Surface-stabilized Ferroelectric Liquid Crystal Research Articles

Electro-optical synaptic characteristics of ferroelectric liquid crystals for artificial intelligence.

We propose electro-optical synaptic devices using surface-stabilized ferroelectric liquid crystals. Typical synaptic characteristics were observed for varying pulse time intervals, numbers of pulses, and signal voltages. Plasticity only occurred when pulses were applied at intervals shorter than the response time of the ferroelectric liquid crystal. Moreover, the plasticity increased with a higher pulse voltage and number of pulses. This demonstrates the importance of repeated learning. The synaptic weights required to make connections through learning in an artificial neural network can be determined by tuning the pulse signal. We discuss the high-speed computational potential of optical neuromorphic devices using liquid crystals.

A new generation of ferroelectric liquid crystals with homogeneous and shock-stable orientation

ABSTRACT This research is focused on ferroelectric liquid crystal compositions with a wide temperature range of the SmC* phase and the design of mechanically stable, fast switching surface-stabilised ferroelectric liquid crystal display with improved electro-optical properties. The work involves the synthesis of high-viscosity achiral compounds and the creation of mixtures based on them that are resistant to mechanical shocks. For this reason, the viscosity of the compositions and the length of achiral molecules were investigated for their impact on mechanical stability of the alignment. For our investigation we synthesised new achiral smectic C compounds containing four or five attached phenyl rings, so called 4,4’-di-substituted quaterphenyls 4,4’-di-substituted-pentaphenyls which have in series-connected benzene rings, but not separated by bridging fragments. We found that the switching of FLCs with such compounds is much faster compared to mixtures containing compounds with the same quaterphenyl or pentaphenyl fragment in chiral molecules. New ferroelectric liquid crystal compositions with optimised parameters have been devised. This work demonstrated that surface-stabilised ferroelectric liquid crystal display could be developed that had switching times less than 10 μs, was stable against mechanical deformations and was independent of alignment material and pre-tilt angle.

Weak anchoring resolved substrate interface and bulk mode processes in surface stabilized ferroelectric liquid crystal

Dielectric spectroscopy is employed to investigate the behaviour of bulk mode (phason mode, or Goldstone mode (GM) in ferroelectric liquid crystals (FLCs)) and surface phenomenon (partially unwound helical mode) in surface stabilized FLC (SSFLC) sample cells. The surface effect influences the basic characteristic of GM phenomenon to a great extent, even if the surface anchoring energy of the cell is kept very weak. The effect of different measuring and bias fields on the GM and surface processes has been studied in smectic C* (SmC*) phase and also at transition temperature (Tc (SmC* to SmA*)) of SmC* and smectic A* (SmA*) phases. It has been observed that in surface stabilized geometry, there exists two molecular dynamics: one near the surface of the electrodes and another in the bulk of the cell which contributes separately in the dielectric properties. The phason mode is also observed at Tc (SmC* to SmA*) and a few degrees above Tc (SmC* to SmA*) i.e. in SmA* phase of FLC in thin cell of surface stabilized geometry. The application of weak and medium measuring electric fields reverses the varying trend of GM frequency process at the phase transition from SmC* to SmA* phase. The study is intended to explore the surface effect along with the surface and bulk interface that influence the bulk response in SSFLC state.

Correlation between alignment geometries and memory effect in a surface-stabilized ferroelectric liquid crystal.

Memory effect in weakly aligned surface stabilized ferroelectric liquid crystal (SSFLC) material has been investigated by electro-optical and dielectric spectroscopy in three configurations of alignment: antiparallel, 90^{∘} twisted, and unaligned planar samples. It has been observed that two types of molecular dynamics exist in antiparallel rubbed cell in which memory effect is observed for longer duration than in other samples. One dielectric relaxation process is near the surface of the electrode and a second is in the bulk of the SSFLC. Both the molecular dynamics contribute in the switching process and affect the memory phenomenon in surface stabilized geometries. However, a single dielectric process is observed in twisted geometry in which the sample is showing shorter memory effect than in antiparallel and this is compared with unaligned samples also having cell thickness less than the pitch value of FLC. In an unaligned sample, a single dielectric process is observed and the smaple does not show memory effect at all. The investigation is significant to understand the anomalies occurring in memory observations in various geometries.

