Nematic Liquid Crystal Film Research Articles

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Generally, macroscopic molecular orientation state in liquid crystal layer can be described by Ossen and Frank's continuum theory. However, the validity of applying the continuum theory to nematic liquid crystal thin film (NLCF) has not been confirmed yet. In this study, we experimentally examined NLCF by employing spectroscopic ellipsometer. As a result, theoretical and experimental result represent a good agreement when NLCF thickness is from 60 to 250 nm. Furthermore, the influence of surface roughness on alignment film surface and glass substrate was also examined.

Singular optical reordering of liquid crystals using Gaussian beams

We introduce the concept of singular optical reordering of birefringent elastic media usingGaussian beams. Theoretical and experimental results are reported in the particularcase of a uniformly aligned nematic liquid crystal film illuminated at normalincidence by a circularly polarized beam. The longitudinal component of thelight field is demonstrated to be at the origin of cylindrically symmetric singularreorientation of the optical axis that can be described by the superposition of a radialand azimuthal elastic distortion field. Moreover, the handedness of the overallreorientation pattern is controlled by the handedness of the incident beam circularpolarization.

Full Optical Control of Holographic Gratings Realized in Composite Materials Containing Photosensitive Liquid Crystals

We report on the characterization of an all optical switching effect that takes place when a UV light beam irradiates an azo-POLICRYPS diffraction grating. This high quality polymeric structure is utilized to confine and stabilize a well aligned nematic liquid crystal (NLC) film doped with a small percentage of azo-NLC molecules. The optical control of the grating diffraction efficiency is obtained by exploiting the photochemical phase transition between the nematic and the isotropic phases of the liquid crystal, based on the photoisomerization of the azobenzene guest molecules. The switch ON-OFF of the grating diffraction efficiency is triggered by a UV pump laser and is investigated by means of a low power red probe beam.

Polarization-tunable chiral nematic liquid crystal lasing

A chiral nematic liquid crystal laser with arbitrarily controllable polarization was demonstrated by combining polymeric cholesteric and nematic liquid crystal films. The lasing polarization states can be continuously varied by applying voltages from 1.25 to 2 V. Right-handed circular polarization is switched to left-handed circularly polarization through −45° linearly polarized states. The trajectories of the Stokes parameters on the Poincaré sphere are used to analyze the variation in lasing polarization. The change in polarization is simulated and the simulation agrees closely with the experimental data.

Composite Holographic Gratings Containing Light‐Responsive Liquid Crystals for Visible Bichromatic Switching

Polymeric microstructures, produced in a multi-step chemico-physical process, confine and stabilize a well-aligned nematic liquid crystal (NLC) film, which is doped with a high-performance mesogenic azobenzene dye, sensitive in the visible range.

Study of selectively reflecting characteristics of polymer stabilised chiral nematic liquid crystal films with a temperature-dependent pitch length

Chiral propane-1,2-di-(4-(4-heptylcyclohexyl)benzoate) (PPCB) and 1-phenylethane-1,2-di-(4-(4-heptylcyclohexyl) benzoate) (PEPB) were synthesised and used as chiral dopants to induce chiral nematic liquid crystals (N*-LCs). Since the helical twisting power of the former first decreased and then increased as a result of thermally induced inversion helicity sense, and those of the latter changed little with increasing temperature, the pitch lengths of the N*-LCs prepared from (N-LC)/PPCB/(R)-PEPB and N-LC/PPCB/(S)-PEPB mixtures decreased and increased with increasing temperature respectively. Then, based on these two kinds of N*-LCs, polymer stabilised liquid crystal films were obtained, in which the bandwidth of the selective reflection spectra increased with increasing temperature.

Color‐ and Reflectance‐Tunable Multiple Reflectors Assembled from Three Polymer Films

Tunings of reflectance and colors of multiple photonic bandgaps are demonstrated in the whole visible region by changing the polarization state of incident light. Two tunable systems are assembled from the same three materials of an anisotropic polymeric nematic liquid crystal film and two kinds of isotropic polymer film. The only difference between the two systems is the stacking sequence.

