Deng-Ke Yang

Cholesteric Liquid-Crystal Polymer Dispersion for Haze-Free Light Shutters

A new dispersion involving a polymer in low concentration with a cholesteric liquid crystal is reported. Two types of light modulators from these materials are described as a normal mode shutter which is opaque (light scattering) in the field‐OFF state and transparent in the field‐ON state and a reverse‐mode shutter with the opposite field conditions. The transparent state of both cells is haze‐free for all viewing angles making the material attractive for window applications and direct view displays.

Cholesteric reflective display: Drive scheme and contrast

We studied the electro‐optical response of a bistable cholesteric texture (BCT) display to ac voltage pulses. The material can be driven into states where planar and focal conic textures coexist at zero field and gray scale memory is achieved. According to the properties of the BCT display we designed two drive schemes; one for binary operation and the other for gray scale operation. We made a 320×320 pixel reflective display with a resolution of 80 lines/in. on a passive matrix. Measurement in an integration chamber showed that the display has higher contrast and better viewing angle than a reflective super twisted nematic display.

Light-Driven Reversible Handedness Inversion in Self-Organized Helical Superstructures

We report here a fast-photon-mode reversible handedness inversion of a self-organized helical superstructure (i.e., a cholesteric liquid crystal phase) using photoisomerizable chiral cyclic dopants. The two light-driven cyclic azobenzenophanes with axial chirality show photochemically reversible trans to cis isomerization in solution without undergoing thermal or photoinduced racemization. As chiral inducing agents, they exhibit good solubility, high helical twisting power, and a large change in helical twisting power due to photoisomerization in three commercially available, structurally different achiral liquid crystal hosts. Therefore, we were able to reversibly tune the reflection colors from blue to near-IR by light irradiation from the induced helical superstructure. More interestingly, the different switching states of the two chiral cyclic dopants were found to be able to induce a helical superstructure of opposite handedness. In order to unambiguously determine the helical switching, we employed a new method that allowed us to directly determine the handedness of the long-pitched self-organized cholesteric phase.

Polymer networks formed in liquid crystals

Abstract Monomers with reactive double bonds were mixed with liquid crystals and polymerized under UV irradiation. The polymer networks formed are anisotropic and consist of fibrils. The orientation of the polymer networks depend on the orientation of the liquid crystals during polymerization. Optical and scanning electron microscopy (SEM) were used to study the polymer networks.

Light‐Directing Omnidirectional Circularly Polarized Reflection from Liquid‐Crystal Droplets

Constructing and tuning self-organized three-dimensional (3D) superstructures with tailored functionality is crucial in the nanofabrication of smart molecular devices. Herein we fabricate a self-organized, phototunable 3D photonic superstructure from monodisperse droplets of one-dimensional cholesteric liquid crystal (CLC) containing a photosensitive chiral molecular switch with high helical twisting power. The droplets are obtained by a glass capillary microfluidic technique by dispersing into PVA solution that facilitates planar anchoring of the liquid-crystal molecules at the droplet surface, as confirmed by the observation of normal incidence selective circular polarized reflection in all directions from the core of individual droplet. Photoirradiation of the droplets furnishes dynamic reflection colors without thermal relaxation, whose wavelength can be tuned reversibly by variation of the irradiation time. The results provided clear evidence on the phototunable reflection in all directions.

DUAL FREQUENCY CHOLESTERIC LIGHT SHUTTERS

We developed a dual frequency cholesteric light shutter which utilizes a cholesteric liquid crystal; the dielectric anisotropy of the material changes from positive to negative when the frequency of applied voltage is increased. The shutters are switched to the transparent homeotropic texture by low frequency voltages, and switched to the reflecting planar texture by high frequency voltages. We designed a waveform which reduced the transition time from the homeotropic texture to the planar texture to less than 100 ms.

