Control of liquid crystal pretilt angles by using organic/inorganic hybrid interpenetrating networks

Refractive-type varifocal liquid-crystal Fresnel lenses for smart contacts

We demonstrate the implementation of a low-power, low-profile, varifocal liquid-crystal Fresnel lens stack suitable for tunable imaging in smart contact lenses. The lens stack consists of a high-order refractive-type liquid crystal Fresnel chamber, a voltage-controlled twisted nematic cell, a linear polarizer and a fixed offset lens. The lens stack has an aperture of 4 mm and thickness is ∼980 µm. The varifocal lens requires ∼2.5 V_RMS for a maximum optical power change of ∼6.5 D consuming electrical power of ∼2.6 µW. The maximum RMS wavefront aberration error was 0.2 µm and the chromatic aberration was 0.008 D/nm. The average BRISQUE image quality score of the Fresnel lens was 35.23 compared to 57.23 for a curved LC lens of comparable power indicating a superior Fresnel imaging quality.

A Novel Approach for the Creation of Electrically Controlled LC:PDMS Microstructures

This work presents research on unique optofluidic systems in the form of air channels fabricated in PDMS and infiltrated with liquid crystalline material. The proposed LC:PDMS structures represent an innovative solution due to the use of microchannel electrodes filled with a liquid metal alloy. The latter allows for the easy and dynamic reconfiguration of the system and eliminates technological issues experienced by other research groups. The paper discusses the design, fabrication, and testing methods for tunable LC:PDMS structures. Particular emphasis was placed on determining their properties after applying an external electric field, depending on the geometrical parameters of the system. The conclusions of the performed investigations may contribute to the definition of guidelines for both LC:PDMS devices and a new class of potential sensing elements utilizing polymers and liquid crystals in their structures.

Nematic Structures under Conical Anchoring at Various Director Tilt Angles Specified by Polymethacrylate Compositions

Dependence of the director tilt angle of nematic liquid crystal (LC) under conical anchoring from the two-component polymer mixture composition has been studied. We varied the ratio of poly(isobutyl methacrylate) (PiBMA), which specifies a conical anchoring for the nematic liquid crystal LN-396, and poly(methylmethacrylate) (PMMA) assigning a tangential alignment for the same nematic. An oblique incidence light technique to determine a tilt angle has been used. It has been shown that the tilt angle increases from 0∘ to 47.7∘ when PiBMA:PMMA ratio changes in the range 30:70 to 100:0. The specific optical textures viewed under the polarizing microscope and proper orientational structures have been considered for various compositions of the polymer films. An electric field action on the formed orientational structures has been investigated. The obtained results are promising for the application in various electro-optical LC devices with a conical anchoring in which the director tilt angle is a crucial parameter: a controlled diffraction gratings, an electrically operated achromatic rotators of linear light polarization, etc.

UV-treated ZnO films for liquid crystal alignment

UV treatment has applied to modify the surface wettability of ZnO films for controlling the liquid crystal alignment.

Tunable Surface Wettability of ZnO Nanoparticle Arrays for Controlling the Alignment of Liquid Crystals

The control of the liquid crystal (LC) alignment is very important for both academic research and practical applications. LC molecules aligned on the ZnO nanoparticle arrays (ZnO NPAs) are demonstrated and the pretilt angles of LCs can be controlled by using ZnO NPAs with different surface wettability. The wettability of ZnO NPAs fabricated by the solution-based hydrothermal method can be controlled by changing the annealing temperature of the as-prepared ZnO NPAs. The measurements of the energy-dispersive spectra and photoluminescence have shown that the chemical properties of ZnO NPAs have been changed with the annealing temperature. Our results show that the pretilt angle of LCs can be tuned continuously from ∼0 to ∼90° as the contact angle of water on ZnO NPAs changes from 33 to 108°.