Calculation and Prediction: An Effective Security Device Consisted of Photo Alignment Nematic Liquid Crystal Films and Polarizers

The paper reports the methods for making a novel optical device consisted of photo alignment nematic liquid crystal films and polarizers. The color shift of the device depends on not only the viewing angle but also the viewing orientation. The retardation and intensity of the reflected light are theoretically calculated on the basis of Jones Matrix, and a spectral-RGB transition program is introduced to display colorful results directly. The theoretical results show excellent agreement with experimental data. It is an effective method for predicting interference color and optimizing key parameters of the device. The device can be used to protect valuable documents against fraud because of its special optical variable effects which is caused by interference of polarized light.

High-diffraction-efficiency holographic gratings in C_60-doped nematics

Holographic gratings were recorded in C60-doped nematic liquid crystals (NLCs) by two-beam coupling. The asymmetric energy transfer dependence on the polarity of the applied voltage and multiple diffractions in two-beam coupling indicated that the gratings were photorefractive and worked in the Raman-Nath regime. The diffraction efficiencies (DEs) of the main diffraction orders were investigated at different applied voltages. The diffractive intensity distribution was asymmetric, and a first-order DE as high as 66% (as high as 84% excluding the influence of the scattering) was obtained from the 20 microm thick C60-doped NLC film.

Micro Manipulation with Optical Responsive Cholesteric and Compensated Nematic Liquid Crystal

We have developed photoresponsive liquid crystalline films (cholesteric liquid crystal and compensated nematic liquid crystal) having azobenzene compound to manipulate micro particles on the surface by light irradiation. When few solid particles (polystyrene bead) were placed on the surface of the film, movement of the solid particles was observed upon irradiation of UV (365 nm) or visible light (>420 nm). Only rotational motion of the particles was observed on the cholesteric liquid crystal film. Both translation and rotation was observed on the compensated nematic liquid crystal film. So the mode of motion (either rotation or both translation and rotation) was changed by changing the film from cholesteric film to compensated liquid crystal film. The rotational direction of the solid was controlled by using chiral azobenzene compound with right or left handed twisting ability.

Identification of optical nonlinearities of dye-doped nematic and polymer-dispersed liquid crystals using Z-scan technique

Abstract This study investigates the nonlinear optical properties of azo-dye-doped nematic and polymer-dispersed liquid crystal (ADDPDLC) films with nano-sized LC droplets using the Z-scan technique, which is a simple but powerful technique for measuring the optical Kerr constants of materials. The results indicate that the optical Kerr constant ( n 2 ) of the azo-dye-doped nematic LC (ADDLC) film is large because of the photoisomerization effect and the thermal effect. Therefore, the optical Kerr constant of this material can be modulated by varying the temperature of the sample and the direction of polarization of incident laser. The range of n 2 modulated is from −5.26 × 10 −3 to 1.62 × 10 −3 cm 2 /W. The optical Kerr constants of ADDPDLC films at various temperatures are also measured. The experimental results reveal that liquid crystals in the ADDPDLC film strengthen the nonlinearity. The n 2 of the ADDPDLC film is maximal at ∼35 °C, because of the decrease in the clearing temperature of the ADDPDLC films. The clearing temperatures of the liquid crystals (E7), and the ADDPDLC film used in this work were found to be 61 °C and 43 °C, respectively.

Light-Induced Nonlinear Rotations of Nematic Liquid Crystal Droplets Trapped in Laser Tweezers

We report on optically induced rotations of nematic liquid crystal droplets. We show experimentally that bipolar nematic liquid crystal droplets trapped in laser tweezers having circular polarization can exhibit nonlinear rotational motion, unlike optically trapped solid birefringent micro-plates. Such nonlinear rotations are retrieved by analyzing the polarization dynamics analysis of the light beam after it has passed through the droplet. The occurrence of complex dynamics is only found for large enough trapping power and droplet diameter. The analogy with optically induced nonlinear rotations in nematic liquid crystal films is briefly discussed.

