Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material

We present a new kind of UV-cured holographic grating that consists of polymer slices alternated with pure nematic films. By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increases diffraction efficiency.     

Dielectric properties of an ytterbium-based nematic liquid-crystal complex

The dielectric properties of an yttrium-based paramagnetic thermotropic nematic liquid-crystal complex have been studied. The conditions for the measurement of the magnitude and sign of the dielectric anisotropy of the mesophase have been determined. The relaxation of the components of the relative permittivity of a macroscopically homogeneously oriented sample has been observed. A high polarity of the metallomesogenic complex has been revealed.     

Diffraction properties of highly birefringent liquid-crystal composite gratings

We have fabricated electrically switchable holographic gratings, using Polaroid Corporation's DMP-128 photopolymer filled with the nematic liquid crystal E7. It is shown that a coupled-wave theory that includes the effects of the birefringence of the liquid crystal must be used to explain the diffraction properties of these anisotropic volume gratings. Furthermore, a detailed comparison of theory and experiment provides information about the alignment of the liquid crystal within the polymer host.     

Optically switchable, polarization-independent holographic polymer dispersed liquid crystal (H-PDLC) gratings

An optically switchable, polarization-independent holographic polymer dispersed liquid crystal (H-PDLC) transmission grating is demonstrated by adding azobenzene-LC and chiral molecules into the H-PDLC formulation. The optical switchable mechanism is from the trans-cis photoisomerization of the doped azobenzene-LC, which modulates the refractive index of the LC rich area. The dependence of the diffraction efficiency of the H-DPLC grating without chiral molecules on light polarization suggests that the orientation of LC directors within the droplet is ellipsoidal and uniaxial. However, the addition of chiral molecules into the H-PDLC formulation helps the formation of isotropic and non-uniaxial LC directors within the droplets. The polarization properties of the grating are investigated and analyzed by the coupled and modified coupled wave theory with a model of sinusoidal dielectric modulation. The results show that the addition of chiral molecules changes the LC phase from nematic to chiral-nematic, where the grating efficiency, which is modulated by the photoinduced phase transition, is independent of the polarization of incident light. Our findings may help improve optical systems that utilize non-polarized light.     

扭曲向列相液晶电光效应的研究

分析了扭曲向列相液晶的电光效应.采用溴钨灯作为光源,运用光栅单色仪选择380~1 100 nm波段的光进行照射,测量在0~8 V电压条件下扭曲向列相液晶对于不同波长入射光的透射率.     

High-resolution photorefractive gratings in nematic liquid crystals sandwiched with photoconductive polymer film

Photorefractive gratings with high grating resolution were observed in the 20 μm thick low-molar-mass nematic liquid crystal (NLC) cell with a separate photoconductive (PC) poly(N-vinylcarbazole) layer. An orientational grating with a grating spacing of 1.9 μm was produced. It is believed that a space–charge field with small fringe spacing forms in the PC layer and its evanescent component penetrates into the NLC layer. The penetrated evanescent field drives the NLC to reorient, and consequently the orientational grating forms. The model indicates that the modulated field exists in several hundred nanometers near the surface, and thus the orientational grating is not full of the NLC film, which is consistent with the observed phenomena of the multiple diffractions. Besides, asymmetric two-beam coupling of 11.2% was achieved for the grating with a grating spacing of 1.9 μm, and a net gain coefficient of larger than 62 cm⁻¹ was obtained.     

聚合物薄膜电光效应研究

介绍聚合物DR/PMMA薄膜材料做成光学法布里-珀罗腔结构,通过观察透射率随外加电场变化,以确定其非线性光学特性参量,在室温和575.0nm时的三阶极化率x~⑶(-ω;ω,0,0)=(5.3-i6.0)×10~((-11)_(θs.u))和二次电光系数R=-(8.5-i9.7)×10~(-19)m~2/V~2.     

Asymmetric transmissive behavior of liquid-crystal diffraction gratings

We have used computer simulations to predict that the beam-steering efficiency of a common liquid-crystal diffraction grating will depend on which side is presented to the incident beam. The finite-difference time-domain method and the Helmholtz-Kirchhoff diffraction integral were utilized to simulate the performance of an idealized configuration of the grating.     

Dielectric properties of liquid-crystal azomethine polymer with alkyl-substituted side chain

Physics of the Solid State - Relaxation processes in side-chain liquid-crystal azomethine polymer have been studied using dielectric spectroscopy in wide ranges of temperatures and frequencies. Two...     

