In-line liquid-crystal microcell polarimeter for high-speed polarization analysis

We report a type of high-speed microcell polarimeter that utilizes microelectrodes, liquid-crystal films, and ultrathin high-contrast polarizers, all integrated between the tips of two optical fibers. When combined with optimized nematic liquid-crystal materials, this compact (2.5 cm x 0.5 cm x 0.5 cm) device offers excellent optical properties and continuous, high-speed operation at > 2 kHz with moderately low operating voltages. It requires no bulk optical elements, and it shows excellent performance when implemented for the measurement of degree of polarization in 10-Gbit/s test systems. Polarimeters based on this design have promising potential applications in polarization analysis for high-speed optical communication systems.     

Observation of self-diffraction by gratings in nematic liquid crystals doped with carbon nanotubes

Diffraction gratings were studied in cells of the homogeneously aligned liquid-crystal E7 doped with multiwall carbon nanotubes. These phase gratings were induced by interference modulation of two coherent optical beams, in conjunction with an applied dc field that was perpendicular to the unperturbed director axis. Self-diffraction was observed at all angles of incidence of the writing beams, including normal incidence. A superior nonlinear-index coefficient of 5x10(-2)cm(2)/W was obtained after passage of a 44-mW/cm(2) beam through a film with a grating constant of 18 mum under an external voltage of 15 V. The observed phenomenon depends strongly on the applied dc field, and the memory effect in a nematic film depends strongly on the grating constant.     

Orientational optical torque in a nematic liquid crystal,caused by trans- and cis-isomers of low- and high-molecular compounds

The effect of light on the conformational composition of low- and high-molecular (fifth-generation dendrimer) azobenzene dyes in a nematic host is studied. The orientational optical response of the nematic liquid crystal (NLC), caused by the presence of these compounds in the nematic host, is measured. The contributions of isomers to the orientational optical torque affecting the NLC director are determined. The results obtained suggest that the increase in the optical orientation efficiency in NLCs when using high-molecular dopants instead of low-molecular ones is unrelated to the difference in their conformational composition in the light field, but is caused by deceleration of orientational motion of chromophores.     

High-damage-threshold static laser beam shaping using optically patterned liquid-crystal devices

Beam shaping of coherent laser beams is demonstrated using liquid crystal (LC) cells with optically patterned pixels. The twist angle of a nematic LC is locally set to either 0 or 90° by an alignment layer prepared via exposure to polarized UV light. The two distinct pixel types induce either no polarization rotation or a 90° polarization rotation, respectively, on a linearly polarized optical field. An LC device placed between polarizers functions as a binary transmission beam shaper with a highly improved damage threshold compared to metal beam shapers. Using a coumarin-based photoalignment layer, various devices have been fabricated and tested, with a measured single-shot nanosecond damage threshold higher than 30?J/cm2.     

Spin nematic and antinematic states in a spin-3/2 isotropic non-Heisenberg magnet

The ground state phase diagram of a general isotropic spin-3/2 system with nearest-neighbor exchange is shown to contain unconventionally ordered spin nematic and antinematic states, as well as usual ferro- and antiferromagnetic phases. The two nematic phases have spontaneously broken rotational symmetry characterized by the long-range order of the nematic director u, as well as the broken time-reversal symmetry described by the pseudospin vector σ. Nematic phase differs from antinematic one by the type of ordering in σ vectors (uniform versus staggered). The ferromagnet-nematic and antiferromagnet-antinematic phase boundaries exhibit enhanced Sp(4) symmetry and correspond to the recently studied effective theory for spin-3/2 cold gases. We discuss optical properties and topological defects in the nematic phases.     

向列相液晶中强非局域空间光孤子的实验观察

通过实验对向列相液晶中的非局域空间光孤子的传输进行了研究。理论上基于非线性的液晶孤子传输方程,采用高斯形式的试探解,得到了孤子传输的解析解,以及对液晶中的非局域孤子和Snyder等提出的强非局域孤子模型的结果进行了比较。实验上,观察了空间光孤子在向列相液晶中的传输,找到了不同束宽下空间光孤子的临界功率。比如束宽为2μm时临界功率为2.0mW。观察了向列相液晶中空间光孤子的弛豫过程,并注意到液晶分子的响应时间长达几秒钟,液晶中的孤子形成速度较慢。     

Kinetics of the isotropic–nematic phase transition in melted multi-component liquid crystal mixtures upon cooling

The kinetics of the phase separation and the nematic phase growth in two melted commercial multi-component liquid crystal mixtures upon cooling was studied using polarising optical microscopy and IR spectrometry. The droplets of the nematic phase revealed in the optical images across the phase transition were segmented and treated statistically. In the resulting size histograms of mixture B, two overlapping statistical ensembles related to two co-existing nematic phases were recognised; these phases were shown to be different in their chemical structures. In mixture A, any separation within the nematic phase was not found. The statistical ensembles of the nematic droplets were successfully described using the principles of irreversible thermodynamics. Analysis of the mean droplet diameter as a function of time allowed recognition of two regimes of the nematic phase evolution: (1) nucleation and rapid nucleus growth and (2) nucleus coalescence. Both the regimes were quantitatively described with the universal law for the cluster growth.     