Voltage-controlled optical switch in planar nematic liquid crystal film

This study presents an integrated device that consists of a directional coupler and an electro-optic switch. The device is designed to include a nematic liquid crystal cell, comprising a grating-like electrode. Applying the appropriate voltage to the cell yields a periodically distributed refractive index. An incident polarized beam will couple to an adjacent channel if it is parallel to the channel. The coupling efficiency is controlled by applied voltage. An obliquely injected polarized beam will be reflected and refracted in the channel, and propagated along a curved path. The route of the beam can be controlled by applying the voltage. A multiport routing was achieved for voltage modulation. In addition, the distribution of refractive index is also investigated by employing conoscopic technique experimentally and numerically.

Investigation of multiple holographic recording in azo-dye-doped nematic liquid-crystal film

We report multiple holographic recording and optical address recognition schemes in Methyl Red-doped liquid-crystal film without an applied electric field. Ten gratings are recorded at a single location of a 5 mm2 area by using optical multiplexing methods, and the diffraction efficiencies are studied for multiplexed gratings. Diffraction behavior of angular and peristrophic multiplexed gratings is discussed, and two methods that involve recording angles and peristrophic rotation angles are presented for optical recognition.

Synthesis and helical twisting property of polymerizable chiral dopant with temperature-dependent solubility in liquid crystal

In this study, right-handed dicinnamate isosorbide was synthesized via the esterification reaction between optically active isosorbide and cinnamate. The chiral dopant was characterized by FT-IR, 1H NMR, elemental analysis, SEM, UV absorption spectrum. After dissolving in a nematic liquid crystal mixture, the chiral dopant exhibited a temperature-dependent solubility in the chiral nematic liquid crystal mixture. Meanwhile, a relatively high value of helical twisting power of the polymerizable chiral dopant was determined. The results show that the chiral dopant has great potential in achieving a polymer stabilized chiral nematic liquid crystal film with a broad-band selective reflection.

Conical soliton escape into a third dimension of a surface vortex

We present an exact three-dimensional solitonic solution to a sine-Gordon-type Euler-Lagrange equation that describes a configuration of a three-dimensional vector field n constrained to a surface p-vortex, with a prescribed polar tilt angle on a planar substrate and escaping into the third dimension in the bulk. The solution is relevant to characterization of a schlieren texture in nematic liquid-crystal films with tangential (in-plane) substrate alignment. The solution is identical to a section of a point defect discovered many years ago by Saupe [Mol. Cryst. Liq. Cryst. 21, 211 (1973)], when latter is restricted to a surface.

Ordering transitions in micrometer-thick films of nematic liquid crystals driven by self-assembly of ganglioside GM 1

We report an investigation of the self-assembly of the monosialoganglioside (GM 1) at interfaces formed between aqueous solutions of 10 μM GM 1 (at 25 °C) and micrometer-thick films of the nematic liquid crystal (LC) 4′-pentyl-4-cyanobiphenyl (5CB). We observe the process of spontaneous transfer of GM 1 onto the interfaces to be accompanied by continuous ordering transitions within the micrometer-thick films of the LC. At saturation coverage, the GM 1 orders the LC in an orientation that is perpendicular to the interface, an orientation that is similar to that caused by phospholipids such as dilauroylphosphatidylcholine (DLPC). This result suggests an interaction between the LC and GM 1 that is dominated by the hydrophobic tails of the GM 1. Relative to DLPC, however, we observe the dynamics of the LC ordering transition driven by GM 1 to be slow (2 h for DLPC versus 100 h for GM 1). To provide insight into the origins of the slow dynamics of the GM 1-induced ordering transition in the LC, we performed two additional measurements. First, we quantified the time-dependent adsorption of GM 1 at the LC interface by using fluorescently-labeled GM 1. Second, we used the Langmuir–Schaefer method to transfer preorganized monolayers of GM 1 from an air–water interface to the aqueous-LC interface. Results obtained from these two experiments are consistent with a physical picture in which the final stages of spontaneous adsorption/ordering of GM 1 at the aqueous-LC interface dictate the dynamics of the LC ordering transition. This rate limiting process underlying the ordering transition was substantially accelerated by heating the system above the phase transition temperature of GM 1(26 °C), suggesting that the phase state of the GM 1 micellar aggregates in bulk solution strongly influences the kinetics of the final stages of ordering/adsorption of GM 1 at the LC interface. Overall, these results and others presented in this manuscript reveal that it is possible to decorate interfaces of a nematic LC with GM 1, and that the assembly of GM 1 at these interfaces impacts the dynamic and equilibrium ordering of the LC.

