Ferroelectric Liquid Crystal Cells Research Articles

Phase transitions in ferroelectric liquid crystals probed by optical second harmonic generation

Phase transitions in thin (1.5–3 μm) planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using the optical second harmonic generation (SHG) technique. This method is well known because of its high sensitivity to the structural and electronic properties of surfaces, interfaces and thin films, as well as to the appearance of the ferroelectric state of a material. It is shown that the studied FLC mixture retains a highly regular structure up to about 80 °C, when a sharp decrease of SHG intensity, associated with the transition of the LC to the nonpolar (N*) phase, is observed. SHG intensity measurements show that the transition from chiral smectic C (SmC*) to chiral smectic A (SmA*) phase which takes place at about 45 °C for the bulk FLC mixture does not influence the ferroelectric state of subsurface FLC layers. At the same time, SHG interferometry measurements show that definite changes in the FLC structure happen above the bulk transition temperature from SmC* to SmA* phase. A DC electric field induced switching of the FLC cells is also observed.

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.

Modelling of addressing schemes in surface stabilized ferroelectric liquid crystal cells using a three variable model in one dimension

The ‘Three-Variable Model in One Dimension’ has been successfully used to model the domain switching process for simple monopolar and bipolar pulses. In this paper, we discuss the development of this model to describe the switching process due to more complicated addressing waveforms. We have taken two contrasting addressing schemes: the Seiko scheme which is a simple example of a low voltage addressing scheme, and the Joers Alvey scheme, a high voltage addressing scheme which utilizes the high biaxiality properties of a ferroelectric liquid crystal (FLC) material. We experimentally simulate the voltage sequence ‘seen’ by a pixel subject to these addressing schemes, and take into consideration the worst case combinations of voltages that could cause switching and non-switching conditions. We show that the ‘Three Variable Model in One Dimension’ is in excellent agreement with experimental data for the Seiko addressing scheme, and can be used to predict the operating regime for the Joers Alvey addressing scheme.

Structure and dynamics of ferroelectric liquid crystal cells exhibiting thresholdless switching.

Surface-stabilized ferroelectric liquid crystal cells are usually bistable, with the spontaneous polarization either up or down, perpendicular to the plane of the cell. In materials with high spontaneous polarization the polarization charge self-interaction leads to a monostable structure that exhibits a thresholdless, "V-shaped" switching of the optical response to an applied voltage. With a model that includes the electrostatic self-energy of the spontaneous polarization, we examine the transition from bistablity to monostability as a function of the polar and nonpolar anchoring energies, the liquid crystal elasticity, and the thickness of the alignment layers. Two bistable and one monostable structures are possible. The bistable state is only obtained when the thickness of the insulating alignment layer and the polar anchoring energy are small, or when the spontaneous polarization is small. From linear stability analysis we get analytical expressions for the limits of stability of the structures. Numerical calculations show that in some ranges of the parameters two structures can coexist. We also present the calculation of the polarization fluctuation modes. The lowest one becomes soft at the continuous transitions among the structures and at the limits of stability.

Application of Photo-Alignment Techniques to Ferroelectric Liquid Crystal Cell

The photosensitive polyimide(PI) films and silane thin film containing chalcone derivative [4-n-propyl-4- (3-propylene hydroxide) chalcone] induced orientation of liquid crystal, which aligned perpendicular to the polarization direction. Chalcone was attached to PI film and silane thin film by surface modification. The anisotropy of alignment layer was induced by irradiating linearly polarized UV(LPUV). The pretilt angle of nematic liquid crystal cells aligned by those films was about 1 °. The photoaligned films were applied for the alignment of antiferroelectric liquid crystal(AFLC) molecules, which needed planar alignment. The effect of linearly polarized UV light after cell had been fabricated was also investigated.

Application of Photo-Alignment Techniques to Ferroelectric Liquid Crystal Cell

The photosensitive polyimide(PI) films and silane thin film containing chalcone derivative [4-n-propyl-4- (3-propylene hydroxide) chalcone] induced orientation of liquid crystal, which aligned perpendicular to the polarization direction. Chalcone was attached to PI film and silane thin film by surface modification. The anisotropy of alignment layer was induced by irradiating linearly polarized UV(LPUV). The pretilt angle of nematic liquid crystal cells aligned by those films was about 1 °. The photoaligned films were applied for the alignment of antiferroelectric liquid crystal(AFLC) molecules, which needed planar alignment. The effect of linearly polarized UV light after cell had been fabricated was also investigated.

