Crystal Reorientation Research Articles

Study of a Holographic Grating based on Dye-Doped Polymer-Ball-Type Polymer-Dispersed Liquid Crystal Films

This study investigates the characteristics of the holographic grating formed in polymer-ball-type polymer-dispersed-liquid crystal (PBT-PDLC) films, doped with a diazo dye (G206). A dye-doped PBT-PDLC sample was fabricated, and used to write a holographic grating. Experimental results indicated that the grating had memory of the polarization of the writing beams. This polarization memory effect was inerasable if the sample was heated to the isotropic phase, and then cooled down to room temperature. Based on these observations, we believe that the memory of the grating effect does not relate to the intrinsic memory in the transmission versus applied voltage curve of PBT-PDLC films, which is thermally erasable. Rather, the effect is due to a feature of the grating, resulting from the reorientation of the liquid crystals through their interaction with the photo-induced adsorption of the doped dyes on the surface of the polymer balls.

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.

Alignment of Photo‐Crosslinkable Copolymer Liquid Crystals Induced by Linearly Polarized Ultraviolet Light Irradiation and Annealing: Effect of Heating Rate

We describe thermally enhanced molecular reorientation of photo-crosslinkable methacrylate copolymer liquid crystal (CPLC) films with a photoreactive 6-[4-(2-cinnamoyloxyethoxy)biphenyloxy]hexyl side group and a photoinactive n-(4-cyanobiphenyloxy)alkyl (nCB, n = 5 or 6) side group. The alignment of the mesogenic groups was induced by irradiation with a linearly polarized ultraviolet (LPUV) light and subsequent annealing at different heating rates. The orientational behavior was investigated by polarized UV and FT-IR spectroscopy. For a film of copolymer P1 (n = 6), both mesogenic groups aligned homogeneously in a direction parallel to the electric vector (E) of the incident LPUV light regardless of the annealing conditions. On the other hand, the heating rate highly influenced the orientational behavior of a film of P2 (n = 5). The out-of-plane alignment of mesogenic side groups was observed when the heating rate after the LPUV light exposure was 90°C/min, while in-plane alignment was more prevalent in the mesogenic groups for the slow heating rate. Tilt angle control of the alignment direction was feasible by the use of oblique LPUV light irradiation. In addition, the inclination angle of the mesogenic groups for the P2 film could be regulated at the same irradiation angle by changing the heating rate.

Effects of Pulsed Electric Fields on a Nematic Device with a Ferroelectric Ceramic Substrate

We report details of the interaction between a thin film of ferroelectric ceramic (lead zirconate-lead titanate) and an overlying layer of nematic liquid crystal. Pulsed electric fields of 1 ms duration and of varying amplitudes and polarity have been used to achieve different degrees of poling within devices in which one substrate includes the ferroelectric ceramic layer. The degree of orientation of the nematic liquid crystal is found to depend on both the amplitude and polarity of the poling voltage employed. The magnitude of the poling voltage necessary to induce reorientation in the liquid crystal compares reasonably with the calculated value. The reorientation of the nematic liquid crystal due to the interaction with poled grains of PZT is stable for at least several months.

Photoinduced Reorientation of Liquid Crystals Doped with a Mesogenic Oligothiophene

We studied the dynamic behavior of director-reorientation induced by linearly polarized light in an oligothiophene-doped liquid crystal (LC) by transmission-mode analyses and the Z-scan measurement. An in-plane reorientation process occurred and LC molecules tended to reorient parallel to the polarization of incident light in the case of low-intensity irradiation. The thermal effect due to absorption affects the reorientation behavior largely in the case of high-intensity irradiation.

Homogeneous and Inhomogeneous Director Dynamics of a Fluorinated Liquid Crystal

The director reorientation of a thermotropic nematic liquid crystal in electric and magnetic fields has been investigated by means of 1H and 19F nuclear magnetic resonance. A new method which uses electric fields to set the desired initial angle of the nematic director with respect to the magnetic field is described. Using this method it was possible to perform reorientation experiments in the magnetic field starting in a large number of initial angles and to investigate the transition from homogeneous to inhomogeneous reorientation processes in detail. It is shown that reorientation processes in the presence of both electric and magnetic fields remain homogeneous for initial angles close to 90°. An analysis of the 1H and 19F NMR spectra shows inhomogeneous processes in the magnetic field beginning at considerably smaller angles. An intermediate region of about 6° where homogeneous and inhomogeneous processes take place at the same time has been found.

