Reorientation Of Liquid Crystal Research Articles

Exploring the Photorefractive Effect in Polymer-Dispersed Liquid Crystals Using Near-Field Scanning Optical Microscopy

Static and dynamic near-field scanning optical microscopy (NSOM) methods are used to study physical phenomena associated with photorefractivity in dye-doped polymer dispersed liquid crystals (PDLCs). Micrometer-sized liquid crystal droplets doped with perylene and N,N'-dioctyl-1,4,5,8-naphthalene diimide (NDI) dyes, which serve as the photoexcitable electron donor and acceptor, respectively, are studied. The droplets are encapsulated within a thin poly(vinyl alcohol) film. Simultaneously recorded topographic, fluorescence, and birefringence NSOM images show the films to be morphologically complex, containing ellipsoidal and collapsed ellipsoidal droplets. The presence of the collapsed droplets suggests the dissolved dyes may be surface active. Nevertheless, the NSOM images prove the vast majority of dye is contained within the well-defined liquid crystal droplets. Data on the local ion migration and liquid crystal reorientation dynamics is obtained by dynamic NSOM imaging methods. A sinusoidally modulated electric field is applied across the sample, between the aluminized NSOM probe and the electrically conductive film substrate. Lockin detection of the modulated optical signal resulting from field-induced liquid crystal reorientation is used to obtain amplitude and phase images of the sample. Comparison of the images recorded before, during, and after photogeneration of ions shows spatial variations in the relative importance of ion generation and migration on the liquid crystal orientation state. Computer simulations of the field-dependent ion migration dynamics and the liquid crystal response are used to better understand the data obtained.

Optical multisoliton generation in nematic liquid crystals.

The nonlocal nonlinearity stemming from molecular reorientation in nematic liquid crystals supports the formation of multiple solitary waves, following the onset of spatial modulational instability from both wide and focused input beams. We report experimental observations of such phenomena at power levels of hundreds of milliwatts in planar cells with a voltage bias.

Measurement of Genuine Azimuthal Anchoring Energy in Consideration of Liquid Crystal Molecular Adsorption on Alignment Film

The photopolymer material of poly(vinyl cinnamate) (PVCi) is known as a liquid-crystal (LC) alignment film whose surface azimuthal anchoring energy is relatively small. However, the results of an experiment on LC reorientation under an electric field suggest that the genuine azimuthal anchoring energy is one order of magnitude higher than that measured by the conventional torque balance method and/or the Néel wall method. Here we clarified that this inconsistency is due to the LC molecular adsorption on the alignment film occurring during the isotropic-nematic phase transition process. In this paper, we propose an improved torque balance method by which the genuine azimuthal anchoring energy can be estimated.

Collective and Individual Molecular Nonlinear Photonics of Liquid Crystals

This review will examine the origins of nonlinear light scattering processes in nematic liquid crystals, and explore various nonlinear photonic processes associated with optically induced director axis reorientation effects. Our theoretical prediction shows that the upper limit of nematic liquid crystal reorientation nonlinearity can be as high as 1000 cm2/W. The supra-nonlinear responses of nematic liquid crystals enable various self-action or electro-optical guiding, mixing, switching and modulation of light with unprecedented low power thresholds. Owing to the broadband birefringence of NLC, we expect to realize optical elements/devices capable of similar multi-functional operations throughout the visible — infrared regime. We also discuss the optical limiting action of isotropic-phase liquid crystal filled nonlinear fiber arrays.

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.

The initial angle solution for the ultra-short laser pulse induced transient twist reorientation in a plane-aligned nematic liquid crystal

The optical phase retardation method was employed to observe the transient twist reorientation effect of a plane-aligned nematic liquid crystal induced by ultra-short laser pulses. The experimental result indicated that, although the electric field intensity of laser pulses is much higher than that needed for twist deformation, it does not ensure the twist reorientation under the laser pulse. This is because the width of laser pulse is very short and the response time for the twist reorientation is very long. To overcome this barrier an optimum angle (45°)between the polarization of the laser pulse and the director orientation in substrates is found by analysis.Using this initial angle, we observed a plane-aligned nematic liquid crystal cell doped with a few o-(D-Thr)N-C2-(TPP) to possess memory effect after the laser pulse pump, which was important in optical data storage and memory display.

Photo-aligned and photo-conductive polymer layers for photorefractive liquid crystal cells of high transmittance

The optical performance of photorefractive cells of liquid crystals strongly depends on the alignment of liquid crystals stabilized by surface states. Two photo-aligned polymer layers with relatively weak anchoring energy were investigated by FT-IR and UV/VIS spectroscopies. The cells without alignment layers showed significant scatterings due to random switching under an applied field. The cells with photo-alignment layers showed the high transmittance because of the high orientation order, but a little decrease in gain coefficient and diffraction efficiency because of the decrease in trap sites. Therefore, it is concluded that the reorientation of liquid crystals onto photo-conduction layer would govern the photorefractivity in two-beam coupling experiments.

