Linear optical switching in a FLC/waveguide composite device

Abstract

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

New ferroelectric liquid crystals with a trifluoromethyl group

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm^-2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.     

Alignment of polymer network-stabilized ferroelectric liquid crystal using a magnetic field

Polymer network-stabilized ferroelectric liquid crystals with homogeneous alignment have been produced in cells without a surface alignment layer. In this technique, a crosslinkable monomer is mixed into a ferroelectric liquid crystal and polymerized in a magnetic field to form a polymer network that will stabilize the alignment of the ferroelectric liquid crystal. The concentration of the monomer is an important factor in achieving alignment of the ferroelectric liquid crystal. Both the morphology of the final composite layer and the molecular alignment of the host FLC are affected by the curing temperature at which the UV curing of the sample is started.     

Alignment of polymer network-stabilized ferroelectric liquid crystal using a magnetic field

Polymer network-stabilized ferroelectric liquid crystals with homogeneous alignment have been produced in cells without a surface alignment layer. In this technique, a crosslinkable monomer is mixed into a ferroelectric liquid crystal and polymerized in a magnetic field to form a polymer network that will stabilize the alignment of the ferroelectric liquid crystal. The concentration of the monomer is an important factor in achieving alignment of the ferroelectric liquid crystal. Both the morphology of the final composite layer and the molecular alignment of the host FLC are affected by the curing temperature at which the UV curing of the sample is started.     

Simultaneous determination of fundamental parameters for ferroelectric liquid crystals

An analysis of an experimental technique which allows for a simultaneous determination of several parameters with special interest for ferroelectric liquid crystals is presented. These parameters are the spontaneous polarization, the rotational viscosity and the switching time. In addition, the susceptibility associated with the soft mode free of contributions related to the helicoidal structure can also be obtained. The experimental results for these parameters for the ferroelectric liquid crystal HDOBACEEC are reported. A comparison between the switching time values deduced from the rotational viscosity and those obtained by optical measurements is performed.     

Shear aligned polymer dispersed ferroelectric liquid crystal devices

Fast switching liquid crystal devices can be produced by forming a dispersion of ferroelectric liquid crystal droplets in a polymer film. Such PDFLCs have been fabricated using a polymerization-induced phase separation technique involving ultraviolet photopolymerization, during which the film was sheared to obtain a uniform orientation of the liquid crystal medium. These birefringence devices show fast response times (sub-millisecond), optimum tilt angle (22.5°), and good contrast (∼ 30:1) at room temperature, using ferroelectric switching. We studied the tilt angles, response times and contrast ratio as a function of voltage and temperature to determine the effects of the preparation parameters on the electro-optic behaviour of these devices. Using a ferroelectric liquid crystal with long helical pitch, such devices appear to be bistable.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

Restricted polymer-stabilised electrically suppressed helix ferroelectric liquid crystals

The electrically suppressed helix ferroelectric liquid crystal (ESHFLC) electro-optical mode is characterised by the high contrast and fast response time at low driving voltage. However, the ESHFLC demands several constraints on the alignment layer and the cell gap. However, due to the fundamental characteristics of the ferroelectric liquid crystals, the cone angle decreases at higher temperatures, which is the biggest hurdle for this electro-optical mode to find the real application. In this article, we disclose a restricted polymer stabilisation (with very small polymer concentration in the mixture) for the ESHFLC without pushing other vital parameters out of the range of the constraints for ESHFLCs. The restricted polymer-stabilised ESHFLC shows large temperature range with stable high contrast, fast response time and low driving voltage, thus, it could find application in a range of the modern display and photonic devices.     

Comparative analysis of basic physical properties of a ferroelectric liquid crystal and a polymer dispersed ferroelectric liquid crystal

A polymer film of polyvinylbutyral with dispersed droplets of a ferroelectric liquid crystalline mixture (FLC309c) has been prepared and characterized. The collective processes have been studied by dielectric relaxation spectroscopy in the frequency range 10 Hz to 13 MHz. In comparison with the FLC309c mixture, the polymer dispersed ferroelectric liquid crystal (PDFLC309c) based on FLC309c exhibits a Goldstone-like mode relaxation with a much higher relaxation frequency, but a smaller dielectric strength than the Goldstone mode observed for the FLC309c mixture. The spontaneous polarization of PDFLC309c decreases by nearly one order of magnitude in comparison with FLC309c, while the tilt angle decreases by 20%. Considering these results, we believe that a non-switching region exists near the polymer boundaries and that significant deformations of the helical structure occur due to stronger anchoring.     

