Reorientation Of Liquid Crystal Research Articles

Plasmonic nanohole electrodes for active color tunable liquid crystal transmissive pixels.

Plasmonic pixels have been shown to offer numerous advantages over pigment-based color filters used in modern commercial liquid crystal (LC) displays. However, wideband dynamic tunability across the visible spectrum remains challenging. We experimentally demonstrate transmissive electrically tunable LC-nanohole pixels operating across the visible spectrum with unpolarized input light. An ultrathin Al nanohole electrode is designed to exhibit a polarized spectral response based on surface plasmon resonances. An output analyzer in combination with a nematic LC layer enables pixel color to be electronically controlled through an applied voltage across the device, where LC reorientation leads to tunable mixing of the relative contributions from the plasmonic color input. The nanostructured Al layer, acting as a combined electrode, polarizer, and functional color filter, is highly promising for electro-optic display applications.

Optically-driven switching of a planar nematic liquid crystal cell with parallel rubbing

This letter reports on the switching of a planar nematic liquid crystal cell with parallel rubbing of the alignment layers, under the application of a voltage, when there is initially an optical field. The voltage application over the liquid crystal in such a cell leads normally to the formation of multiple domains because there is the two switching directions are equivalent. However, an incident optical field under an angle will locally reorient the director and break the symmetry between the equivalent switching directions. The subsequent application of a voltage pulse amplifies the tilt angle and leads to the formation of a dominant domain, with an order of magnitude larger size than the optical beam profile. Several switching conditions are demonstrated for different incident angles of the beam. It is shown that the final switching direction of the entire cell is determined by the tilt angle of the optical field. The lensing effects due to the modified director distribution in the domain walls is analyzed qualitatively. Full Text: PDF ReferencesI. C. Khoo, Liquid crystals (2nd ed. Hoboken (NJ), Wiley, 2007) CrossRef A. Zolotko, V. Kitaeva, N. Kroo et al. OCBP. JETP Lett. 32, 158?162 (1980). DirectLink J. Beeckman, K. Neyts, X. Hutsebaut X, et al. "Simulations and experiments on self-focusing conditions in nematic liquid-crystal planar cells", Opt Express, 12, 1011? 1018 (2004). CrossRef M. Peccianti, C. Conti, G. Assanto, et al. "Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells", Appl. Phys. Lett. 77, 7 ? 9 (2000). CrossRef N. Kravets, A. Piccardi, A. Alberucci et al, "Bistability with Optical Beams Propagating in a Reorientational Medium", Phys. Rev. Lett. 113, 023901 (2014) CrossRef A. Piccardi, N. Kravets, A. Alberucci et al, "Voltage-driven beam bistability in a reorientational uniaxial dielectric", APL Photonics 1, 011302 (2016). CrossRef V. Nersesyan, T. Brans, F. Beunis, R. Drampyan , J. Beeckman, K. Neyts, "Light-controlled reorientation of nematic liquid crystal driven by an electric field", Liquid crystals, 43, 1422-1430 (2016). CrossRef J. Beeckman, K. Neyts, W. Cort, et al. "Non-linear light propagation and bistability in nematic liquid crystals", Proc SPIE 7414, 74140K (2009). CrossRef

Oriyentatsiyna nestiykist' dyrektora v planarniy nematychniy komirtsi z kerovanymy mezhovymy umovamy v elektrychnomu poli

The planar-planar director reorientation in a nematic liquid crystal (NLC) cell by an external static electric field has been studied. The gliding of the NLC director easy axis over the polymer substrate due to the interaction between the easy axis and the electric field is taken into account. The contribution of this interaction to the surface free energy of the system is assumed to be linear in the electric field, if the elastic fragments of substrate polymer molecules possess their own dipole moments. If the dipole moments of the elastic fragments of polymer molecules are induced by the electric field, the corresponding contribution is taken to be quadratic in the electric field. Depending on the character of the interaction between the easy axis and the electric field, the orientation instability of the director is shown to either have a threshold or not. In both cases, the dynamics of the director and the easy axis has been analyzed, by starting from the field switching-on time moment, during the transition of the system to a stationary state, and until the field is switched-off and the system returns to the initial homogeneous state.

