V-shaped Switching Research Articles

Response to an applied electric field in an antiferroelectric 1/2 subphase: The role of thermal fluctuations.

The response to an applied electric field in the q_{T}=1/2 subphase of the MC881-MC452 binary mixture system is studied by using thick homeotropically aligned cells. In the ordinary antiferroelectric SmC_{A}^{*} and 1/2 (sub)phases, some nonplanar asymmetric distortions in the antiferroelectric unit cell structure produce induced polarization in the applied field direction, starts to unwind the helix from the beginning, and tends to align the averaged tilt plane direction parallel to the applied field. In the 1/2 subphase under consideration, however, the helix resists being deformed at the beginning and then the thresholdlike steep increase of birefringence Δn occurs in the transition from 1/2 to unwound SmC^{*} at a field of less than 0.5V/µm; we conclude that the thermal fluctuations play an important role in promoting the director flip-flopping in a single layer under the applied field and bring about additional induced polarization, which counteracts the aforementioned ordinary induced one and prevents the helix from unwinding. This suggests that the Langevin-like director reorientation is the mechanism of the V-shaped switching which was actually observed in the thin films of Mitsui mixture [Phys. Rev. Lett. 87, 015701 (2001)0031-900710.1103/PhysRevLett.87.015701] and must have been used in prototyped thresholdless antiferroelectric liquid-crystal displays.

4-9 Two-domain texture obtained in ferroelectric LC cells showing V-shaped switching.

4-9 Two-domain texture obtained in ferroelectric LC cells showing V-shaped switching.

Multi-Objective Optimization of an Outer-Rotor V-Shaped Permanent Magnet Flux Switching Motor Based on Multi-Level Design Method

This paper proposes an outer-rotor V-shaped permanent magnet flux switching (V-PMFS) motor for potential applications in in-wheel direct drive system. However, the cogging torque and torque ripple of the outer-rotor V-PMFS motor are relatively high, which may lead to mechanical vibration and acoustic noise, especially in low speed operation. To improve the overall performances of the outer-rotor V-PMFS motor, a multi-objective optimization method based on multi-level design method is proposed. According to the effect of the design variables on the optimization objectives, sensitivity analysis method is used to divide the design variables into two levels. Then, the two levels of design variables are optimized with two different optimization methods. After optimization, electromagnetic performances between initial and optimal motors are compared. Finally, a prototype motor is manufactured and tested. Both electromagnetic performance analyses and experimental results verify the validity of the proposed optimization method.

Analog switching in the nanocolloids of ferroelectric liquid crystals

Nanoparticle (NP) dispersion in liquid crystals (LCs) results in significant changes in the physical properties of the existing LC mixtures. Two ferroelectric liquid crystals (FLCs), 5F6T and 6F6T, have been studied for analog switching. The 5F6T sample is doped with titanium dioxide (TiO2) NPs of two different concentrations of the same average particle size and another FLC 6F6T is systematically doped with barium titanate (BaTiO3) NPs of two different average particle sizes at the same concentration. The frequency and temperature dependence of the coercive voltage of FLC nanocolloids has been studied. The V-shaped switching was observed in the case of nano-doped FLCs. The value of inversion frequency for the 5F6T+1.0 wt.% TiO2 doped sample is 30 Hz while it is 24 Hz for the 6F6T+0.5 wt.% BaTiO3 (particle size 5–10 nm) doped sample. The conductivity measurements show that the conductivity of doped samples is higher than the conductivity of their parental FLCs and can be considered the main reason for the V-shaped switching in the FLC nanocolloids, which was initially absent in their parental FLCs.

Development of simultaneous measurement system of birefringence, optical rotational power, and transmission spectra for chiral liquid crystal phases

A novel experimental setup used to measure the important optical properties of liquid crystal materials is proposed. The setup allows us to measure electric-field-induced birefringence, optical rotational power, and transmission spectra consecutively. This system can be widely applied to characterize liquid crystal materials including blue phases, ferroelectric liquid crystals, and other chiral phases. We adopted this system to study the phase transition behavior of a V-shape switching ferroelectric liquid crystal mixture and made an important correction of experimental results previously reported by Sandhya et al. [Europhys. Lett. 90, 56005 (2010)]. This finding proves the advantage of this system compared with the measurement method using individual systems.

Electro-optic and dielectric investigations of a perfluorinated compound showing orthoconic antiferroelectric liquid crystal

We observed microsecond order response ranging from 100 to 600μs with respect to temperature for a single component fluorinated rigid core antiferroelectric liquid crystals having highly tilted phases. In the antiferroelectric region, we observed V-shaped electro-optic switching in a 4μm indium tin oxide coated homogeneously aligned cell having PS, ranging from 100 to 200nC/cm2. Dielectric studies have revealed an interesting phase that exists during heating in the temperature region between 74°C and 84°C. From dielectric results studied we calculated activation energies associated with different relaxation modes using Arrhenius law.

