In situ observation of smectic layer reorientation during the switching of a ferroelectric liquid crystal

A thin plastic cell containing the ferroelectric liquid crystal ZLI3654 (4 μm) was placed edge-on to a pin-hole collimated horizontal X-ray beam. In this way, the smectic layers were brought into register. Subsequently, triangular voltage waves with various peak (V_p) values were applied across the cell and diffraction photos were obtained during the application of the alternating voltage. Up to V_p = ± 30 V, no significant movements in the initial tilted orientation of the smectic layers with respect to the surfaces (chevrons) could be observed. During the application of V_p = ± 32 V an increasing fraction of the smectic layers changed their initial tilt angle with respect to the cell surfaces to make larger tilt angles. At a slightly higher voltage, the layers became upright (bookshelf structure). Upon removing the voltage and short circuiting the cell, the quasi-bookshelf structure was sustained. The new method described here can be used in combination with a fast detector for time resolved experiments.     

Dynamics of the smectic layer reorientation of ferroelectric liquid crystals

Detailed experimental results of a systematic investigation of the dynamics of the in-plane smectic layer reorientation in SmC* ferroelectric liquid crystals on application of different types of asymmetric electric fields are reported. The reversible reorientation of smectic layers is characterized as a function of field asymmetry, electric field amplitude, frequency, cell gap and temperature. On the basis of the observed behaviour we discuss a phenomenological interpretation of the smectic layer reorientation in terms of dominant influences director switching, convection due to ionic motion and liquid crystal substrate interactions which limit the rotation to the amount of twice the tilt angle.     

Bistable electro-optical switching in the smectic I* phase of a ferroelectric liquid crystal

The ferroelectric switching behaviour of the highly ordered smectic I* phase has been investigated in the mixture which shows a S*_I phase at room temperature. The bistability was obtained in a 3.5μm thick cell. Director switching and the reorientation processes have been studied by applying symmetric square and triangular wave pulses. It has been found that an asymmetric switching occurs in the smectic I* phase for low electric fields due to the hexagonal ordering of the molecules in the layer. This asymmetric switching was confirmed by optical microscopy and four stable states have been observed for low electric field. For higher electric fields only one state is stabilized which results in symmetric switching by both methods in the smectic I* phase.     

Effect of alignment layer on V-shaped switching in a chiral smectic liquid crystal

The influence of the alignment layer on V-shaped switching in thin homogeneous cells of a chiral smectic liquid crystal were studied by means of electro-optic and switching current measurements.Severalpolyimides withdifferent chemicalstructures wereusedas thealignment layer in thin homogeneous cells; V-shaped switching was observed with some of them and W-shaped switching with others. It was also shown that the switching current constantly flows during the V-shaped transmittance change. In order to clarify this effect of the alignment layer on V-shaped switching, polyimide films with various polarity and thickness were examined. It is concluded that thick alignment layers assist liquid crystal materials in realizing V-shaped switching, even if the materials are not ideal for this process.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

The director structure and electroclinic fast optical switching in a grating-coupled ferroelectric smectic liquid crystal cell

Optical excitation of guided modes in a liquid crystal layer using grating-coupling gives sharp features in the angle-dependent reflectivity data. These features are strengthened by using a metallized grating to enhance coupling to the guided modes in the liquid crystal. In the present study the liquid crystal has a smectic A phase which exhibits fast electroclinic switching. Combining the sharp features in the reflectivity together with the electroclinic effect leads to fast, high contrast, optical switching which may open up potential for novel device structures.     

Topological ferroelectric bistability in a polarization-modulated orthogonal smectic liquid crystal

We report a bent-core liquid crystal (LC) compound exhibiting two fluid smectic phases in which two-dimensional, polar, orthorhombic layers order into three-dimensional ferroelectric states. The lower-temperature phase has a uniform polarization field which responds in an analog fashion to applied electric field. The higher-temperature phase is a new smectic state with periodic undulation of the polarization, structurally modulated layers, and a bistable response to applied electric field which originates in the periodically splay-modulated bulk of the LC rather than by surface stabilization at the cell boundaries.     

Field-induced reorientation dynamics of a surface stabilized ferroelectric liquid crystal mixture

Time-resolved measurements of electro-optical response and Fourier-transform infrared absorption have been employed to study the submolecular motion of a surface stabilized ferroelectric liquid crystal (SSFLC) mixture during a field-induced reorientation process. All observed submolecular groups were found to rotate in unison about the layer normal in a steady d.c. field. In a transient situation, the FLC director can respond rapidly to a pulsed driving field. However, the orientation distribution was found to spread at first and then slowly converge to the new direction of the FLC director. When the field is switched off, all the core groups relax to a steady-state direction with varying relaxation times. The results reveal that submolecular fragments of different molecular species in the FLC mixture move correlatively in a steady d.c. field and during a field-induced reorientation process.     

Director reorientation dynamics in chevron ferroelectric liquid crystal cells

We present sample numerical solutions of the equation of motion that governs the dynamics of molecular orientation in ferroelectric liquid crystal cells with chevron layer structure. We show that the chevron structure significantly influences the director field, the chevron interface providing surface stabilization on a plane interior to the FLC layer. Assuming non-polar nematic-like elasticity in the vicinity of the chevron interface, we have modelled the effects of applied field on cells with purely non-polar cell boundary interactions that have uniform director orientation at zero field, and on cells in which the cell walls are strongly polar and the zero-field states are splayed. The simulations with strongly polar surfaces give bistable operation with the two states having fixed orientations at the FLC-solid surfaces, different orientation of P at the chevron interface, and P splayed in either the upper or lower portion of the cell. A monostable state can arise when the chevron interface is asymmetric, i.e. located away from the middle of the cell. Experimental results on asymmetric chevron cells qualitatively confirm the calculated switching scenario.     

