Response Of Ferroelectric Liquid Crystals Research Articles

Dielectric response of ferroelectric liquid crystals in samples of finite thickness

The structure in real planar samples of ferroelectric liquid crystals as well as the collective modes characterizing these structures, are strongly affected by surface anchoring of molecules. This is reflected in dielectric properties, which become significantly dependent on the sample thickness. The dielectric strength and the relaxation frequency of the characteristic modes (pinned Goldstone mode for the thick samples and twisted mode for the thin ones) are determined and found that for both types of structures the dielectric strength increases and the relaxation frequency decreases with increasing the sample thickness. We proved theoretically predicted thickness dependences of various physical parameters for new ferroelectric liquid crystalline compound. We concluded the experimental results are in accordance with the theory taking into account the real structure of the samples.

Chiroptic response of ferroelectric liquid crystals triggered with localized surface plasmon resonance of achiral gold nanorods

The chiroptic response of a multi-component molecular mixture [chiral smectic C (SmC*) liquid crystal] modified by achiral capped gold nanorods (GNRs) has been reported. The dispersion of the GNRs (radius rGNR = 5 × 10−9 m; length L = 4.5 × 10−8 m) with two concentrations (0.1 wt. % and 0.5 wt. %) resulted in the concentration dependent change in the circular dichroism (CD) spectrum of the SmC*-GNR complex. Three effects could be responsible for the change in the chiroptic properties of the SmC* liquid crystal. The first one is the stabilization of the helical superstructure due to the GNRs. The second one is the change in the angle between the transition electric and magnetic dipole moments near the absorption bands due to the influence of the longitudinal surface plasmon resonances (LSPRs) excited in nanorods. The third effect is the induction of chiral current in gold nanorods by the chiral component of the mixture. The latter phenomenon leads to the broad CD band at the wavelength of the LSPR.

Exfoliation of graphene oxide and its application in improving the electro-optical response of ferroelectric liquid crystal

Near complete exfoliation and reduction of lyophilized graphene oxide (GO) has been carried out at temperature as low as 400 °C. The structural characterizations of the reduced GO have been performed using X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy techniques. The morphological studies were carried out using scanning electron microscopy. The synthesized GO finds an application in improving the switching performance of a liquid crystal (LC) mixture by remarkably modifying the physical properties, such as spontaneous polarization and rotational viscosity of the ferroelectric LC (FLC) material which in turn resulted into faster response of the FLC. The present study explores the possibility of low temperature thermal reduction of GO along with its application in improving the properties of LC based display systems.

Enhancement of electro-optical response of ferroelectric liquid crystal: the role of graphene quantum dots

We present a pioneer study depicting a significant improvement in the photoluminescence (PL) intensity and display of an expeditious electro-optic response of ferroelectric liquid crystal (FLC) when doped with graphene quantum dots (GQDs). Significant threefold enhancement in PL intensity of GQDs/FLC composite material can be ascribed to the additive combination of emissions from GQDs and FLCs. Furthermore, promptness in electro-optical response by a factor of 34% can be attributed to the lowering of rotational viscosity of the FLC material due to the incorporation of GQDs. These results would certainly be helpful in realisation of highly luminescent and faster generation of LC systems.

Optical response of ferroelectric liquid crystals doped with metal nanoparticles

As-synthesized deformed helix ferroelectric liquid crystal (FLC) material with high spontaneous polarization and rotational viscosity is characterized. The FLC material is then systematically doped with various noble metal nanoparticles (MNPs) such as Ag, Au, and Pt in the size range of 5 nm. The optical behavior of MNP doped FLC material is studied via electro-optic and photoluminescence measurements. The observed optical response has been discussed on the basis of the interaction of plasmons in MNPs with the electromagnetic wave from the incident light that traverses through the FLC molecules.

Molecular Alignment of Ferroelectric Liquid Crystal in Wide-Gap Cell for Liquid Crystal Lens

The high-speed response of ferroelectric liquid crystals (FLCs) is advantageous for optical devices such as an electrically controlled lens. We have investigated the molecular alignment structure of FLC in a wide-gap cell for application to the lens. As a result, it has been found that a zigzag defect-free FLC medium can be obtained by utilizing alignment films, which possess a smooth surface and give rise to the defect-free situation in a narrow-gap cell, and by cooling from the nematic to the smectic phase at a low rate of <1 °C/min. However, the transformation of the smectic layer structure has been observed under an electric field, particularly under a very low electric field in the wide-gap cell. We have attempted to suppress this transformation, which considerably degrades the optical performance of FLC, by the polymer stabilization technique.

Spontaneous polarization and electro-optic behaviour of a ferroelectric liquid crystal cell fabricated with polyimide Langmuir-Blodgett films

The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.

