Dual-frequency Nematic Liquid Crystal Research Articles

Dielectric behaviour of binary dual-frequency nematics with low crossover frequencies

Binary dual-frequency nematic liquid crystals synthesised at the Military University of Technology were measured by dielectric means. All the investigated materials exhibit low crossover frequency. They were composed of only two molecules with longitudinal and transverse dipole moments. The main difference is the molecular weight of the molecules. Results of relaxation parameters of detected mode and crossover frequency are shown. The influence of structure on ion conductivity is discussed. The broad phase transition from an isotropic phase to a nematic one was observed.

Molecular and Intramolecular Motions in Dual-Frequency Nematic Liquid Crystal, at Extra Low Temperatures, Observed by Means of Dielectric Spectroscopy

New dual frequency nematic liquid crystal mixture has been prepared and investigated by means of dielectric spectroscopy in a wide temperature range (from 120°C to −100°C). The molecular structures of compounds used to prepare the final mixture and mixture's composition influence its dielectric properties. Wide temperature investigations show how molecular and intramolecular motions change with temperature. It is possible at low temperatures to slow down intramolecular motions so much that they are visible in dielectric spectroscopy. At −80°C molecular motions disappear from dielectric response. At −100°C all thermally activated motions are too slow to be detected by means of dielectric spectroscopy.

Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to −60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.

Symmetrical optical response in the hybrid-oriented twist structure of a dual-frequency nematic liquid crystal

The reorientation dynamics of the director, which affects the optical response of a dual-frequency nematic liquid crystal under the action of an electric field in cells with hybrid-oriented twist structure, has been studied in the framework of continuum theory. The proposed model makes it possible to find the angular distribution of the director for arbitrary values of both the elastic constants and the initial tilt angles. Hydrodynamic flow was neglected in the calculations. It is shown that, for a specific liquid crystal, the turn-on and turn-off times of the cell can be equal when sinusoidal signal pulses with low and high frequencies are applied to it at definite voltages.

Dielectric Properties of Compounds Creating Dual-Frequency Nematic Liquid Crystals

In this paper we report how dielectric spectroscopy can help in creating of dual-frequency nematic liquid crystals. Dual-frequency nematic liquid crystals is new class of liquid crystal materials. Such mixture is usually formed by a combination of many components (even more than 10), which can be split into two groups: molecules having large transverse dipole moment and molecules with a large longitudinal dipole moment. The behavior of a base (parent) mixture, functional admixtures and nal dual-frequency nematic liquid crystals mixture is investigated by dielectric spectroscopy in wide frequency (100 Hz 10 MHz) and temperature ranges. This allows us to nd out why the dual-frequency liquid crystal has an important feature: positive and negative dielectric anisotropy at di erent frequencies. We present parameters of molecular motions around short (S-mode) and long (L-mode) molecular axes observed in investigated materials and discuss how the creation of nal dual-frequency nematic liquid crystals mixture can modify molecular relaxations.

Frequency-Controlled Routing of Self-Confined Beams in Nematic Liquid Crystals

We demonstrate the electro-optic control of self-confined beams via frequency modulation in dual-frequency nematic liquid crystals. Proper design of comb electrodes in a planar cell allows the maximization of the overall steering angle when only walk-off variations are used. A span angle as large as two times the maximum walk-off angle is achieved.

Frequency-controlled deflection of spatial solitons in nematic liquid crystals

We demonstrate frequency-controlled angular steering of light-induced solitary waveguides using dual frequency nematic liquid crystals. Specially designed comb-shaped electrodes allow the maximization of the in-plane electro-optic angular deviation of the soliton Poynting vector, modulating the resulting walk-off up to an overall deflection of 13° when the frequency of the applied voltage goes from 1 to 100 kHz.