A sub-hertz frequency dielectric relaxation process in a ferroelectric liquid crystal material

Dielectric studies of the first order phase transition of a ferroelectric liquid crystal material having the phase sequence chiral nematic to smectic C* have been performed using thin (2.5 mum) cells in the frequency range 0.01 Hz to 12 MHz. For planar alignment, one of the cell electrodes was covered with a polymer and rubbed. Optically well defined alignment was obtained by applying an a.c. field below the N*-SmC* transition. Charge accumulation was enhanced by depositing a thick polymer aligning layer for the alignment of the liquid crystal molecules. A sub-hertz frequency dielectric relaxation process is detected in smectic C*, in the chiral nematic and a few degrees into the isotropic phase, due to the charge accumulation between the polymer layer and the ferroelectric liquid crystal material. The effect of temperature and bias field dependences on the sub-hertz dielectric relaxation process are reported and discussed.     

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.     

New ferroelectric liquid crystals with a trifluoromethyl group

Abstract

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm^?2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.     

Domain manipulation of ferroelectric BaTiO3 thin films: application to the local reorientation of liquid crystal

A method is reported for local alignment of nematic liquid crystal (NLC) molecules. It consists in the poling of small areas of ferroelectric thin films using scanning probe microscopy. A liquid crystal deposited onto such a surface is aligned via a dipole-dipole interaction. The ferroelectric films are first characterized using X ray diffraction, atomic force microscopy and ellipsometry. The domain manipulation and local poling of the film are achieved and characterized using an electrostatic microscopy type set-up. A hybrid nematic liquid crystal cell (NLC-OFC: nematic liquid crystal - oxide ferroelectric cell) is then constructed and its alignment inspected using polarizing microscopy (in reflection mode). The reorientation is explained by invoking a simple interaction between the dipole moment of the LC and the surface electric field generated by the poled area. In addition, a complimentary experiment is performed to determine the depth to which the poled area affects the liquid crystal alignment. This consists of measuring the deflection of a collimated beam (optical soliton) propagating across the poled area.     

Mechanism of electrooptic switching time enhancement in ferroelectric liquid crystal/gold nanoparticles dispersion

It is well established that incorporation of nanoparticles (NPs) in the structure of ferroelectric liquid crystals (FLCs) leads to a decrease in their electrooptic response time. Several approaches have been suggested to explain this effect (decrease in rotational viscosity of FLCs, ions enhanced localised electric field, dipole–dipole interaction among NPs and FLC molecules, FLC ordering). In this article, we will report the role of the voltage divider formed by the structural elements of a FLC cell based on ferroelectric liquid crystal/gold nanospheres (FLC/GNSs) dispersion in enhancement of the switching time. Using the impedance spectroscopic measurements, it was demonstrated that the dispersing of GNSs leads to the increase in the voltage drop on FLC/GNSs layer in comparison with the pristine FLC one. Consequently, the electrooptic response time of the FLC/GNSs cell is faster than that of the pristine one. However, the rotational viscosity of the FLC does not depend on the presence of the GNSs.     

Helical unwinding in polymer-dispersed ferroelectric liquid crystals

We demonstrate that ferroelectric liquid crystals dispersed in a polymer matrix can form uniaxially aligned elliptical droplets. The alignment is controlled by mechanical shear during the polymerization of a UV-curable adhesive. The possibility of prealignment makes polymer-dispersed ferroelectric liquid crystals suitable for application in flexible electro-optical displays. In contrast to surface stabilized ferroelectric liquid crystal displays, the electro-optical effect in our system is due to the deformed-helix ferroelectric (DHF) effect.     

Soft mode and related behaviour in the SmA and SmC* phases of a ferroelectric liquid crystalline polymer by dielectric spectroscopy

Anomalous dielectric relaxation behaviour is observed in the ferroelectric liquid crystalline polymer (viz. ferroelectric copolysiloxane (R)-COPS 11-10) around the ferroelectric SmC* to paraelectric SmA phase transition. Measurements have been performed on sample of thickness ~10 mum in indium-tin-oxide coated cell in the frequency range 10 Hz to 13 MHz. With increase of temperature, a gradual shift of the soft mode frequency towards the higher frequency side was observed, while a decrease in the relaxation strength was seen with the corresponding increase in temperature. The shifts of the soft modes in the SmC* and SmA phase are considered to be due to change in the viscosity of the polymer, as an increase in viscosity increases fluctuations of the coupling between the dipoles in the network even far from the paraelectric-ferroelectric phase transition. Application of a bias field causes a shift of the critical frequency towards the higher frequency side, while the dielectric strength ( Δε) decreases under the bias field. The Cole-Cole fitting parameters obtained from the best fit of the dielectric constant data are found to be consistent with other similar materials. Another relaxation mode (molecular mode) was also observed which comes into play in both the smectic phases (SmC* and SmA) and contributes to the dielectric permittivity.     