Performance enhancement using a non-uniform vertical electric field and polymer networks for in-plane switching of multi-pretilt, vertically aligned liquid crystal devices

A simple and reproducible alignment method for fabricating vertically aligned (VA) liquid crystal (LC) cells with a multi-pretilt structure is developed. A non-uniform vertical electric field is employed in the LC/monomer mixed cells during the photocuring process, and two pretilt domains with a functional small pretilt angle (∼1.6°) in the stabilized VA LC/polymer cells are achieved. The enhanced electro-optical performance of the cell driven by an in-plane switching field is demonstrated. Compared to the pure cell, the 2 wt.% pretilt angle cell shows 36%, 64%, and 76% improvement in the optical switch, the gray-level rise time, and the gray-level fall time responses, respectively, which are obtained at a low driving voltage (≤12  V). When applied to LC devices, the proposed method not only effectively benefits the LC molecular alignment, but it also significantly boosts the electro-optical performance.

Orientating layers with adjustable pretilt angles for liquid crystals deposited by a linear atmospheric pressure plasma source

A method for controlling the pretilt angles of liquid crystals (LC) was developed. Hexamethyldisiloxane polymer films were first deposited on indium tin oxide coated glass plates using a linear atmospheric pressure plasma source. The films were subsequently treated with the rubbing method for LC alignment. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy measurements were used to characterize the film composition, which could be varied to control the surface energy by adjusting the monomer feed rate and input power. The results of LC alignment experiments showed that the pretilt angle continuously increased from 0° to 90° with decreasing film surface energy.

Photo-configurable embossed liquid crystal alignment layer with high azimuthal anchoring strength

Herein we describe a photo-alignment layer of improved azimuthal anchoring energy comparable to conventional rubbing method. In order to address the inherent low anchoring stability of photo-alignment layer, we applied embossing technique to conventional photosensitive polymer film, based on the cinnamoyl photoreactive groups, to introduce physical micro-groove effect for additional anchoring energy. From this, 2.5 × 10⁻⁴ J/m² of azimuthal anchoring energy was achieved, which is considered as synergistic effect from both photoinduced chemical interaction and physical microgroove alignment. In this study, we conducted systematic study on change in anchoring energy as a function of both aspect ratio of embossed pattern and UV exposure dose. We also demonstrated fabrication of sophisticated multi-domain structure of LC cells and discussed theoretical interpretation through LC simulation.

UV-driven in-plane rotation of a liquid crystal director in poly(vinyl cinnamate) films having microscale grooves

The micropatterned poly(vinyl cinnamate) (PVCi) alignment layers are fabricated by a solvent-assisted micromolding method. Without UV irradiation, the alignment layer can induce the unidirectional liquid crystal (LC) orientation, which is influenced by topography-based anchoring energies. With photodimerization of PVCi by UV irradiation, we could modulate the anchoring energies caused by chemical interactions of the alignment layer. It is observed that these two different contributions compete against each other in determining LC rotation on the surface of the alignment layer. The rotation angle of LC directors could be controlled from 45° to 70° by simply changing the UV exposure dose.

Nanoparticle-doped polyimide for controlling the pretilt angle of liquid crystals devices

The pretilt angles of liquid crystal molecules can be controlled by using conventional polyimide (PI) alignment material doped with different concentrations of Polyhedral Oligomeric Silsequioxanes (POSS) nanoparticles, which have been observed to spontaneously induce vertical alignment. The addition of POSS in the homogenous PI changes the surface energy of the alignment layer and generates a variable pretilt angle. Experimental results demonstrate that the pretilt angle thetap is a function of the POSS concentration, and can be tuned continuously over the range of 0 degrees <thetap<90 degrees. The polar anchoring energy of POSS/PI alignment layer is almost constant regardless of the POSS concentration. This technique is very simple and is compatible with methods familiar in the current LCD industry.

Generation of pretilt angles of liquid crystals on cinnamate-based photoalignment layer by a simple directional peel-off process

As an alternative to the rubbing method, the photoalignment method has been extensively developed for liquid crystal displays owing to its various advantages including the absence of any contamination problems, feasibility of multi-domain structure formation, and low temperature processing capability. However, cinnamate-based photo-reactive materials that are widely used in photoalignment suffer from several limitations including low pretilt angle and limited control of tilted-up direction of the liquid crystals compared to that of the rubbing process. In this work, we demonstrate a simple and convenient method to generate the pretilt angle, compared to that achieved via the rubbing method, by means of a simple directional peel-off process using a photo-reactive cinnamate oligomer having flexible molecular chain.