Advanced bistable cholesteric light shutter with dual frequency nematic liquid crystal

In this paper, a direct switching between a transparent (or reflecting) planar (P) state to an opaque (or transparent) focal conic (FC) state and vice-versa of a polymer free bistable cholesteric light shutter without any homogeneous polyimide (PI) layer, is demonstrated based on the sign inversion of dielectric anisotropy of dual frequency liquid crystal (DFLC). The direct switching was achieved by applying square wave field at low (1 kHz) and high (50 kHz) frequency. As a result, the DFLC light shutter sustains bistable bright and dark states in electric field off state and exhibits excellent electro-optic performance. The direct switching from the FC to P states not only supports more uniform P state but also significantly reduces switching voltage by eliminating the high field homeotropic (H) state required for the switching in the conventional polymer stabilized cholesteric texture (PSCT) light shutter. The driving voltage applied to make a transition from the P to FC one is relatively low (3Vp-p/µm). Further, switching time from FC to P state was reduced drastically with homeotropic PI layer. Results show that dual frequency cholesteric liquid crystal (DFCLC) light shutter holds a great promise for use in energy efficient display devices and switchable windows.

Specific features of attenuated light transmission by liquid-crystal twist cells in constant and alternating electric fields

Optical transmission characteristics of dual-frequency nematic liquid crystal (NLC) twist cells with different alignment layers (rubbed polyimide and obliquely deposited cerium dioxide) have been studied in constant and alternating electric fields. It has been established that a change in the optical (twist effect) threshold and dynamic range of attenuated transmission depend both on the boundary conditions (that influence the screening of applied voltage) and on the parameters of the applied electric field. The maximum dynamic range (49.5 dB) has been obtained in the cell with a CeO2 alignment layer controlled by a constant potential. In the case of an alternating electric field, the dynamic range decreases because of reduced effective voltage.

Pretilt Angle Effect on Response Time of Dual Frequency Liquid Crystal

Dynamics of splay and bend deformations of the dual frequency nematic liquid crystal were investigated. Response times for 2π phase retardation were determined when a sinusoidal bias voltage was applied to cells. Experimental dependences of response times at 1 kHz and 30 kHz frequencies of electric fields from the director pretilt angles in interval from 40 to 75 degrees were found. It was shown the influence of electric field waveform on a threshold and response times of cells. This study could be helpful for device designing based on a dual frequency nematic liquid crystal.

Screening Effect of a-C:H Alignment Layer and its Influence on Characteristics of LC Cells

The influence of the hydrogenated carbon alignment layer electrostatic screening effect on characteristics of dual frequency nematic liquid crystal was investigated. The screening effect was declined as the result of the threshold voltage reduction from 6 V to 1 V connected with the fourfold decrease in alignment layer thickness. The pretilt angle rising and the phase retardation reduction were observed at the same time. It is shown that hydrogenated carbon alignment layer application is able to change electrical properties of the cells as well as to accelerate the switching time at the wavelength of 1.55 μm.

Chiral Flexoelectro-Optic Effect in a Surface-Polymer Assisted Cholesterics with Uniform Lying Helix

Fast-switching flexoelectric cholesteric liquid crystal displays that can be operated in two modes: amplitude (flexoelectric effect, in-plane-switching) and phase (dielectric coupling, out-of-plane switching) have been developed. The device comprises of a small amount of polymer network localized at the substrate surfaces and short-pitch cholesteric liquid crystal whose helical axis laid in the direction parallel to the substrates. The surface-localized polymer network stabilizes the helical axis in the plane parallel to the substrates at zero voltage. The response times for amplitude and phase switching of a polymer-stabilized cholesteric display with 2 microns cell gap are 200 μs at 3.5 V/μm and 3 ms at 12.5 V/μm, respectively. Using a dual-frequency switchable nematic liquid crystal, we are able to improve the contrast of the amplitude switching by obtaining a larger deviation angle of helical axis with a high voltage and in the same time, suppressing the helix unwinding.

Dual frequency operation of a blue phase liquid crystal

The authors demonstrate dual frequency switching in a cubic blue phase liquid crystal. The dual frequency blue phase was prepared by adding a strongly twisting chiral dopant to a dual-frequency nematic liquid crystal mixture. The crossover frequency appeared at few hundred kHz in the blue phase, and sub-millisecond response was obtained for dual-frequency operation, which was comparable to conventional voltage operation. This new class of material is advantageous over conventional blue phases in polarization-independent phase modulation devices, since a larger lattice deformation can be obtained.