Transformation of cholesteric orientational structures and optical textures induced by the electric field–driven ionic modification of surface anchoring

The reorientation of a cholesteric liquid crystal with a large helical pitch induced by the electric field–driven modification of surface anchoring is investigated. In the initial state, the liquid crystal cell has a homeotropic alignment of the director. An applied dc electric field produced a twisted homeoplanar structure of the cholesteric.

Electrically tunable all-dielectric optical metasurfaces based on liquid crystals

We demonstrate electrical tuning of the spectral response of a Mie-resonant dielectric metasurface consisting of silicon nanodisks embedded into liquid crystals. We use the reorientation of nematic liquid crystals in a moderate applied electric field to alter the anisotropic permittivity tensor around the metasurface. By switching a control voltage “on” and “off,” we induce a large spectral shift of the metasurface resonances, resulting in an absolute transmission modulation of up to 75%. Our experimental demonstration of voltage control of dielectric metasurfaces paves the way for new types of electrically tunable metadevices, including dynamic displays and holograms.

Fast Switching and Low Operating Vertical Alignment Liquid Crystal Display With 3-D Polymer Network for Flexible Display

Vertical alignment liquid crystal (LC) cell driven by in-plane field with 3-D polymer network exhibited no pooling mura under an external mechanical pressure; however, the operating voltage of the device was increased, because the polymer network hinders field-induced reorientation of LC. In this paper, we adopted a modified cell structure in which a counter electrode on top substrate of the conventional mode is existed, to improve upon this drawback. The proposed device in which the polymer network is formed in bulk of vertically aligned LC layer shows a very fast response time of 2 ms (rise + decay), 57% reduction in operating voltage, and also keeps image quality although the cell is curved.

Electrically reconfigurable split ring resonator covered by nematic liquid crystal droplet.

In this paper, an electrically reconfigurable split ring resonator (SRR) covered by sessile droplet of nematic liquid crystal (LC) is demonstrated experimentally. The magnetic resonance of single SRR decreases gradually by 237 MHz as external bias voltage is applied, resulting from increasing fringing capacitance due to liquid crystal molecular reorientation along local electric field distribution. The transmission phase can be modulated by more than 100 degrees. Furthermore, frequency tuning range of SRR increases with the droplet height, because of the significant enhancement for SRR capacitance difference between LC states with/without bias voltage. This work will be of interest for the development of reconfigurable metasurface and related application.

Effect of surface treatment on molecular reorientation of polymer-stabilized liquid crystals doped with oligothiophene

Irradiation of dye-doped liquid crystals (LCs) with linearly polarized light leads to molecular reorientation, which manifests functional properties for various nonlinear optical (NLO) applications. Material designs with lower light intensity thresholds for molecular reorientation have been explored, and nematic LCs have been one of the most attractive choices because of the high NLO properties. Here we present a different approach to reduce light intensity for reorientation by modifying a substrate surface that controls initial molecular orientation in polymer-stabilized nematic LCs doped with oligothiophene. The surface of the glass substrate was treated with various concentrations of a silane coupler. Water contact angle measurement and analysis of samples using polarized optical microscopy revealed that surface anchoring in the initial state decreased as the silane coupler concentration decreased. The threshold intensity was successfully reduced by 30% simply by optimizing the silane coupler concentration. This finding clearly indicates that weak surface anchoring is key to the reduction of light intensity for molecular reorientation. The molecular orientation of polymer-stabilized nematic liquid crystals (PSLCs) doped with oligothiophene can be easily manipulated using irradiation with linearly polarized light above a threshold intensity. The photoresponsive behavior depended on the initial molecular orientation, as controlled by the surface treatment of PSLC cells. The threshold light intensity for molecular reorientation was successfully reduced by 30% by optimizing the silane coupler concentration because of the decrease in surface anchoring. Thus, weak surface anchoring leads to a reduced threshold light intensity for molecular reorientation.