Soft mode and related behaviour in the SmA and SmC* phases of a ferroelectric liquid crystalline polymer by dielectric spectroscopy

Anomalous dielectric relaxation behaviour is observed in the ferroelectric liquid crystalline polymer (viz. ferroelectric copolysiloxane (R)-COPS 11-10) around the ferroelectric SmC* to paraelectric SmA phase transition. Measurements have been performed on sample of thickness ～10 μm in indium-tin-oxide coated cell in the frequency range 10 Hz to 13 MHz. With increase of temperature, a gradual shift of the soft mode frequency towards the higher frequency side was observed, while a decrease in the relaxation strength was seen with the corresponding increase in temperature. The shifts of the soft modes in the SmC* and SmA phase are considered to be due to change in the viscosity of the polymer, as an increase in viscosity increases fluctuations of the coupling between the dipoles in the network even far from the paraelectric-ferroelectric phase transition. Application of a bias field causes a shift of the critical frequency towards the higher frequency side, while the dielectric strength (δε) decreases under the bias field. The Cole-Cole fitting parameters obtained from the best fit of the dielectric constant data are found to be consistent with other similar materials. Another relaxation mode (molecular mode) was also observed which comes into play in both the smectic phases (SmC% and SmA) and contributes to the dielectric permittivity.     

Molecular and collective modes in ferroelectric liquid crystals studied by dielectric spectroscopy

The complex dielectric permittivity has been measured for a ferroelectric liquid crystal in the range 102-109Hz. Six different relaxations have been obtained and characterized: soft mode (SmA* and SmC* phases), Goldstone mode (SmC* phase), rotation around molecular long axis, rotation around molecular short axis, ferroelectric domain mode (SmC* phase) and an internal motion associated with a polar group. Strengths and frequencies of these modes have been obtained for the different phases for different bias fields. Using these results together with spontaneous polarization and molecular tilt measurements we have also obtained the rotational viscosities associated with the soft mode and the Goldstone mode. We explain the results in the light of the so-called Landau extended model, concluding that the biquadratic coupling between polarization and tilt is quite important with regard to the bilinear coupling. This fact has been used to explain the noticeable increase of the activation energy of the frequency of the mode related to the rotation around the molecular long axis at the SmA*-SmC* phase transition.     

A new class of high birefringence smectic C liquid crystals. 3,3'-Difluorodiphenyldiacetylenes

Ferroelectric smectic C* liquid crystals have been shown to exhibit high speed, multistate electro-optic switching, particularly when incorporated into the surface stabilized ferroelectric liquid crystal (SSFLC) light valve. Certain applications utilizing IR modulation, waveguide, and fibre-optic devices can benefit from a birefringence higher than the value of Δn = 0·15 seen in typical FLC materials. By incorporating the highly conjugated diacetylene moiety into an FLC core, an increase in birefringence to greater than Δn = 0·3 is achieved. Fluorination of the core induces a strong preference for the tilted smectic C phase as compared to the unfluorinated materials which typically show the nematic phase. The effect of different tail substitutions is examined in the 3,3'-difluorodiphenyldiacetylene system.     

The optical tensor configuration in a surface stabilized ferroelectric liquid crystal determined by using half leaky guided modes

The optic tensor configuration in a surface stabilized ferroelectric liquid crystal cell is investigated using optical excitation of half leaky guided modes. A thin ferroelectric liquid crystal layer is confined between a high index pyramid, with an index greater than the maximum of the liquid crystal, and a glass substrate having an index less than the minimum of the liquid crystal. Using standard attenuated total reflection experimental procedures, over a small angle range a series of sharp resonant peaks are recorded in the s-polarized reflectivity using p-polarized incident light. These peaks are extraordinarily sensitive to details of the optical tensor configuration within the cell. Fitting theoretically modelled reflectivities from multilayer Fresnel theory to the data allows determination of near surface alignment, bending of the chevron, surface tilt angle and biaxiality. To give a clear physical explanation for the great sensitivity of the technique, the electromagnetic field component distributions in the cell are also presented and analysed. The results confirm that the half leaky guided mode method has enormous potential for the study of the optic tensor configuration in liquid crystal layers.