How the initial tilt angle of the director of a dual-frequency liquid crystal affects the electrooptic characteristics of cells

This paper discusses cells filled with a dual-frequency nematic liquid crystal (DNLC) in which the initial tilt angle θp of the director was varied in the interval from 40° to 75° by varying the surface relief of the orienting layers. The experimental dependences of the time to change the phase lag by 2π at a wavelength of 0.65 µm on angle θp for longitudinal and transverse deformation of the DNLC layer have been obtained for the first time. It is shown that transverse elastic deformation is a slower process than longitudinal elastic deformation. The ON or OFF time of the cells as a result of the process of longitudinal elastic deformation occupied 1–2 ms when a field is applied with frequency 1 kHz, and was 3–9 ms for transverse deformation by applying an electric field with frequency 30 kHz.

Reorientation dynamics of a dual-frequency nematic single crystal with quasi-homeotropic structure

This paper discusses the features of the switching dynamics of the phase lag of light by 2π and 4π in cells with a dual-frequency nematic liquid crystal as a result of bend and splay deformation of layers with oblique homeotropic orientation by means of an electric field with frequency 30kHz and as a result of relaxation when a low-frequency field of 1kHz is applied. The slope angle of the director was varied from 36° to 84° by means of cerium oxide layers obliquely deposited in vacuum. Studies of the surface texture of the orienting layers by means of atomic-force microscopy showed that this is achieved as a result of variation of the surface nanotexture.

Dynamic characteristics of dual-frequency nematic liquid crystal with quasi-homeotropic twist structure

Dynamic characteristics of a liquid crystal (LC) cell with a quasi-homeotropic twist structure formed in a dual-frequency nematic liquid crystal (DFNLC) layer with the director pretilt angle increased to 60° have been experimentally studied. The cell was switched from the off to on state using a 30-kHz electric field, while the reverse (off/on) switching was effected by a 1-kHz field. An increase in the director pretilt angle allowed the switch-on time of a 6.4-μm-thick DFNLC cell to be reduced to 1 ms and the relaxation (switch-off) time, to 0.5 ms.

Electrically controlled relaxation at twist deformation of a dual-frequency nematic liquid crystal

The relaxation of a dual-frequency liquid crystal at the twist effect and the influence of the external electrical circuit parameters on the relaxation process in the case of a large initial inclination angle (44) of the director are studied. It is found that oscillation arising at the trailing edge of the modulator’s electro-optic response considerably increases the time of relaxation due to the action of a high-frequency electric field. The influence of the electric field on the relaxation time is stronger, the thinner the liquid crystal layer. It is experimentally shown that the duration of the interval between the removal of low-frequency voltage from and the application of high-frequency voltage to the modulator affects the relaxation time.

Dielectric relaxation dynamics in a dual-frequency nematic liquid crystal doped with C.I. Acid Red 2

The dielectric relaxation dynamics of a dual-frequency liquid crystal-dye composite were studied and the effect of the photoinduced isomerization of the azo dye on the dielectric spectrum of the cell under illumination by an argon-ion laser operating at wavelength of 514.5 nm and irradiance of 550 mW cm −2. It was found that the illumination process has a profound influence on the dielectric spectrum of the dye-doped dual-frequency liquid crystal in the low-frequency range.

Influence of the alignment layer and the liquid crystal layer thickness on the characteristics of electrically controlled optical modulators

The screening effect of the amorphous hydrogenated carbon (a-C:H) alignment layer and its dependence on the thickness of a dual-frequency nematic liquid crystal (NLC) layer have been studied. Optimization of the a-C:H layer thickness allows a threshold voltage for the optical S-effect to be reduced and the characteristic switching time and relaxation time of 0.5 and 2.5 ms, respectively, to be obtained for a phase retardation of 2π at a wavelength of 0.86 μm.

Dynamics of twist effect in a dual-frequency nematic liquid crystal

The dynamics of the electrooptical 90° twist effect in a dual-frequency nematic liquid crystal is investigated for wavelengths of 0.65 and 1.55μ m. It is shown that the boundary conditions of the interaction between the phases affect the optical threshold of the twist effect, the contrast, and the working voltage range. The switching time of the twist effect from the off to the on state upon a variation of the amplitude of a rectangular dc voltage pulse from 15 to 50 V changes from 1.5 to 0.3 ms for a thickness of the nematic crystal layer of about 7 μ m. The minimal time of switching from the “on” to the “off” state was 3 ms in the case when relaxation of molecules in a cell with asymmetric boundary conditions was controlled electrically. The dynamic range of transmittance variation at a wavelength of 1.55 μm extended to 30 dB.