Effect of surface treatment on molecular reorientation of polymer-stabilized liquid crystals doped with oligothiophene

Abstract

Irradiation of dye-doped liquid crystals (LCs) with linearly polarized light leads to molecular reorientation, which manifests functional properties for various nonlinear optical (NLO) applications. Material designs with lower light intensity thresholds for molecular reorientation have been explored, and nematic LCs have been one of the most attractive choices because of the high NLO properties. Here we present a different approach to reduce light intensity for reorientation by modifying a substrate surface that controls initial molecular orientation in polymer-stabilized nematic LCs doped with oligothiophene. The surface of the glass substrate was treated with various concentrations of a silane coupler. Water contact angle measurement and analysis of samples using polarized optical microscopy revealed that surface anchoring in the initial state decreased as the silane coupler concentration decreased. The threshold intensity was successfully reduced by 30% simply by optimizing the silane coupler concentration. This finding clearly indicates that weak surface anchoring is key to the reduction of light intensity for molecular reorientation. INTRODUCTIONLiquid crystal (LC) materials have had a great impact on our modern information society. 1-3 Various LC materials and devices including LC displays, 3 smart windows, 4 reconfigurable optical elements 5 and tunable optical metamaterials 6 have been realized. 2 In these applications, light modulation is the most important factor and is achieved by orientational changes of LC molecules using an external field.Molecular reorientation of conventional LCs has been performed by an electric field. However, molecular reorientation triggered by an optical field has attracted attention because it enables the development of all-optical devices. 7,8 Such photoinduced molecular reorientation includes photochemical and photophysical processes. Photochemical processes control LC orientation through a photochemical reaction, for example, photoisomerization, photocrosslinking or photodegradation. [8][9][10][11][12][13][14] In contrast, a photophysical process exploits the nonlinear optical (NLO) effects of an LC. 7,15 When a homeotropic LC is vertically irradiated with linearly polarized light, the interaction between the optical field and the LC molecules generates a torque to rotate the molecular director along the polarization direction, leading to homogeneous (in-plane) orientation. On the other hand, rotation of the molecular director is disturbed both by the interaction between LCs and the glass substrate surface, a process which is also called surface anchoring, and by bulk elasticity. These torques reach a balance that determines a specific light intensity that allows

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals

“…The difference in the threshold intensity, which reflects the sensitivity of molecular reorientation, is derived from the dielectric anisotropy of each sample. According to previous work, the threshold intensity is determined by a balance of four torques: opticalelectric torque, elastic torque, dye torque, and surface anchoring [15]. The increase in the optical-electric torque promotes the molecular reorientation.…”

“…Instead of the anthraquinone dyes, Zhang et al reported the usage of oligothiophene dyes to enhance the sensitivity of the LC molecular reorientation [12]. To further decrease the light intensity required for the OFT in oligothiophene-doped LCs, various approaches focusing on the incident light, alignment processing, and materials have been proposed [13][14][15][16][17][18][19]. Recently, we focused on the material design and achieved the effective molecular reorientation by doping oligothiophene dyes with ester moieties [17,18].…”

“…[12]. [13][14][15][16][17][18][19]. Recently, we focused on the material design and achieved the effective molecular reorientation by doping oligothiophene dyes with ester moieties [17,18].…”

Nonlinear Molecular Reorientation of Polymer-Stabilized Dye-Doped Liquid Crystals by Depolarized Laser Beam

“…A host nematic LC, 5CB (4-cyano-4′-pentyl biphenyl), was obtained from Merck Ltd., Tokyo, Japan. A4CB (4-[4-(4′-cyanobiphenyl)oxy]butyl acrylate) was mixed to host LC as a monoacrylate according to a previous report (A4CB:5CB = 10:90, molar ratio) [29]. The dye molecule, TR5 (5,5′′-Bis-(5-butyl-2-thienylethynyl)-2,2′:5′,2′′-ter thiophene) was synthesized as reported previously [30] and doped into the mixture of 5CB and A4CB at the concentration of 0.1 mol%.…”

“…The reorientation of LC molecules in optical field is enhanced by adding a small amount of absorbing dye molecules, which generally causes a photochemical reaction, to the LCs [15][16][17][18][19][20][21][22][23][24]. On the other hand, molecular reorientation without a photochemical reaction, termed photophysical process, has also been explored [25][26][27][28][29] because of exhibiting a well-defined threshold intensity of nonlinear molecular reorientation. We found that homeotropic-aligned LCs doped with an oligothiophene-dye (TR5) efficiently underwent the photophysical molecular reorientation with lower light intensity compared with previously reported dye-doped LCs [30].…”