICAL NONLINEARITIES OF LIQUID CRYSTALS FOR OPTICAL-TERAHERTZ-MICROWAVE APPLICATIONS (Invited Review)

“…There have also been recent studies of another phase of chiral nematic liquid crystals, namely, Blue-Phase Liquid Crystals (BPLC) and their polymer-stabilized forms (PSBPLC) that presented promising polarization-and alignment-free alternatives [5,[92][93][94][95][96][97][98][99] for nonlinear optics as well as electro-optics. In this unique class of optical materials formed by mixing chiral and achiral nematics (with or without polymer stabilization), the molecules self-assemble into tightly wound defect-spirals that form 3-D cubic or BCC lattices with sub-wavelength lattice constants.…”

A model for the Pockels effect in distorted liquid crystal blue phases

“…5,[10][11][12][13] It is the Kerr effect that has been of particular interest in the context of display devices because it operates on a time scale of 10 À5 -10 À4 s, which is much quicker than the time scale of field-induced phase changes and electrostriction. [14][15][16] Further, the effective Kerr coefficient in the BPs is remarkably large compared to conventional materials: values 10 2 -10 4 times larger than for nitrobenzene have been reported. [16][17][18] The Pockels effect, despite being of considerable technological importance in other materials, 19 is not usually considered in the context of the BPs.…”

“…[14][15][16] Further, the effective Kerr coefficient in the BPs is remarkably large compared to conventional materials: values 10 2 -10 4 times larger than for nitrobenzene have been reported. [16][17][18] The Pockels effect, despite being of considerable technological importance in other materials, 19 is not usually considered in the context of the BPs. This is because the symmetries of the undistorted BP structures are such that the Pockels effect is forbidden.…”

“…In this case, the linear electro-optic behavior is characterized by the Pockels tensor (similar to the way in which the quadratic electro-optic behavior is characterized by the Kerr tensor, often approximated as a single Kerr constant 5,9,14,16 ). However, while this macroscopic approach is of great utility, it says nothing about the underlying physical origin of the effect.…”