Optical properties of metamaterial-based devices modulated by a liquid crystal

Due to the fact that it is possible to manipulate light with photonic crystals (PCs), PCs hold a great potential for designing new optical devices. There has been an increase in research on tuning the optical properties of PCs to design devices. We presented a numerical study of optical properties of metamaterial-based devices by liquid crystal infiltration. The plane wave expansion method and finite-difference time-domain method for both TE and TM modes revealed optical properties in photonic crystal structures in an air background for a square lattice. E7 type has been used as a nematic liquid crystal and SrTiO₃ as a ferroelectric material. We showed the possibility of the metamaterials for a two-dimensional photonic crystal cavity on a ferroelectric base infiltrated with a nematic liquid crystal.     

Light-induced changes of the refractive indices in a colloid of gold nanoparticles in a nematic liquid crystal

It was shown that irradiation of a nematic liquid crystal doped with metal nanoparticles in the visible near the plasmon resonance band led to strong thermal changes of the refractive indices. The effect was studied by recording of dynamic optical gratings in the colloid. Nanoparticles "worked" as effective nano-heaters in a matrix causing the order parameter decrease around the particles. A large nonlinearity parameter (n (2) ≈ 10(-2) cm(2)/kW and fast response (≈ 0.7 ms), with no detectable particles' aggregation and excellent photo- thermo-stability make these colloids potentially attractive nonlinear optical media. Application of a dynamic holography technique allowed measuring the coefficients of thermal conductivity of the liquid crystal along the director k (||) = (0.4 ± 0.02) W m(-1)K(-1) and perpendicular to the director k (⊥) = (0.2 ± 0.01) W m(-1)K(-1).     

Optical response from dual-frequency hybrid-aligned nematic liquid crystal cells

Dual-frequency hybrid-aligned nematic liquid crystal cells and the influence of the parameters of a control electric field on their optical response are studied. It is found that the harmonic oscillations of the optical transmission in such cells are observed in the interval between low frequency-to-high frequency voltage switchings unlike in conventional twisted nematic cells. A V-shaped bistable optical response is obtained by successively applying sinusoidal electric fields with frequencies of 1 and 30 kHz to a twisted nematic cell. For a liquid crystal layer 8 ??m thick and an applied voltage of 50 V, the response time is 10 ms. In a hybrid-aligned twisted-nematic cell with a large initial tilt angle of the director (about 70°), the V-shaped optical response is observed when the inclined homeotropic state is switched to the twisted state by applying a 30-kHz field. The initial structure of the layer recovers as a result of natural elastic relaxation, and the response time increases roughly fourfold.     

向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

Improvement of the switching frequency of a liquid-crystal spatial light modulator with optimal cell gap

In the application of a nematic liquid-crystal (LC) spatial light modulator, we derived the formula of retardation dynamic response of the device by solving the Erickson-Leslie equation. Then, the response time of the 2π phase change can be expressed as a function of the LC cell gap. The theoretical and experimental results all indicate that the response time of 2π first decreases and then increases with the LC cell gap increasing, and there is an optimal cell gap to obtain the shortest response time. Therefore, the method of optimizing the cell gap shows potential to improve the switching frequency for all type of nematic LC optical device with specific modulation quantity.     

Orientation phenomena in nematic liquid crystals under a periodic compressional deformation

The orientation behavior of homogeneous planar layers of nematic liquid crystals with open and closed ends in the field of compressional deformations caused by an acoustic effect is studied. The mechanisms determining the connection of the optical response of a nematic liquid crystal (the variable component of an optical signal and its spectrum) with the acoustic parameters (the oscillation amplitude and frequency, and the amplitude of sound pressure) and the layer thickness are revealed. The factors responsible for the mechanism and modes of acoustooptic conversion are considered. It is demonstrated that, by varying the layer thickness, it is possible to implement different modes of signal conversion. The possibility of designing a new modification of a sound receiver based on a nematic liquid crystal and the specific features of this design are discussed. Its advantages over conventional sound pressure receivers based on nematic liquid crystals are indicated, in particular, the absence of limitation of the frequency of the received signal in the low-frequency range.     

Onset of parametric electroconvection in a nematic liquid crystal

The behavior of a nematic liquid crystal (NLC) in an external ac electric field is studied in terms of the weak electrolyte model. Since electroconvection of certain types of NLCs can occur in a constant field in an oscillatory mode, a quasi-periodic response of a nematic to the external action is possible in an ac field, as well as a sharp reduction of the electroconvective stability thresholds at resonance frequencies. Stability maps for NLCs are plotted on the plane of amplitude-frequency parameters of the external field. Phase portraits are constructed for subharmonic and synchronous perturbations. The optical response of a NLC in the ac electric field is studied; the time dependences of the relative intensity of light, which passed through the nematic liquid crystal cell, are obtained.     