Simultaneous Near-Field Optical Birefringence and Fluorescence Contrast Applied to the Study of Dye-Doped Polymer-Dispersed Liquid Crystals

The local optical and electrooptical properties of dye-doped polymer-dispersed liquid crystal (PDLC) thin films are explored by near-field scanning optical microscopy (NSOM). Detailed information on the mechanisms for dye alignment and electric-field-induced dye reorientation in localized sample regions is sought. Dye reorientation is predicted to occur by two distinct processes: (i) via its interactions with the liquid crystal host and (ii) via the direct interactions of its permanent and induced dipole moments with the applied field. The liquid crystal is doped with a nearly isotropically aligned BODIPY dye so that contributions of both reorientation mechanisms can be explored. An order parameter of 0.10 ± 0.02 is measured for BODIPY in bulk aligned samples, suggesting that host−guest interactions are very weak. Simultaneously recorded birefringence (transmission) and fluorescence NSOM images confirm that the dye is apparently not well aligned, even on submicrometer length scales. Surprisingly, NSOM studies of dye reorientation prove that host−guest interactions still play a substantial role in controlling the dye reorientation process. The field dependence of dye reorientation is found to be correlated with liquid crystal reorientation, varying spatially across individual droplets. Analysis of the simultaneously recorded birefringence and fluorescence data shows that the dye is strongly reoriented via its interactions with the reorienting liquid-crystal host and also via its independent interactions with the applied field. The former mechanism is manifested as a rotation of the dye alignment axis while the latter leads to a field-dependent increase in the local dye order parameter. “Bistable” alignment of the dye in the nematic host is proposed as a possible explanation for the apparent coupling of the two reorientation mechanisms.

On the local and average energy release in polarization switching phenomena

We derive an expression for the energy release during polarization switching in ferroelectric/ferroelastic materials with piezoelectric coupling. The energy release density is proportional to the local changes of the remanent and material properties due to crystal reorientation, and depends on the local electromechanical fields before and after switching. Its equivalent in a representative volume element is applied to constitutive modeling of repolarization and analysis of switching stability in a ferroelectric material.

STH centers in LiF and NaH crystals: ab initio calculations

An attempt has been made to compare between the energetic properties of the self trapped hole STH center in LiF and NaH isoelectronic crystals, using an embedded cluster model and second order Moller-Plesset perturbation correction of ab initio theory. LiF and NaH clusters were embedded in simulated Coulomb fields that closely approximate the Madelung potentials of the host crystal and STH reorientation, band structure and adsorptivity of atomic H were examined. LiF and NaH clusters in crystals were found to exhibit distinct differences in the energetic properties. While the off-center configuration in the bulk of LiF was more stable than the on-center configuration, the on-center configuration in the bulk of NaH was more stable. The activation energy barriers were calculated to be 0.07 and 0.30 eV for bulk and surface diffusion of STH in LiF, and 0.66 and 0.60 eV for bulk and surface diffusion of STH in NaH. On basis of X 2 − bond elongation, LiF surface crystalline potentials were found to be less important than those of NaH. The order of stability of atomic H over LiF and NaH surfaces was totally reversed under the effect of STH imperfection. The STH enhances the adsorptivity of atomic H by ca. 4.49 and 3.85 eV on LiF and NaH surfaces respectively, and changes the nature of adsorption from physical adsorption to chemical adsorption. The dramatic increase in adsorption energies was found to be attributed to spin pairing rather than the differences in energy gaps between adsorbate and substrate electrons in the course of adsorbate substrate interactions.