Orientational transition in the cholesteric layer induced by electrically controlled ionic modification of the surface anchoring

ABSTRACTA reorientation of cholesteric liquid crystal with a large helix pitch induced by the electrically controlled ionic modification of the surface anchoring has been studied. In initial state, the cholesteric helix is untwisted completely owing to the normal surface anchoring specified by the cations adsorbed at the substrates. As a result, the homeotropic director configuration is observed within the cell. Under the action of DC electric field, one of the substrates becomes free from the layer of surface active cations, therefore, setting the planar surface anchoring. The latter, in turn, leads to the formation of the hybrid chiral structure. The threshold value and dynamic parameters have been estimated for this process as well as the range of control voltages, which do not allow the electrohydrodynamic instabilities. The twisted hybrid director configuration observed in the experiment has been analysed by means of the simulation of polarisation change of light propagating through the cholesteric layer with asymmetric (planar and homeotropic) surface anchoring on the cell substrates.

Dynamics of a director reorientation and optical response of polymer films filled with a liquid crystal under strong electric field

The dynamics of the director reorientation in nematic liquid crystals (NLC) confined by cylindrical cavities of porous polymeric films under strong electric field E with has been investigated theoretically. The main attention was paid to the specific mode of field application characterized by abrupt changes of the applied voltage's polarity. In experiments with porous films filled with a liquid crystal 5CB such mode resulted in appearance of strong peak -like decreasing of an optical transparence of the films. Two mechanisms of such unusual response based on assumption of electrically induced motion of ions and overall motion of a liquid were considered and applied to explain experimental results.

Nematic liquid crystal reorientation around multi-walled carbon nanotubes mapped via Raman microscopy.

We have studied the formation of topological defects in liquid crystal (LC) matrices induced by multiwalled carbon nanotubes (MWCNTs) and external electric fields. The defects are ascribable to a distortion of the LC molecular director in proximity of the MWCNT surface. The system is analyzed macroscopically using spectroscopic variable angle ellipsometry. Concurrently, confocal micro-Raman spectroscopy is used to study the system state at the microscale. This allows to acquire a three-dimensional, spatially-resolved map of the topological defect, determining scale length variations and orientation topography of the LC molecules around the MWCNT.

Electro-optical Kerr effect of azo-dye-doped liquid crystals using nulled intensity method

ABSTRACTThe electro-optical Kerr effect and pre-transition behavior have been investigated for different concentration of dimethyl-aminoazobenzene dye-doped nematic liquid crystal by using the nulled intensity method. Measurement results have showed that the molecular reorientation of liquid crystals induced by photoisomerization of azo dyes come about to increase an amount of Kerr constant at low percentages of dye concentration. Additionally, the results of pre-transition temperature which were obtained by extrapolating the invers Kerr constant as a function of temperature have been found to be in good agreement with predictions of the Landau-De Gennes model.

Light-controlled reorientation of nematic liquid crystal driven by an electric field

ABSTRACTThe joint influence of optical and (quasi-)static electric fields on the orientation of liquid crystal gives rise to peculiar effects. In this article we report on the generation of transient domains in liquid crystals, which are an order of magnitude larger than the size of the optical field profile. The formation of such a domain is due to the fact that the initially present optical field reverses the pre-tilt, and the voltage that is then applied gives rise to an amplification of the tilt angle. The resulting reorientation of the director strongly depends on the starting conditions of the preliminary present optical field. We demonstrate different switching conditions, depending on the relation between the incident angle of the beam and the pre-tilt angle. The resulting refractive index profiles give rise to lensing effects.

Effect of an electric field on the orientation of a liquid crystal in a cell with a nonuniform director distribution

The electric field-induced reorientation of a nematic liquid crystal in cells with a planar helicoidal or a homeoplanar structure of a director field is studied theoretically and experimentally. The dependences of the capacitances of these systems on the voltage in an applied electric field below and above the Freedericksz threshold are experimentally obtained and numerically calculated. The calculations use the director distribution in volume that is obtained by direct minimization of free energy at various voltages. The inhomogeneity of the electric field inside a cell is taken into account. The calculation results are shown to agree with the experimental data.