Investigation of the Electroclinic Effect at the Liquid Crystal-Substrate Interface

We present a model of the electroclinic effect near the Smectic A-Smectic C transition, within a Surface Stabilised Ferroelectric Liquid Crystal cell. We propose an internal chevron structure within the cell, as had been suggested by previous x-ray studies 1,2 . We tested this model by comparing it with experimental results, which involved a total internal reflection experiment to probe the liquid crystal-substrate interface, and a transmission experiment to probe the behaviour within the bulk of the cell. This model shows very good agreement with the experimental data, for both the liquid crystal-substrate interface and within the bulk of the cell

Second Harmonic Generation Studies of Phase Transitions in Ferroelectric Liquid Crystals

Phase transitions and switching behaviour in thin (1.5 - 4 µm) planar cells of a ferroelectric liquid crystal (FLC) mixture are studied by optical second harmonic generation (SHG). A pronounced anisotropy of the SHG response shows a high ordering degree of the FLC molecules in the cells. We studied the electric field-induced switching of the FLC cells and their temperature dependence by means of SHG. This shows the existence of a 10-20 nm thick frozen sub-surface layer of FLC molecules, which cannot be switched by an electric field well above the bulk saturating field and at significantly high temperatures.

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.

Modelling Switching and Optics in Ferroelectric Liquid Crystal Microdisplays

Switching in surface stabilised ferroelectric liquid crystal (SSFLC) cells takes place through the formation and evolution of domains, which can grow up to many tens of microns in size. Domain growth can be described by the Avrami model, but this relies on a statistical distribution of the number of nucleated domains, and hence a large number of domains are required for this model to succeed. We discuss the limitations of this model, for example in a microdisplay pixel, which is between 10 20 microns in size, and therefore only a few domains exist within each pixel. We also investigated the effects of light scattering during partial switching. Light scattering through domains is modelled using the Finite Difference Time Domain (FDTD) method, and shows good agreement with experimental results.

OPTICAL TRANSMISSION AND POLARIZATION SWITCHING PROPERTIES OF A FERROELECTRIC LIQUID CRYSTAL CELL WITH A STABLE TWISTED STATE

Ferroelectric liquid crystal cells with a stable twisted state were prepared. Dynamic processes of molecular reorientation in a stable twisted state under an applied electric field have been investigated by observing the optical transmittance and polarization switching current with simultaneous observation of the texture change.

Phenomenological Analysis in Fabricating Defect-Free Ferroelectric Liquid Crystal Displays with C-1 or C-2 Uniform States

The most important issue in developing liquid crystal displays (LCDs) utilizing ferroelectric liquid crystals (FLCs) is how to obtain defect-free FLCDs. In this report, a phenomenological theory of transition between C-1 and C-2 states in FLC cells is suggested. Free energy that includes bulk elastic and surface parts has been theoretically simulated depending on parameters such as surface topography, anchoring energies, thickness of the LC medium and so forth. The critical conditions of the state transition from C-1 to C-2 have been determined. Based on this theory, we succeeded in fabricating defect-free FLCDs with uniform C-2 states by rubbing and that with uniform C-1 states by the photoalignment technique.

40.3: Advanced Electronic Color Switch for Time‐Sequential Projection

AbstractWe present a novel thin‐film based electro‐optical concept of a fast responding electronic color switch with optimal transmission and a color gamut meeting NTSC standard. The versatile and compact polarization color switch consists of ferroelectric liquid crystal cells and photo‐aligned, non‐absorbing liquid crystal polymer thin‐films.

Special phase matching of second-harmonic generation in helical ferroelectric liquid crystal cells.