Mechanical twinning of jadeite – an indication of synseismic loading beneath the brittle–plastic transition

The orientation distribution of mechanically twinned jadeite crystals in a metagranite from the Sesia Zone, Western Alps, is evaluated to derive the orientation of the principal stress directions for a prominent stage of ductile deformation at ca. 300 to 350 °C that has been interpreted to reflect synseismic loading and postseismic creep by Kuster and Stockhert (1999). It is tested whether the orientation distribution can be used to obtain information on the magnitude of differential stress if the critical shear stress for twinning is known. Assuming that the critical shear stress of 140 MPa determined by Kolle and Blacic (1982) for mechanical twinning of clinopyroxene in the (100) [001] system holds true for jadeite, and assuming a homogeneous stress field as a first approximation, differential stresses of the order of 1–2 GPa are inferred for metagranites with ca. 15% jadeite, and of 0.5 GPa for jadeitite with 80% of jadeite. Notwithstanding the uncertainty for the critical resolved shear stress for jadeite, these unexpectedly high values are suspected to be due to a combination of the following effects: (1) an inhomogeneous stress field in the polyphase material with curved stress trajectories, (2) stress concentration in jadeite due to load transfer from the quartz matrix, and (3) minor reorientation of the jadeite crystals in the flowing quartz matrix. Hence, absolute values of differential stress cannot be derived from the orientation distribution of twinned jadeite in polyphase rocks. Notwithstanding the failure in deriving reliable absolute values, a short period with exceptionally high stress must have occurred, and is fully consistent with the microstructural record of other minerals. It is proposed that the mechanical twinning of jadeite was caused by a short-lived stress peak and flow at laboratory strain rates related to quasi-instantaneous loading of the uppermost level of the ductile lower crust during brittle failure of the brittle upper crust in a major seismic event.

Laser-induced surface and bulk reorientation of the director in azo-dye-doped liquid crystal cells

The director reorientation in the liquid crystals (LCs) cells initiated by the trans– cis isomerization of azo-dyes was studied with the aid of pump–probe–beam experiments. Two different types of the samples were used, namely, a planar aligned LC layer, doped with an azo-dye, placed between substrates coated with rubbed poly(vinyl alcohol) (PVA) films, and an undoped LC layer placed between substrates, one of which (input) was coated with an untreated azo-polymer film and other one was coated with rubbed PVA film. The cells were excited with nanosecond pulses of the second harmonic of an Nd:YAG laser. By comparing the reorientation dynamics with the help of dynamic holography method in both samples it was established that the values of development times of the orientational effects were the same. The reorientation develops within 15–17 μs and the efficiency of the orientational gratings is 13–15%. In both kinds of samples the director orientation on the output surface with the rubbed PVA coating was not changed after irradiation. Contrary, the orientation of the director on the input substrate was changed in both cases. In case of a LC cell with azo-modified surface the reorientation is due to the photoisomerization of the azo-polymer film. In case of the cell with azo-dye-doped LC layer the photoisomerization of dye in the bulk of LC together with PVA film rearrangement on input substrate exert an influence on the director reorientation.

Photorefractive Effect in Nematic Liquid Crystal Cell with Sandwiched Structure

ABSTRACTOptical responses and orientational photorefractivities of nematic liquid crystal cells doped with fullerene were investigated with a sandwiched structure. The photorefractive effect was attributed to the reorientation and switching of liquid crystals using a photoalignment layer for vertical alignment. When nematic liquid crystals were injected at a nematic phase, a better cell texture could be obtained due to the capillary flow-induced orientation effect. Samples with this method showed the higher photorefractive effects than those injected at an isotropic phase. Therefore, the state of molecular ordering is important for the orientational photorefractive effects.

Role of dye structure in photoinduced reorientation of dye-doped liquid crystals

The giant optical nonlinearity associated with photoinduced molecular reorientation of dye-doped nematic liquid crystals has been measured for a homologous set of dyes belonging to the anthraquinone family, dissolved both in a polar and in a nonpolar liquid crystal host. We found a strong sensitivity of the nonlinearity merit figure to the detailed structure of the dye substituent groups. Our results provide some insight into the molecular mechanisms underlying this phenomenon, whose full understanding will finally require a detailed picture of guest–host intermolecular interactions and their dependence on the molecule electronic state.

A Thiophene Liquid Crystal as a Novel π-Conjugated Dye for Photo-Manipulation of Molecular Alignment

DIRE—the dye-induced reorientation effect—occurs when the optical-field-induced reorientation of liquid crystals (LCs) is enhanced by dyes such as the thiophene-based oligomer TR5 shown in the Figure. The synthesis of TR5 is described and its DIRE characterized. It is shown that TR5 provides a new way of controlling LC alignment using low-intensity light (see also cover).