Modeling of molecular reorientation in nematic liquid crystals

Modeling of molecular reorientation in nematic liquid crystals

Electrically Tunable Quasi-3-D Mushroom Plasmonic Crystal

This paper reports an electrically tunable plasmonic crystal incorporating a nematic liquid crystal (LC) layer on the top surface of quasi-3-D mushroom plasmonic nanostructures. The presented plasmonic crystal is formed by an array of polymeric mushroom nanoposts with gold disks at the top and perforated nanoholes in a gold thin film at the bottom. The coupling between surface plasmon polariton (SPP) and Rayleigh anomaly (RA) is observed in experiments with quasi-3-D plasmonic crystals, and verified by simulations. The coupled SPP-RA resonance mode has its electric field vector prominently normal to the surface of the plasmonic nanostructures, and extends into the surrounding medium. This feature makes the coupled resonance sensitive to molecular reorientation in LC, and thus, is useful for designing index modulation-based tunable plasmonic crystal devices. Therefore, by applying external voltages across the LC layer, the SPP-RA resonance mode shows a redshift of 8 nm with a 35% change in the amplitude.

Light controlled phase shifter for optofluidics.

We report the light-induced control of optical phase shift by a liquid crystal cell made with LiNbO3:Fe crystals as substrates. We show that a phase shift of a few πs can be easily reached for two orthogonal polarizations due to liquid crystal reorientation driven by the photovoltaic electric field originated in the substrates. This effect is exploited to get all-optical switching with a contrast of about 80%, by 2 s pump pulse.

Electrically tunable liquid crystal laser using a nanoimprinted indium-tin-oxide electrode as a distributed feedback resonator.

We demonstrated electrical tunability of a liquid crystal (LC) laser using a nanoimprinted indium-tin-oxide (ITO) film as a distributed feedback (DFB) resonator, a transparent electrode, and an alignment layer for LCs. From the field-induced reorientation of LCs and changes in effective refractive indices of guided laser modes, lasing emission is tuned by 6 nm at low applied voltage of 8.0 V. This is because the LC laser with the nanoimprinted ITO electrode has no additional insulating layers for lasing performance. The present system is based on the functional electrode and its active control provides various applications and advances in laser technology.

Enhanced diffraction properties of photoinduced gratings in nematic liquid crystals doped with Disperse Red 1.

Diffraction properties of photoinduced gratings recorded by overlapping two coherent beams at 532 nm in nematic liquid crystals doped with Disperse Red 1 were investigated with a probe beam at 632.8 nm. The grating was formed due to the alignment of dye molecules that leaded to the reorientation of the liquid crystal phase. The diffraction efficiency of the photoinduced grating was found to increase rapidly when the sample temperature was close to the clearing point in the nematic phase and a nearly 30-fold enhancement of the first-order diffraction efficiency was obtained. The pretransitional enhancement of the diffraction efficiency was discussed in terms of the reorientation of liquid crystals, optical nonlinearity effects and the onset of critical opalescence near the nematic-isotropic phase transition. Moreover, a peak shift of diffraction efficiency towards the lower temperature was observed with the increase of recording light intensity, which was attributed to laser induced photochemical disordering.

Effect of dopant nanoparticles on reorientation process in polymer-dispersed liquid crystals

The analysis of the experimental data of the nanoscale powders application for doping polymer-dispersed liquid crystals (PDLC) was represented in this work. A model based on the separation of the liquid crystals reorientation process on the surface mode and the volume mode was proposed and tested. In the research the wide-spread model mixture PDLC were used. But alumina nanoparticles were the distinctive ones obtained by electron beam evaporation. The proposed model allowed to conclude that the nanoparticles localization at the surface of the droplets (as in the Pickering emulsion) lead to the variation of the connection force between the liquid crystals and the polymer. The effect of nanoparticles resulted in an acceleration of the reorientation process near the surface when the control field is turned on and in a deceleration when it is turned off. The effect for the different size particles was confirmed.