Dielectric relaxation spectroscopy and electro-optical studies of a new, partially fluorinated orthoconic antiferroelectric liquid crystal material exhibiting V-shaped switching

In this paper we report the results from detailed electro-optical and dielectric studies in various antiferroelectric and ferroelectric phases of an orthoconic antiferroelectric liquid crystal (OAFLC) material . The material possesses high tilt and high spontaneous polarisation. Such an OAFLC, because of its high tilt, provides an excellent dark state. The material exhibits V-shaped switching in the SmC* phase. Dielectric studies reveal the existence of another phase during heating in the range between 78.6 and 92°C which did not appear in the DSC curve and in polarising microscopy. This phase has been identified as the SmCγ* phase and is extremely sensitive with respect to the cell conditions, aligning material, purity, etc. Three dielectric modes have been assigned in the above-mentioned temperature range and their origins are discussed.

Enhancing the electro-optical properties of ferroelectric liquid crystals by doping ferroelectric nanoparticles

The effects on the physical and electro-optical properties of ferroelectric liquid crystals (FLCs) after the doping of a dilute suspension of ferroelectric nanoparticles (BaTiO3) have been studied. Due to the permanent electric dipole moments of the ferroelectric nanoparticles, the spontaneous polarisation of FLCs with low doping concentration was about twice that of pure FLCs, in addition to a significant improvement in the dielectric properties, the response time and the V-shaped switching in the chiral smectic C (SmC*) phase. The results obtained point the way to an alternative for improving the applicability of FLCs without resorting to chemical synthesis.

Influence of Network Stabilization on the Dielectric and Electrooptical Properties of Ferroelectric Liquid Crystal FELIX-M4851/100

Dielectric and electrooptical properties as well as phase transition behavior in a network-stabilized ferroelectric liquid crystal were studied. The influence of polymerized network on these physical parameters were characterized. All measurements were performed upon adding monomer to the ferroelectric liquid crystal (FLC) mixture after in situ polymerization with UV radiation. We observed hysteresis-free symmetric V-shaped switching in a 2 µm cell. It was observed that both the Iso → N* and smectic A (SmA) → smectic C* (SmC*) phase transition temperatures were depressed to more ordered phase with increasing monomer concentration. The decrease was more prominent in the case of the SmA → SmC* phase transition. The Goldstone mode dielectric strength ΔεG in the SmC* phase was drastically reduced in the presence of networks. Soft mode dielectric strength was also reduced considerably. A small increase of pitch was observed with increasing monomer concentration. Spontaneous polarization was dramatically reduced in the presence of polymer networks.

Effect of Alignment Layers on the Response Time in a Half V-Shaped Ferroelectric Liquid Crystal Cell

In this paper, we have studied the influence of alignment layers on the switching characteristics in various half V-shaped switching ferroelectric liquid crystal (HV-FLC) cells. By comparing various combinations of alignment layers, we found that an HV-FLC cell with oppositely aligned polarization dipoles has the shortest response time. Such a cell not only has a faster switching characteristic but also has better alignment and is easily fabricated by a simple manufacturing procedure.

Evaluation of the Phase Modulation During V-Shaped Switching in a Smectic Liquid Crystal Device

In this work, V-shaped liquid crystals are proposed for phase modulation optical devices. The aim is to measure the relationship between phase shift and the applied voltage by an experimental set-up based on an interferometer. The optical retardation of the liquid crystal medium could then be determined simply by measuring the phase difference of the waves of the two arms. By supplying a voltage, the liquid crystal molecules are reoriented and the effective refractive index of the liquid crystal changes. Preliminary results show that the devices may be used such as phase modulators, but the phase modulation is polarization sensitive.

V-shaped electro-optic response observed in a chiral ferroelectric smectic liquid crystal

We report on the observation of V-shaped switching in a ferroelectric liquid crystal cell over a wide range of temperatures. Results of the optical transmittance in the visible region give us the helical pitch for various temperatures of the ferroelectric liquid crystalline compound used. We show that the helical pitch, in addition to the spontaneous polarization (PS) and thickness of the alignment layer of the cell, is an important factor in giving V-shaped switching. A longer or helical compensated mixture gives a better V-shaped switching.

Improved Equivalent Circuit Model for V-Shaped, Thresholdless Switching Ferroelectric Liquid Crystals

For V-shaped, thresholdless switching ferroelectric liquid crystals (FLCs), the impedance divider induced by the multilayer structure of FLC cells and the drive circuit play an important role in switching characteristics. In this paper, an equivalent circuit model that can be applied to conventional circuit simulators is proposed for the optical response prediction and drive circuit optimization of V-shaped FLCs. The model is improved from the original model of Moore and Travis; however, the impedance divider is taken into account, and both polar and nonpolar surface anchoring energies are considered to make the model more preferable. The model is then utilized to investigate thresholdless switching characteristics. Simulation results show that the hysteresis inversion frequency fi increases more than one thousand fold with the drive circuit and then decreases with REXT following the relation log fi = -alog REXT + b, and a (b) increases from 0.43 to 0.46 (2.46 to 2.66) as the amplitude of triangular voltage increases from 4 to 10 V, agreeing with experimental results. Also, the same optical transmissions are plotted as different coordinates, as a function of voltage dropping on liquid crystal layer and of drive voltage, and the results show that genuine V-shaped switching is only observed when the transmission is plotted as a function of drive voltage, coinciding with the model suggested by Blinov et al.

