Peng Zenghui

Calculating the dielectric anisotropy of nematic liquid crystals: a reinvestigation of the Maier-Meier theory

This paper investigates the average dielectric permittivity () in the Maier–Meier theory for calculating the dielectric anisotropy (Δe) of nematic liquid crystals. For the reason that of nematics has the same expression as the dielectric permittivity of the isotropic state, the Onsager equation for isotropic dielectric was used to calculate it. The computed shows reasonable agreement with the results of the numerical methods used in the literature. Molecular parameters, such as the polarizability and its anisotropy, the dipole moment and its angle with the molecular long axis, were taken from semi-empirical quantum chemistry (MOCPAC/AM1) modeling. The calculated values of Δe according to the Maier–Meier equation are in good agreement with the experimental results for the investigated compounds having different core structures and polar substituents.

Rotational viscosity of a liquid crystal mixture: a fully atomistic molecular dynamics study

Fully atomistic molecular dynamics (MD) simulations at 293, 303 and 313 K have been performed for the four-component liquid crystal mixture, E7, using the software package Material Studio. Order parameters and orientational time correlation functions (TCFs) were calculated from MD trajectories. The rotational viscosity coefficients (RVCs) of the mixture were calculated using the Nemtsov–Zakharov and Fialkowski methods based on statistical-mechanical approaches. Temperature dependences of RVC and density were discussed in detail. Reasonable agreement between the simulated and experimental values was found.

Improvement of Response Performance of Liquid Crystal Optical Devices by using a Low Viscosity Component

Difluorooxymethylene-bridged (CF2O) liquid crystal (LC) with low viscosity is prepared and used as a fast response LC material. When the material is mixed with isothiocyanato LCs with high birefringence, the viscoelastic coefficient of the mixture decreases evidently and, accordingly, the response performance increases. While the concentration of CF2O LCs is about 7%, the LC mixture approximately maintains high birefringence and exhibits a fastest response performance that is 14% higher than that of pure isothiocyanato LCs. Therefore, the LC material and mixing method could find useful applications in optical devices.

Order parameters of liquid crystal on the rubbing surfaces of alignment layers

Liquid crystal (LC) alignment is most important in LC devices. In this paper, we quantitatively analyze the LC scalar order parameters on the rubbed surface of an alignment layer. Careful measurement of dichroic infrared absorbance is performed. The result gives the evidence that the order parameter of LC just on the rubbed alignment film is only 1/3–1/2 that in the LC bulk.

The review of liquid crystal wavefront corrector with fast response property

Liquid crystal wavefront corrector (LCWFC) is an optical device that can modulate wavefront of incident light, and shows advantages of high density pixels, high reliability, and high precision accuracy, and can be used in the field of optical beam shaping, optical information processing and beam transforming. All these application model show broad application prospect of LCWFC. When LCWFC is used in the optics systems, a fast response speed is always required, such as 1 ms. In optical application, specific modulation depth is always required. If the modulation depth is 1 λ , the response time of the commercial LCWFC is slow, such as, 10 ms, which block the application of LCWFC. The response speed of LCWFC always depends on two elements, the properties of LC material and the parameters of LC device. Firstly, fast response liquid crystal material with high birefringence and low viscosity was studied and reviewed in our research group. Secondly, the optimization of LC device parameters and driving method was reviewed. By the research in LC material and device, the response time of LCWFC can be fast to sub-millisecond, which will highly enhance the application region and depth of LCWFC.

Wide-Range Position-Tuning Lasers in Cholesteric Liquid Crystal

A wedge liquid crystal (LC) cell is designed and manufactured, and a dye-doping cholesteric LC laser formed by mutual diffusion of the cholesteric LC with different pitches. A laser that is tunable in the 558-624 nm range is obtained under moderate optical pumping, with a tuning range of 66 nm and a laser spectral tuning resolution of 1 nm, so as to achieve the spatial position of a wide range of tunable lasers. The laser threshold varies at different positions in the device, and the lasing thresholds of the dye-doping cholesteric LC cell at 40 and 9 mu m are 18 and 25 mu J/pulse, respectively. The density of the photonic states is simulated in the experimental sample, and the result is in good agreement with the photonic band gap in our experiment, which not only explains the low-threshold laser at the band gap edge, but also predicts the experiment.

High closed loop correction accuracy with a liquid crystal wavefront corrector

We investigate the accurate control of a liquid crystal wavefront corrector. First, the Gamma correction technique is adopted to amend the nonlinear phase modulation. Then, the control method and wavefront reconstruction are considered. Lastly, a closed loop correction experiment is carried out and a high correction accuracy is obtained with peak to valley (PV) of 0.08? (? = 632.8 nm), the wavefront phase rms 0.015?, as well as the Strehl ratio of 0.99. The diffraction-limited resolution is achieved.

Effect of molecular weight on liquid crystal photoalignment by photosensitive polyester containing thrifluoromethyl moieties

We investigate the liquid-crystal (LC) alignment direction on photoalignment films formed from photosensitive polyester containing thrifluoromethyl moieties (PPDA) with various molecular weights by crossed polarized optical microscopy. It is found that LC alignment behaviour changes with molecular weight of PPDA. The LC alignment on PPDA irradiated films with the highest molecular weight is homogeneous, while those with low and intermediate molecular weights are homeotropic. However, surface morphologies show weak dependence on molecular weight. The surfaces are smooth and there is no clear morphological anisotropy on these aligned films observed by an atomic force microscope. The surface energies of the irradiated films are also measured by using an indirect contact-angle method where both surface energy and its polar component increase with increasing molecular weight. Different polar surface energies can be considered as a main reason for different alignment characteristics.

Alignment behavior of liquid crystals on the crosslinked film of monomers with fluorinated groups

A series of photosensitive monomers, including hexafluorobiphenol A dicinnamate ester (6F-BADE) and fluorinated dicinnamate esters (FDE-n, n=2, 3, 4), were synthesized. These monomers could be crosslinked under irradiation of linearly polarized ultraviolet light. FTIR spectra and GPC spectra were used to measure the results in real time. The results indicated that the crosslinking reaction was [2+2] cycloaddition. The photoalignment films formed by monomers under the ultraviolet light could induce different types of liquid crystal (LC) alignments: 6F-BADE and FDE-n films resulted in vertical and parallel alignments, respectively. AFM images of the surfaces of the alignment showed no obvious anisotropy on the surfaces of these films. Finally, it was found that, using semiempirical method AM1 of quantum mechanics, the difference of the polarities among different monomer molecules was large, which was considered to be the main cause resulting in different LC alignments.