表面液晶-垂直腔面发射激光器温度特性的研究

本文利用向列相液晶层作为激光偏振调控单元,涂覆于垂直腔面发射激光器（VCSEL）表面,测量并分析了不同温度下VCSEL正交线偏振光的阈值电流、峰值光功率和I-P特性. 实验结果表明：温度为293 K时,涂覆液晶后激光偏振第一跳变点和第二跳变点之间的电流值ΔI增大了2.2 mA,比无液晶时增大1倍. 温度为313 K、注入电流为3 mA时,两种正交线偏振光的光功率差ΔP由133.6 μW增大到248.8 μW,进一步增加了线偏振光的各向异性. 表面液晶层的引入有效地扩大了VCSEL的正交线偏振态稳定范围和光功率差,为实现液晶VCSEL高温单偏振稳定的设计和器件制备提供了理论和实验基础. 关键词： 垂直腔面发射激光器 向列相液晶 偏振态     

平行排列丝状液晶在超短脉冲激光激励下发生瞬态扭曲形变的最佳偏振角

利用光学相位延迟法，观察到平行排列丝状液晶在超短脉冲激光（10ns）激励下的瞬态扭曲形变.实验结果表明，虽然脉冲激光的电场强度远远大于液晶发生扭曲形变的阈值，但并不能保证液晶会发生瞬态扭曲形变.这主要是因为脉冲激光的激励时间非常短，而液晶分子发生扭曲形变的响应时间相对较长，从而使其来不及形变.分析发现：激励激光的偏振方向与平行排列丝状液晶分子的最佳夹角是45°.利用此条件，观察到掺微量D型苏氨酸卟啉的平行排列丝状液晶在超短脉冲激光激励后具有记忆效应，这在光存储和记忆显示方面有很大的实用价值.     

Lyotropic mixture made of potassium laurate/1-undecanol/K2SO4/water presenting high birefringences and large biaxial nematic phase domain: a laser conoscopy study

The lyotropic liquid crystalline quaternary mixture made of potassium laurate (KL), potassium sulphate, 1-undecanol and water was investigated by experimental optical methods (optical microscopy and laser conoscopy). In a particular temperature and relative concentrations range, the three nematic phases (two uniaxial and one biaxial) were identified. The biaxial domain in the temperature/KL concentration surface is larger when compared to other lyotropic mixtures. Moreover, this new mixture gives nematic phases with higher birefringence than similar systems. The behavior of the symmetric tensor order parameter invariants σ (3) and σ (2) calculated from the measured optical birefringences supports that the uniaxial-to-biaxial transitions are of second order, described by a mean-field theory.     

High-resolution, high-speed photorefractive incoherent-to-coherent optical converter

We demonstrate a photorefractive incoherent-to-coherent optical converter driven by ultraviolet light that provides a 35-mus response time and an optical resolution of 124 line pairs/mm. The device, implemented in KNbO(3) , operates with a modulating intensity of 85 mW/cm(2) , which corresponds to an optical switching energy per bit of 0.5 pJ. A conversion rate of the order of 90 Gbits/(s cm(2)) is achieved. The conversion between the ultraviolet light and the visible laser beam at lambda=532 nm occurs through anisotropic Bragg diffraction at a modulated interband photorefractive grating. Our device has a better optical resolution and conversion rate than optically addressed solid-state spatial light modulators based on the photorefractive effect and multiple quantum wells, and it is also faster than devices based on liquid crystals.     

Nonlinear optical responses of Sudan IV doped liquid crystal by z-scan and moiré deflectometry techniques

The nonlinear optical properties of a nematic liquid crystal doped with an azo dye were investigated. The magnitude and the sign of the third-order nonlinear refractive index, n₂, were measured using both single-beam z-scan and moiré deflectometry techniques. The measurements were performed at 532 nm using a Nd:Yag CW laser. The positive sign showed that sample which was studied possess a self-focusing optical nonlinearity. Rather close agreement was found between the values for n₂ obtained by z-scan and by moiré deflectometry. The nonlinear refractive index was found to be in the order of 10^− 6 (cm²/W) and the nonlinear absorption coefficient to be β = 0.171 (cm/W).     

Design and characterization of hydrogen bonded ferroelectric liquid crystals: A study of light modulation in nematic and smectic orderings

A novel series of hydrogen bonded ferroelectric liquid crystals (HBFLC) have been isolated. Phase transition temperatures obtained by differential scanning calorimetry (DSC) and textural observations elicited by polarizing optical microscope (POM) studies are discussed. Phase diagram is constructed from the above data. Salient feature of the present work is observation of a light modulation due to field induced transitions (FiT) in nematic and smectic C* phases. Tilt angle measurements in various FiT of smectic C* phase are discussed. One of the mesogens in nematic phase behaves like an optical modulator; helical pitch measurements support the above observation.