Temporal electrical response of chiral smectic liquid crystal displays with V/W-shaped electrooptical characteristic

AbstractChiral smectic liquid crystal cells showing V-shaped electrooptical switching have been reported as one of the most promising technologies for high-end display applications. In this work, time-resolved electrical behaviour of these devices has been obtained through a set of systematic measurements. The electrical equivalent circuit has been derived, a number of simulations at different frequencies have been performed using commercial software for analogue circuits. Performance of this electrical model to account for time domain variations of switching currents in chiral smectic LC displays with V/W-shaped electrooptical response has been analyzed as well.

Electrooptical Properties of Diacrylate Based Polymer Network Stabilized Ferroelectric Liquid Crystals

Polymer network stabilized ferroelectric liquid crystals were produced by mixing a ferroelectric liquid crystal with a small amount of photo-cross-linkable diacrylate monomer and in situ polymerization of the monomer in homogeneously aligned ferroelectric liquid crystal environment. The addition of the diacrylate monomer into the ferroelectric liquid crystal causes dramatic reduction in spontaneous polarization and decrease in response speed. The formation of diacrylate polymer network in the ferroelectric liquid crystal can improve the response speed. The V-shape switching can be achieved by properly increasing the amount of polymer network.

The Role of Spontaneous Polarization and the Thickness of Alignment Layers on the V-Shaped Switching in FLC Cells

Ferroelectric liquid crystal mixtures with the various values of spontaneous polarizations were prepared, and the substrates were coated with different thicknesses of aligning agent. The frequency dependence of the electrooptic response of these materials in cells with various thickness of alignment layers has been measured. The coercive voltage as well as the contrast ratios are calculated. Our results support the “electrostatic model”, according to which both the value of spontaneous polarization and the thickness of the alignment layer play important roles in the V-shaped switching.

V-Shaped switching and interlayer interactions in ferroelectric liquid crystals

Linear electrooptic responses in ferroelectric liquid crystals can be achieved via thresholdless switching. The molecular parameters for the design of ferroelectric liquid crystals that may yield a thresholdless response have not been delineated so far. In this article we explore some of the chemical design features that may be utilised in controlling switching processes, and we develop property–structure correlations in order to achieve materials that exhibit thresholdless behaviour.

Study of the SmCα* Phase in the Tokyo Mixture by Conoscopy Using Tilted Cell

We observed the existence of the SmCα* phase in the so-called Tokyo mixture, which has been used for studying V-shape switching by a number of research groups. Conoscopic observations, birefringence measurements using a set-up that employs a photo elastic modulator (PEM) and dielectric measurement were used for studying the behavior of the SmCα* phase. In particular, a conoscopic method using tilted cells was used to observe the conoscopy of the large tilt angle phase. We found that the tilted conoscopic method is very useful for analyzing large tilt smectic phases. The SmCα* phase was observed in a narrow temperature range from 63.0°C to 63.4°C. This is the first report of an observation of this phase in the Tokyo Mixture. It showed uniaxiality at low voltage but changed to biaxiality parallel to the electric field at high voltage in the conoscopic experiment. The absolute value of the biaxiality was quite high, up to 0.003, which means that the unwinding process of SmCα* is different from that of SmC*. This high parallel biaxiality implies the emergence of ferri-like structure, that is, co-existence of synclinic order and anticlinic order, during the field induced unwinding process.

High-Tilt, High-PS, de Vries FLCs for Analog Electro-Optic Phase Modulation

Four new de Vries I-A-C* ferroelectric liquid crystal mixtures have been formulated to have high tilt (45°), high polarization, and low smectic layer shrinkage for use in analog electro-optic phase modulators. The mixtures are based on a high-tilt anti-ferroelectric compound which, when combined with other compounds, results in ferroelectric I-A-C* materials. Three of the mixtures include a new germanium-containing liquid crystal compound. X-ray diffraction measurements show layer contraction as small as 7%, while the temperature-dependence of birefringence shows a sharp increase at the A → C* transition as expected of a de Vries material. Charge control drive produced V-shaped switching with low hysteresis.

Analog low-loss full-range phase modulation by utilizing a V-shaped switched ferroelectric liquid-crystal cell in reflective mode

We have studied the analog (V-shaped switching) mode in ferroelectric liquid crystals in reflective mode for analog phase modulation applications. We have found that several combinations of cell thicknesses and input polarization states exist for which near-lossless analog phase modulation with a range of approximately 2pi rad is obtained, and we demonstrate one such combination experimentally. Despite a slight deviation from the ideal conditions, e.g., the tilt angle was 38 degrees instead of the desired 45 degrees , virtually pure 1.6pi rad phase modulation was obtained; the measured values agree very well with our numerical simulations of the real device.