X-ray diffraction analysis of the layer structure in a ferroelectric liquid crystal with a chiral nematic-chiral smectic C phase transition

A half-V-shaped switching ferroelectric liquid crystal (FLC) is a promising candidate for fast response displays. In the half-V FLC display, a liquid crystal with a chiral nematic-chiral smectic C phase transition is used, and the smectic layer is formed by cooling from N* to SmC* with an applied d.c. field. We studied the layer structure by means of X-ray measurements for two axes (ω and χ). By using a point-focused X-ray tube and optimizing the slit width, we succeeded in the two-axis measurement with a commercial X-ray system. The ω-χ profile of the half-V FLC showed two broad peaks in an arc-shaped high-intensity area. Our interpretation of this result is that the major part of the layer consists of a tilted-bookshelf structure and that the minor part consists of a near-bookshelf structure. Since optical microscopy observations on the half-V FLC cells showed a stripe-shaped texture, we consider that the coexistence of the tilted-bookshelf and the near-bookshelf structures forms the stripe-shaped patterns. The radius of the arc-shaped high-intensity area was nearly equal to the molecular tilt angle. This result can explain why the half-V FLC showed a desirable black appearance in spite of the stripe-shaped texture.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.     

Microsecond studies of layer motion in a ferroelectric liquid crystal device

Abstract

Small angle X-ray scattering has been employed to study dynamically the layer motion in a ferroelectric liquid crystal device on application of low electric fields. Microsecond time resolution was achieved and the use of an area detector in the experiment allowed the examination of layer motion in two orthogonal planes. The X-ray data show that during switching the chevron structure adopted by the layers distorts, implying a variation in the chevron angle. A rotation of the layers in the plane of the device is also observed, coincident in time with the change in chevron angle. The motion of the layers takes place on a ten microsecond time scale and the angular rotation of the layers is approximately 1°.     

Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C liquid crystals

Abstract

A new electrooptic mode of operation of ferroelectric chiral smectic C liquid crystal displays (LCDs) is proposed and demonstrated. The effect, which is called the deformed helical ferroelectric (DHF) effect, is based on the deformation of the helical structure by weak electric fields. In the unbiased device the smectic layers are arranged in the bookshelf geometry with the helix axis parallel to the electrodes [1]. Systems with a very small pitch (<1 μm) and a large tilt angle are especially well suited for this mode. The key characteristics of DHF-LCDs are: (a) low driving fields (1 V_p-pμm^?1 for maximum contrast); (b) grey scale which is approximately linear with the applied electric field; (c) easy alignment even for thick cells using standard wall-aligning methods; and (d) response times at room temperature of 300 μs.     

Stressed states in domain switching of ferroelectric liquid crystal devices

In this paper we show that the director profile of a low pre-tilt surface stabilized ferroelectric liquid crystal passes through quasi-static stressed states during domain switching under direct drive conditions. Using polarized stroboscopic microscopy, we have observed two quasi-static transmission levels during a domain switching transition from dark to light. This is a result of the directors reorienting into stressed profiles both before and after the chevron interface has switched. By modelling the interaction between the elastic forces and the torque from the applied field, we have determined these voltage dependent director profiles and, by calculating their corresponding transmissivities, have shown very good agreement with the experimentally observed values.     

In situ self-assembled photo-switchable liquid crystal alignment layer using azosilane monomer-liquid crystal mixture system

Self-assembled monolayers (SAMs) are a unique approach for the liquid crystal (LC) alignment in electro-optical applications such as displays. Herein, a new methodology for photo-switchable LC alignment layer using an azosilane monomer and LC mixture system in the absence of any other foreign alignment layer is presented. The azosilane monomer spontaneously separated from the host LCs, and formed a stable monolayer network on the substrate surface. Data from X-ray photo-electron spectroscopy (XPS), spectroscopic elipsometry (SE), water contact angle and LC alignment studies confirmed that, in the azosilane and LC mixture system, azosilane makes an in situ SAM that is capable of photo-switchable LC alignment layer on glass and indium tin oxide (ITO) substrates. The LCs are aligned with respect to change in the photo-isomerisation of the azo molecule.     

A racemic layer structure in a chiral bent-core ferroelectric liquid crystal

A fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmC_AP_F*. The layer chirality of this structure alternates from layer to layer despite their being composed of chiral mesogens. Observations of the optical second harmonic generation signal from well-aligned domains confirm that the ground state of this phase is bistable ferroelectric. In addition to the ground state two types of metastable domains are also observed.     

A racemic layer structure in a chiral bent-core ferroelectric liquid crystal

A fluid smectic phase of a chiral bent-core liquid crystal was found to have a ground state structure that is anticlinic in tilt and ferroelectric in polar order, SmC_AP_F*. The layer chirality of this structure alternates from layer to layer despite their being composed of chiral mesogens. Observations of the optical second harmonic generation signal from well-aligned domains confirm that the ground state of this phase is bistable ferroelectric. In addition to the ground state two types of metastable domains are also observed.