Widely Tunable, Photoinvertible Cholesteric Liquid Crystals

Researchers have examined the utility of this mesophase in a range of topical areas including optics (switchable or tunable fi lters), photonics (selective shutters), and lasing (mirrorless feedback cavity). [ 1 , 2 ] Like other liquid-crystal phases, CLCs are responsive to a range of stimuli including temperature, electric fi elds, and light. The color switching or tuning response to each of these stimuli has been recently reviewed. [ 3 ] In particular, photoresponsive CLCs offer the capability of dynamic selective refl ection cued by light itself. Photoresponsive CLCs have been examined for more than forty years. Early examinations looked at photodegradation of halide-containing cholestryl mixtures [ 4 , 5 ] as well as guest–host mixtures with photochromics such as azobenzene. [ 6 ] Vinogradov et al. were the fi rst to demonstrate comparably larger optical responses by employing photoresponsive chiral materials to formulate the CLC mixture. [ 7 ] In this case, a menthone chiral dopant yielded a approximately 400-nm tuning of the refl ection bandgap with exposure to a 442-nm helium–cadmium laser. Convergent to this effort, Feringa, [ 8 , 9 ] Schuster, [ 10–12 ] and others pursued the synthesis of novel photochiral materials, which at the time were focused towards the development of an optical switch. In these examinations, as well as others that have followed, a variety of photochromic chiral materials have been examined including menthone, [ 7 , 13 , 14 ] chiral olefi ns, fulgides, [ 15 , 16 ] azobenzene, [ 17–24 ] and overcrowded alkenes. [ 25–27 ]

Flexible bistable cholesteric reflective displays

Cholesteric liquid crystals (ChLCs) exhibit two stable states at zero field condition - the reflecting planar state and the nonreflecting focal conic state. ChLCs are an excellent candidate for inexpensive and rugged electronic books and papers. This paper will review the display cell structure, materials and drive schemes for flexible bistable cholesteric (Ch) reflective displays.

Thermally, photochemically and electrically switchable reflection colors from self-organized chiral bent-core liquid crystals

We report the synthesis and characterization of two new chiral 1,3-phenylene based five ring bent-core mesogens that combine the unique electro-optic characteristics of banana-shaped molecules with chiroptic properties. Azobenzene moiety incorporated as a linking unit in one of the rigid arms renders trans–cis isomerization property to the molecules while chirality is introduced by tethering chiral aliphatic terminal chains. Both compounds can self-organize into helical superstructure, i.e. cholesteric mesophase, which can selectively reflect light. The novelty of the helical self-organized superstructure reported here lies in its low molecular weight single component molecular system that is truly multifunctional so that the reflection band is tunable by three different external stimuli, i.e. temperature, light and electric field. A red shift in reflection colors is obtained by changing the temperature on cooling and by UV irradiation while a blue shift is seen by electrical field application. Due to the high applicability of azobenzene-doped liquid crystalline systems, we also evaluated the efficiency of these chiral bent-core molecules as chiral transfer agents and found that they behave similar to rod-shaped dopants whose chirality is a consequence of the presence of one chiral center.

Transient Dielectric Study of Bistable Reflective Cholesteric Displays and Design of Rapid Drive Scheme

Transient dielectric measurement is used to study the transitions among the planar, focal conic, and homeotropic states of cholesteric liquid crystals. If the initial state is the field‐induced homeotropic state, at low bias fields, the liquid crystal transforms to the planar state in a sequence of homeotropic‐transient planar–planar; at high bias fields, the liquid crystal transforms to the focal conic state. The homeotropic‐transient planar transition is on the order of 1 ms while the homeotropic‐focal conic transition is on the order of 100 ms. Large hysteresis is observed in the transitions between the homeotropic and the focal conic state. Based on the rapid homeotropic‐transient planar transition and the hysteresis effect in the focal conic‐homeotropic transition, we have designed a drive scheme which can address bistable reflective cholesteric displays at the speed of one line per millisecond.