Stripe patterns in the magnetic reorientation of a glass-forming nematic liquid crystal.

We study spontaneous pattern formation in a glass-forming nematic liquid crystal during the magnetically induced dynamic Fréedericksz transition. Pattern growth rates and wavelengths as functions of the magnetic field are extracted from optical transmission textures of thin planar cells. The characteristics of the observed stripe pattern can be related to viscoelastic parameters of the nematic by means of a linear stability analysis of director fluctuation modes. The viscous properties of the material allow to vary the time scales of the experiment with temperature by orders of magnitude, leaving the spatial structure of the pattern essentially unchanged. We find that the ratios of shear and rotational viscosity coefficients relevant for the pattern wavelength selection remain constant in the temperature range investigated, whereas their absolute values change by almost two orders.

Temperature dependence of the director reorientation of dye-doped nematic liquid crystals subjected to an optical field and its relation with the non-linear absorption coefficient

The temperature behaviour of the torque amplification factor η in the E48-D4 dye-host/liquid crystal-guest dyed system illuminated by a laser beam has been investigated using a single beam Z-Scan experimental technique. A plot of η versus temperature showed a monotonic aspect, different from other anthraquinone dyed liquid crystals. This behaviour, in our case, is consistent with the monotonic dependence on temperature of the linear αo and non-linear β optical absorption coefficients. In particular, the non-linear optical absorption coefficient related to the reorienting process is negative and of the order of-0.25 mm W-1. The temperature dependence of η seems to be directly correlated to β and not to αo. Since drastic conformational modifications on dye molecules are not expected on passing from the ground to the excited state, the role of the electronic structure seems to be dominant in the reorienting process in dyed nematics.

Photoinduced reorientation and thermal effects in an oligothiophene-doped liquid crystal system

Photoinduced reorientation (photon-mode process) and thermal effects in an oligothiophene-doped liquid crystal (LC) system were investigated by self-diffraction measurements in different experimental configurations. The results obtained in homeotropic sample cells were compared with those obtained in homogeneous cells. Optical Freedericksz transition could be induced by low-intensity light in the homeotropic cell, and the induced birefringence was affected by the thermal effect. No photoinduced reorientation effect was observed in the homogeneous cell in the present experimental configuration; however, due to the thermal modulation of the refractive index of the LC, which arises from light absorption, self-diffraction phenomena could also be observed.

The Role of the Orbital Angular Momentum of Light in the Optical Reorientation of Liquid Crystals

In this work we studied experimentally the optical effects produced by the orbital angular momentum of light on the Optical Fréedericksz Transition (OFT) in a nematic liquid crystal film. The measurements were carried out using a linearly polarized astigmatic laser beam with elliptical cross-section. The laser-induced reorientation was found to be strongly dependent on the angle between the polarization and the major axis of the beam profile. A threshold increase, out-of-polarization-plane reorientation and persistent oscillations were found to be the newest features observed in our experimental conditions. We ascribed all these features to the presence of an additional torque due to the orbital angular momentum of light.

The In-Plane Switching in the Nematic Cell

We have analyzed the nematic liquid crystal reorientation in IPS mode and we present the result of theoretical study of this problem. In calculations we used the non uniform electric field profile [1] for interdigital electrode configuration.

Surface-mediated light-controlled Friedericksz transition in a nematic liquid crystal cell

Surface-mediated director reorientation in a pure nematic liquid crystal (LC) cell in the presence of both a dc and low-power optical field has been observed. A surface-dependent lowering of the Friedericksz transition voltage in a planar cell upon light irradiation was found. It is believed that this effect is due to a light-induced change in the near-surface ion concentration in the presence of a dc field and a photosensitive surface. Enrichment in the ion concentration near the surface causes a redistribution of the electric field in the cell and its localization near the surface. As a consequence, the energy of interaction between the LC and the dc field near the surface overcomes the anchoring energy and results in the director reorientation.

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.

EFFECT OF AZIMUTHAL SURFACE LAYER ANCHORING ON FIELD-INDUCED LAYER REORIENTATION IN A SHORT PITCH FERROELECTRIC LIQUID CRYSTAL

The concept of the azimuthal surface layer anchoring (ASLA) is introduced to describe the field-induced layer reorientation in a short pitch ferroelectric liquid crystal (FLC). It was found that for the larger ASLA, the smaller layer reorientation occurs and saturates in the high field regime, especially near both surfaces. The ASLA tends to renormalize the surface interactions which produce the asymmetric director switching. It is suggested that the ASLA acts as a hindered restoring force for the layer reorientation.

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.