Electrically induced dynamic processes in nematic liquid crystals: H1 nuclear magnetic resonance investigations

The director reorientation of some nematic liquid crystals in the presence of both magnetic and electric field is described by a modified from of the Leslie equation. Proton nuclear magnetic resonance experiments observing static director orientations in different angles between the director and the magnetic field are described together with reorientation experiments driven by various electric fields. A new technique to measure Δχ/Δε is presented which gives direct access to the anisotropies without any influence of elastic properties. Using electric fields of different strength and in different angles with respect to the magnetic field the dynamic processes in liquid crystals can be investigated in a very flexible way. A number of experiments investigating the homogeneous director reorientation in the electric field is presented. As one result the rotational viscosity was determined. As a surprising result we found a homogeneous reorientation in the electric field and an inhomogeneous reorientation back in the magnetic field even though the initial angles between the director and the external field were the same.

Letter Surface and bulk reorientation in ferroelectric liquid crystals

Letter Surface and bulk reorientation in ferroelectric liquid crystals

Letter surface and bulk reorientation in ferroelectric liquid crystals

The switching processes in ferroelectric liquid crystals are probed optically using a combination of transmission between crossed polarizers and surface evanescent field reflection techniques. This allows the behaviour of the liquid crystal director reorientation to be studied in the bulk of a device and near the surface simultaneously. Interesting behaviour is observed, and it is found that the processes can be adequately modelled using a simple one-dimensional model to represent the director structure across the thickness of the liquid crystal layer.

Longitudinal extra torque in liquid crystals due to the optical beam shape

Optical reorientation in nematic liquid crystals turns out to depend not only on the intensity and polarization of the pumping laser beam, but also on the shape of the transverse cross-section. A longitudinal extra torque acting on the molecular director appears whenever the beam cross-section shape breaks the system cylindrical symmetry.

Nanometer-scale resolution and depth discrimination in near-field optical microscopy studies of electric-field-induced molecular reorientation dynamics

Electric-field-induced molecular reorientation dynamics in polymer-dispersed liquid crystal (PDLC) films are characterized in detail using near-field scanning optical microscopy (NSOM) methods developed previously [Mei and Higgins, J. Phys. Chem. A 102, 7558 (1998)]. In these experiments, a modulated electric field is applied between the aluminum-coated NSOM probe and an indium-tin-oxide (ITO) substrate. The field causes reorientation of the liquid crystal within the ITO-supported PDLC film. The reorientation process is observed by near-field optical means. In this paper, it is conclusively shown that under appropriate conditions the dynamics observed occur in extremely small volumes, and are substantially confined within the near-field optical regime. The volume in which the dynamics are probed may be controlled by varying the experimental parameters (i.e., field strength and modulation frequency) employed. Conclusive evidence for confinement is obtained from both theoretical arguments and experimental results. Calculations of the electric fields in a model dielectric medium show that the largest fields occur very near the NSOM probe. Experimental observation of spatial variations in the threshold (i.e., the “Frederiks transition”) for liquid crystal reorientation provide further evidence. The most direct evidence is provided by the observation of sub-diffraction-limited resolution in dynamics images of ≈ 1 μm thick samples. Spatial variations in the observed dynamics are interpreted to reflect the energetics of local liquid crystal organization, the details of the reorientation process, and also polymer/liquid-crystal interfacial interactions. Finally, important information on the local rotational viscosity and elastic force constants within individual liquid-crystal droplets is obtained.

Polarized infrared study of a polymer network deformation in a nematic liquid crystal host

Using polarized infrared (IR) spectroscopy we have observed deformation of a polymer network in a liquid crystal host during the reorientation of the liquid crystal by an external electric field. In the system studied, containing 2% BMBB-6 polymerized at zero applied field in the host nematic liquid crystal 6CB, the observed deformation angle was between 20o and 40o 10o. These experimental results provide some of the first conclusive experimental evidence that the polymer network elastically deforms as a direct result of the reorientation of the liquid crystal host.