Sum-Frequency Vibrational Spectroscopy Study of Photoirradiated Polymer Surfaces

Sum-frequency vibrational spectroscopy can be used to study photoinduced structural change at polymer surfaces. Results on rubbed and unrubbed surfaces of poly(vinyl cinnamate) (PVCi), a well-known photopolymer, show that polarized UV irradiation dimerizes rather than isomerizes the protruding cinnamoyl side chains at the surfaces and creates significant surface structural anisotropy needed in many applications. Anchoring energy measurements of a nematic liquid crystal film on PVCi irradiated by low UV dosages yield results in good correlation with the observed spectral anisotropy.

Principles for Manipulation of the Lateral Organization of Aqueous-Soluble Surface-Active Molecules at the Liquid Crystal−Aqueous Interface

We report an investigation of the lateral organization of water-soluble, surface-active molecules within monolayers formed spontaneously at interfaces between aqueous phases and immiscible, micrometer-thick films of nematic liquid crystals (LCs; 4'-pentyl-4-cyanobiphenyl and TL205, a mixture of cyclohexanefluorinated biphenyls and fluorinated terphenyls). Using both anionic (sodium dodecyl sulfate) and cationic (dodecyltrimethylammonium bromide) surfactants, we demonstrate that the nematic order of the LCs can direct monolayers of surfactant in dynamic equilibria with bulk aqueous solutions to phase separate and assume lateral organizations at the interfaces of the LCs that are not seen in the absence of the nematic order. The lateral organization of the surfactants is readily evidenced by the patterned orientations assumed by the LCs and can be manipulated reversibly by changes in the bulk concentrations of the surfactants. Experimental observations of the effects of bulk surfactant concentration, thickness of the film of LC, nematic order, and aqueous electrolyte concentration are placed within the framework of a simple thermodynamic model. The model incorporates the dynamic equilibration of surfactant between the bulk and interface as well as the coupling between the elasticity of nematic LCs and the lateral organization of the water-soluble surfactants within the monolayers. Qualitative agreement is found between the model predictions and experimental observations, thus supporting our conclusion that LCs offer the basis of general and facile methods to direct the lateral organization of interfacial molecular assemblies.

Anisotropic diffraction pattern formation from a nematic liquid crystals film induced by low-power linearly polarized beam

Anisotropic diffraction patterns were observed on far-field screen induced by a weak (0.16 W/cm2) linearly polarized collimated beam passing through a homeotropic C60-doped nematic liquid crystal (5CB) cell. Based on orientational photorefractive effect, two-wave coupling experiment was carried out to induce liquid crystal molecule re-orientation. The form of space-charge field with the Gaussian distribution induced by the Gaussian beam incident on the sample is investigated. The outline of far-field diffraction pattern is concentric ring, and shows symmetrical gaps in the direction perpendicular to the direction of polarization of light. In addition, diffraction pattern changes when the polarization of the incident light is changed, effective nonlinear index coefficient is n20.3 cm2/W. The results showed that our theoretical analyzsis was consistent with the observed experimental phenomena.