Topologically patterning of polyvinyl alcohol microstructures for vertical-/hybrid-aligned nematic liquid-crystal gratings doped with polyhedral oligomeric silsesquioxane nanoparticles

We demonstrate a soft lithography approach for fabrication of a topographically patterned polyvinyl alcohol(PVA) liquid-crystal(LC) alignment layer. This specific approach employs modified micromolding in capillaries for negative replication of the PVA microstructures on indium tin oxide(ITO) substrates from patterned poly(dimethylsiloxane) molds in a single step, leading to planar alignment on the desired regions. By doping with polyhedral oligomeric silsesquioxane nanoparticles, which can induce vertical alignment on bare ITO surfaces,periodic LC phase gratings based on an alternating vertical-aligned/hybrid-aligned nematic geometry are presented as an application, and a theoretical model was used to simulate and examine the experimental results.     

A method for calculating the light extinction coefficients of a polymer layer with small nematic liquid-crystal droplets

A method is proposed for analyzing radiation extinction coefficients of polymer films containing small nematic liquid-crystal droplets with cylindrical symmetry. This method is based on the Rayleigh-Gans approximation and the effective scattering matrix. The orientational ordering of both liquid-crystal molecules in droplets and the droplets in a layer is described by multilevel order parameters. The extinction of radiation in polymer films is analyzed for the cases of oblique and normal incidence of light. Simple and convenient expressions are derived for calculating and estimating the extinction coefficients for the case of normal incidence of light on a layer containing spherical droplets. The error of the method is estimated.     

Surface-anchoring properties related to the distribution of polyimide chains in a twisted nematic liquid-crystal cell

On the basis of a general Rapini and Papoular equation and the unified surface-anchoring energy, surface-anchoring strength is newly defined theoretically as a function of the azimuthal deviation angle of the surface liquid-crystal directors and the statistical distribution of polyimide chains in a typical twisted nematic liquid-crystal cell. Then these are determined experimentally by the surface second-harmonic generation method, which is nonlinear optics, and the optical phase retardation method. We assume that the anisotropy distribution of polyimide chains induced by the rubbing strength is dominated by a Gaussian distribution around the rubbing direction.     

Angular dependent optical transmission of twisted nematic liquid-crystal layers

The angular-dependent light transmission of a twisted nematic layer between crossed polarizers in an electric or magnetic field depends mainly on the induced orientation of the director in the middle part of the layer. Measurements and computations compare favourably.     

Coexisting nematic and smectic-A phases in a twisted liquid-crystal cell

Twisted homogeneously planar-aligned nematic liquid-crystal cells are cooled into the smectic- A phase. The expected defective structure does not form. Instead the cells still show good optical-guiding characteristics. Exploration of the cells using a half-leaky guided-mode arrangement reveals that the liquid-crystal phase separates into three or more regions. Adjacent to both the upper and lower boundaries is a region of highly twisted nematic liquid crystal. In the center of the cell is one or more homogeneous smectic- A regions with smectic layers normal to the cell surfaces, separated by twisted nematic. As the cell is cooled so the smectic- A regions grow in thickness with the nematic regions progressively thinning but with increased twist gradient. A theoretical model of these novel results is presented.     

Dynamic studies of holographic gratings in dye-doped liquid-crystal films

The dynamic behavior of a holographic grating induced in a homeotropically aligned dye-doped liquid-crystal film is investigated. In the presence of an applied dc voltage, photoexcited azo dyes induce a photorefractive grating and then diffuse and are adsorbed onto cell substrates. The reorientation of liquid crystals as a result of adsorbed dyes leads to a phase grating that is phase shifted 90 degrees from the photorefractive grating. Competition of these two gratings induces two-beam coupling of the writing beams, initially transferring energy from beam 1 to beam 2 and then, after a pause, from beam 2 to beam 1.     

Control of anisotropic diffraction in liquid-crystal composite volume gratings

Orientation-controlled anisotropic diffraction gratings are realized by interferometric exposure using composite materials of nematic liquid crystals (LCs) and LC diacrylate monomers. The anisotropic diffraction properties in volume gratings, which dominantly diffract p- or s-polarized light, are shown to be controlled by the rubbed directions of the alignment layers under the control of the photopolymerization temperature. Images of the fringe patterns observed by polarization microscopy show the effects of the alignment layers on the LC orientation during grating formation.     

混合排列向列液晶盒的电光响应

本文考察了一组混合排列向列液晶盒的电光响应特性,重点研究了透射光强随外电压的变化及响应时间与外电场的关系,计算了作为外电压及开启时间函数的双折射。应用液晶的线性响应理论分析了实验结果,理论与实验基本相符。结论指出:适当改变液晶器件的结构,有可能使响应时间加快。