Dielectric Response of Ferroelectric Liquid Crystals in Helical and Twisted Planar Samples

The dielectric spectroscopy has been studied for both twisted and helical structures at the same sample. On cooling below the SmA phase the twisted structure spontaneously appeared. The helical structure could be generated by applying electric field of the frequency 20 Hz. It is shown that the twisted structure exhibits higher relaxation frequency and lower dielectric strength than the helical one. This result can be explained within the present theory providing the existence of weak depolarization fields.

Influence of ions on the "V-shaped" electro-optic response of ferroelectric liquid crystals.

It has recently been shown that in surface-stabilized ferroelectric liquid crystal cells with high spontaneous polarization, the polarization charge self-interaction leads to a "V-shaped" optical response to an applied voltage. The presence of ionic free charges in the liquid crystal changes the internal electric field, and therefore also the spatial dependence of the polarization and optic axis orientation. We have numerically solved the nonlinear ion diffusion equation in the electric field due to external voltage, spontaneous polarization, and ions, and calculated the electro-optic response to a triangular applied voltage. When the period of the driving voltage is smaller than the zero-field diffusion time of the ions across the cell, and larger than the ion transit time in the applied field, an inverse hysteresis in the electro-optic response is obtained. At these time scales ions also cause characteristic changes in the shape of the electro-optic response.

Detailed simulation of the goldstone mode response of ferroelectric liquid crystals in the surface stabilised geometry

The Goldstone mode dielectric response of a ferroelectric liquid crystal material is calculated by comparing the phase and amplitude of the applied sinusoidal electric field with the time dependent displacement field from continuum modelling. Fitting the experimental measurements made in the surface stabilised geometry has yielded values for an isotropic elastic constant and the rotational viscosity of SCE8 as a function of temperature. The dependence of the dielectric strength and the frequency of the Goldstone peak as a function of cell thickness are reproduced by the theory.

Spontaneous Polarization in Tilted Smectics

The rapid electro-optic response of ferroelectric liquid crystals (FLCs)-spontaneously polar phases-makes them candidates for the next generation of liquid crystal displays and a considerable effort has been underway to establish design rules for synthesizing more responsive materials. Herein we describe the key features of a simple theory [D. J. Photinos and E. T. Samulski, Science, 270, 783 (1995)] which suggests that polarity is indigenous to tilted smectics and not a consequence of mesogen electrostatic attributes, e.g. dipole moment and/or molecular chirality. We suggest classes of prototypical FLCs and NMR experiments which could critically test the theory.

Aligning properties and anchoring strength of Ferroelectric Liquid Crystals

Abstract The review of methods to measure Ferroelectric Liquid Crystal (FLC) anchoring strength are given. The methods are based on measuring the hysteresis and electrooptical dynamic response of FLC. The results of measurements are presented. The ways of future investigations in the field are discussed.

Nonlinear effects in the dielectric response of ferroelectric liquid crystals

Abstract The unwinding of the helicoidal structure of ferroelectric liquid crystals by a static electric field is studied theoretically in the constant amplitude approximation. The field dependence of the pitch, of the induced polarization and of the static dielectric susceptibility is evaluated. Close to Tc nonlinear effects in the susceptibility become important and the dielectric susceptibility, which is defined as the susceptibility in the limit of small fields is hard to evaluate experimentally, simply because it is hard to perform experiments at sufficiently small fields. By our calculation we present a way to get around this problem and we show how to determine the Goldstone mode dielectric susceptibility also if comparatively large fields are used in an experiment.

Electro-optic pulse response of ferroelectric liquid crystals

Abstract The electro-optic response of ferroelectric smectic C* liquid crystals has been studied. Anomalous switching behaviour of such materials which possess a negative dielectric anisotropy has been reported. These materials show a minimum in response time at a sufficiently high field. We present results showing the dependency of this minimum upon spontaneous polarisation and the effect of AC bias. Calculations based upon the equation of motion of the director around the cone are presented which describe this effect and its dependence on the relative magnitudes of the spontaneous polarization and dielectric anisotropy of the material. Good agreement with the experimental results is found.

Dynamic Responses of Ferroelectric Liquid Crystals in the Surface-Stabilized Geometry I

The dynamic response of surface-stabilized ferroelectric liquid crystals was studied on the basis of a continuum theory. In this work, polarization electric field was consistently introduced through Poisson's equation, including the polarization charge accompanied by an elastic deformation. An improved anchoring energy consistent with inclined smectic layers relative to the bounding plates is put forward. From numerically calculated results, the polarization current and the hysteresis curves are found to be crucially affected by the inherent chirality and the polarization electric field. Remarkably, it is shown that an increase of the inherent chirality or the negative dielectric anisotropy results in a doubly or a triply peaked polarization switching current under an alternating external field.