Kung-Lung Cheng

A microsecond-response polymer-stabilized blue phase liquid crystal

A polymer-stabilized blue-phase liquid crystal (BPLC) with microsecond response time is demonstrated using a vertical field switching cell. The measured decay time is 39 μs at room temperature (21 °C) and then decreases to 9.6 μs at 44.3 °C. Such a response time is 1-2 orders of magnitude faster than that of a typical BPLC device. The responsible physical mechanisms are the collective effects of short pitch length, strong polymer network, and low viscosity through temperature effect. The on-state voltage of our BPLC device is 44.2 V, hysteresis is below 0.7%, and contrast ratio is over 1300:1.

Critical Field for a Hysteresis-Free BPLC Device

A correlation between the peak electric field and hysteresis of a polymer-stabilized blue-phase liquid crystal (BPLC) is found experimentally. If the peak electric field is below ~ 5 V/μm, hysteresis is negligible. Based on this guideline, we propose elliptical protrusion electrodes to reduce peak electric field which in turn eliminates hysteresis while keeping a high transmittance. Such a hysteresis-free BPLC device is highly desirable for display and photonic applications.

A full-color reflective display using polymer-stabilized blue phase liquid crystal

We demonstrate a full-color-capability reflective display using red, green, and blue sub-pixels of polymer-stabilized blue phase liquid crystal (BPLC) with different pitch lengths. Vivid colors originate from three-dimensional BPLC photonic crystalline structure. Surface alignment plays a key role to generate uniform and saturated colors in the voltage-off state. Analogous grayscale is achieved by the electric-field-induced unwinding of double-twist structure. This working principle is drastically different from the phase retardation effect of conventional liquid crystal displays. Moreover, the submillisecond response time enables crisp video displays without image blurring. Potential applications for reflective 3D display are also analyzed.

High birefringence and high resistivity isothiocyanate-based nematic liquid crystal mixtures

The molecular structures and physical properties of several single‐ and double‐fluorinated isothiocyanatotolane, isothiocyanatocyclohexyltolane, and isothiocyanatoterphenyl compounds are reported. Two eutectic mixtures comprising these compounds are formulated and their properties evaluated. These mixtures exhibit a high birefringence, relatively low viscosity, high resistivity, and good photo and thermal stabilities. Potential applications of these mixtures for spatial light modulators, optical phased arrays, and high speed photonics are discussed.

Heterocyclic benzoxazole‐based liquid crystals

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds 1a–c and a Pd complex 1d derived from benzoxazole as the core group are reported. These compounds were prepared by the ring closure reaction of 4‐alkoxybenzoic acid 4‐[(4‐alkoxy‐2‐hydroxyphenylimino)methyl]phenyl esters 6 in the presence of lead(IV) acetate. All the compounds were characterized by 1H and 13C NMR spectroscopies and elemental analysis. The phase behaviour of these mesogenic compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All the compounds 1a exhibited nematic (N) and/or smectic C (SmC) phases, as expected for rod‐like molecules; however, the compounds 1b and 1c exhibited crystal phases. For those compounds 1a having shorter carbon chains (n = 1, 3, 4) nematic phases were observed, whereas for compounds having longer carbon chains (n = 6, 7, 8, 10, 12, 14) smectic C behaviour was also observed at lower temperatures. The greater aspect ratio (l/d) of compounds 1a compared with 1b and 1c was found to be required for the observation of liquid crystallinity. The fluorescent properties of these compounds were also examined. All λmax peaks of the absorption and photoluminescence spectra of compounds 1a–1c occurred at c. 316–322 nm and 371–382 nm, respectively. The quantum yields of some compounds were relatively low, and also slightly solvent‐dependent.

Low voltage and high contrast blue phase liquid crystal with red-shifted Bragg reflection

We demonstrated a low voltage and high contrast polymer-stabilized blue phase liquid crystal (BPLC) by shifting the Bragg reflection to a red wavelength. To retain high contrast ratio, a left-handed BPLC is sandwiched between a right-handed circular polarizer and a left-handed circular analyzer. The driving voltage is reduced by 35% as compared to that of a transparent BPLC, while keeping submillisecond response time. The proposed approach would accelerate the emergence of BPLC for next-generation display and photonic applications.

Dielectric dispersion on the Kerr constant of blue phase liquid crystals

Dielectric dispersions on the Kerr constant of two polymer-stabilized blue phase liquid crystals (BPLCs) are investigated. An extended Cole-Cole model is proposed to fit the experimental results and good agreement is obtained. As the electric field frequency increases, Kerr constant decreases and the associated dielectric heating effect gradually increases. These results will undoubtedly affect the high frequency operation of BPLC devices.

Dynamic response of a polymer-stabilized blue-phase liquid crystal

Fast response time is the most attractive feature of polymer-stabilized blue phase liquid crystals (PS-BPLCs). We have investigated the dynamic response of a PS-BPLC under various electric fields and found that the response time becomes slower as the applied electric field exceeds a critical field. Further analyses of experimental data reveal that two relaxation processes are involved. Possible mechanism is proposed to explain the behavior of each process. These results provide useful guidelines for achieving fast response time without hysteresis.

Diluter Effects on Polymer-Stabilized Blue Phase Liquid Crystals

Diluter effects on the phase transition temperatures, Debye relaxation frequency, and electro-optics of polymer-stabilized blue phase liquid crystal (BPLC) are investigated. Some diluters not only significantly decrease the BPLC response time but also slightly enhance the Kerr constant. For practical applications, optimal diluter concentration should be considered in order to balance the needs between response time and operation voltage.

Liquid crystal based sensors for the detection of cholic acid

The concentration level of cholic acid is a biomarker for the early diagnosis of liver and intestinal diseases. We present a biosensor platform based on the anchoring transition of 4-cyano-4′-pentylbiphenyl (5CB) liquid crystals at surfactant-laden 5CB/aqueous interfaces for the detection of cholic acid (CA) in aqueous solution. In the biosensor platform, the competitive adsorption of CA at the surfactant-laden 5CB/aqueous interfaces can trigger a homeotropic-to-planar anchoring transition of the 5CB at the interface, which can be easily observed using a polarizing optical microscope. We find that the detection limit of the 5CB based biosensors for CA depends on the pH and ionic strength of the aqueous phase and the headgroup of the surfactants.