The helical structures in ferroelectric liquid crystals can be utilized to realize a special phase matching for second-harmonic generation (SHG) when two counter fundamental waves propagate along the helical axis and the wavelength of SHG is near the photonic (selective reflection) band edge. On the basis of the exact theory [Drevensek-Olenik and Copic, Phys. Rev. E 56, 581 (1997)], a simple analytical description is derived and some characteristic features of the special phase matching are shown. (1) Special phase matching is definitely achieved under particular combinations of polarization. (2) The SH spectrum is related to a subsidiary oscillating structure in the selective reflection spectrum. The maximum SH intensity is realized at the first dip of the oscillation near one of the edges in the selective reflection band. (3) The thickness (d) dependence of the maximum SH intensity is d4 in thick cells, while it is d2 for conventional phase matching. (4) The linewidth for the SH peak is d(-3) dependent, which is much narrower than in conventional phase matching.

Modelling of switching in surface-stabilized ferroelectric liquid crystal cells using a three-variable model in one dimension

The switching process in ferroelectric liquid crystal devices takes place through the formation and evolution of domains, which is modelled here using a three-variable approach. This approach includes variation of the director profile in one dimension, through the thickness of the cell. Here we discuss details of the model which are necessary in order to reproduce the domain nucleation and switching times as a function of applied voltage for both monopolar and bipolar pulses. We show that the three-variable modelling of SSFLC switching in one dimension produces excellent comparisons with experimental data for both bipolar and monopolar pulses.

Spontaneous formation of horizontal chevrons in smectic-C* liquid crystals

We report an unusual structural formation observed in surface-stabilized ferroelectric liquid crystal cells with asymmetrically treated surfaces of 3-Glycidoxypropyl trimethoxysilane and nylon. On cooling the material W415 from the smectic-A* to the smectic-C* phase, the bookshelf smectic layers spontaneously rearrange to form a horizontal chevron structure, manifested optically as a periodic texture of uniform stripes. The polarization is perpendicular to the boundary plates and reverses direction from stripe to stripe, while the director is oriented in the plane of the cell and alternates symmetrically about the stripe direction. In an applied field the cells show a V-shaped (analog) electro-optic response.

Characterization of the Local Layer Structure of a Narrow Wall in a Surface Stabilized Ferroelectric Liquid Crystal Using Synchrotron X-Ray Micro-Diffraction

Synchrotron X-ray microbeam small angle diffraction experiments were carried out for the characterization of the local layer structure of the narrow wall of a zig-zag defect in a surface stabilized ferroelectric liquid crystal cell. A series of the rocking curve (ω) and the azimuthal (χ) intensity distribution profiles were measured as a function of the vertical position across the narrow wall. The local layer was deflected at the wall in the χ direction. For the inclined narrow wall making an angle of a few degrees to 10° with respect to the rubbing direction, the deflected layer bent in both ω and χ directions. At the narrow wall running parallel to the rubbing direction, the layer bent only in the ω direction. The local layer structure was discussed in relation to the surface anchoring effect and a uniformly bent layer structure was proposed to explain experimental results.

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.

Switching behaviors of ferroelectric liquid crystal cells with a polyimide alignment layer

The spontaneous polarizations, response and decay times, and light transmittances of ferroelectric liquid crystal cells were observed. When a field was applied, the spontaneous polarization of ferroelectric liquid crystal by a rubbing method was higher than that was by a LB technique. In the cases of response and decay times, the FLC cell treated by the rubbing method at once was faster than that by the LB method. Especially, the response time of a ferroelectric liquid crystal cell of having an alignment layer treated by the LB 1 layer after rubbing, was similar to that by rubbing, but decay time fell in between the cells of a rubbed surface and of a LB film.

Optical characterization of the director profile in a ferroelectric liquid crystal cell with homeotropic alignment

The optic tensor configurations (director profiles) in surface stabilized ferroelectric liquid crystal cells with homeotropic alignment are considerably simpler than in cells with the more conventional homogeneous alignment and, with the exception of thin surface regions, are essentially that of an unconstrained or “free” FLC sample. Previously the half-leaky guided mode (HLGM) technique, in which angle dependent reflectivity only is measured, has been used to characterize the director profile in such cells. The extended HLGM technique reported in this paper enables detection of both angle dependent reflectivity and transmissivity data. Combined analysis of these features significantly enhances the sensitivity of the original HLGM technique enabling the director profile in cells with homeotropic alignment to be explored in even greater detail.