Superior spatial resolution of surface-stabilized ferroelectric liquid crystals compared to nematic liquid crystals for wide-field-of-view holographic displays

We examined the spatial resolution characteristics of surface-stabilized ferroelectric liquid crystals (SSFLCs) to produce an amplitude-type spatial light modulator with 1 μm pitch pixels for wide-field-of-view holographic displays. We compared the resolution of SSFLCs and parallel-aligned nematic liquid crystals (NLCs) using small-pitch stripe-shaped electrodes, and concluded that SSFLCs are superior. The enhanced resolution of SSFLCs is attributable to the symmetrical LC molecular alignment achieved by applying opposite-polarity direct-current voltages; NLCs are aligned using alternating-current voltages.

Ferroelectric Liquid Crystals: Physics and Applications

Electrooptic modes with fast response and high contrast ratio are highly desirable in modern photonics and displays. Ferroelectric liquid crystals (FLCs) are especially promising for fulfilling these demands by employing photoalignment technology in FLC cells. Three electrooptic modes including surface stabilized FLC (SSFLC), deformed helix ferroelectric (DHF) mode, and electrically suppressed helix (ESH) mode are reviewed with the corresponding electrooptic effects like bi- and multi-stable switching, continuous modulation of grayscale or phase, and high contrast switching. The general operation principles FLC electrooptic modes are described, and then the characteristics of each modes for potential applications are summarized. With the advantages of controllable anchoring energy, the photoalignment provides FLC samples with uniform alignment and high contrast ratio. The fast FLCs with a high resolution and high contrast can be used in the next generation display including field sequential color FLC microdisplays, as well as switchable 2D/3D televisions.

Application of thermodynamic model to study the ferroelectric properties of biphenyl alkyloxy benzoate homologous series of SSFLC

ABSTRACTBased on a thermodynamic model, as developed by us [Singh A, Singh S. Thermodynamic model for the description of smectic C* and smectic A–smectic C* phase transition properties. Phase Transit. 2010;83:205–222; Singh S, Singh S. Thermodynamic model for the investigation of phase transition properties of homologous series of nAL and nHL ferroicmesogens. Phase Transit. 2014;87:427–437], we have studied relative temperature dependences of spontaneous polarization and tilt angle for three members C10, C11 and C12 of a homologous series, biphenyl alkyloxy benzoate, of surface stabilized ferroelectric liquid crystal (SSFLC) material. We found that the order parameter coupling terms make crucial contributions to the transition parameters and have to be included in free-energy density expansion. The values of these parameters depend on the length of terminal alkyl chain. A close agreement between theory and experiment has been found.

An effect of chiral dopants on mesomorphic and electro-optical properties of ferroelectric smectic mixture

ABSTRACTThe aim of the presented research was an assessment of an effect of chiral dopants on the physical and electro-optical properties of a ferroelectric smectic mixture which is a key factor of elaboration of working ferroelectric liquid crystal (FLC) mixtures of tailored properties, especially with the tilt angle value near 22.5° in a broad temperature range. Testing FLC mixtures were prepared using the common pyrimidine-based parent mixture F4 doped with three different chiral dopants. The thermal characteristic of the optical tilt angle , tilt angle calculated by means of X-ray diffraction method , spontaneous polarisation and layer spacing of obtained FLC mixtures were studied. Based on the above measurements and computer calculations, the relationship between spontaneous polarisation and tilt angle was discussed. Electro-optical performance of the Surface Stabilised Ferroelectric Liquid Crystal (SSFLC) structures was observed. The relation between the molecular properties of chiral dopants and the electro-optical and physical properties of obtained FLC mixtures is considered in frames of the extended microscopic model of the spontaneous polarisation in FLCs.

Development of ferroelectric liquid crystals with low birefringence

ABSTRACTWe propose an approach for development of ferroelectric liquid crystals (FLC) with low birefringence Δn. Two basic principles have been used to get lowering of Δn: selection of molecules with short chains of conjugation as components for achiral matrix and averaging of local refractive indices by FLC helical structure. FLC mixtures with low birefringence (0.07 < Δn < 0.10 at wavelength 589.3 nm of sodium line) were elaborated and investigated. They consist of an achiral matrix including both nematic and smectic liquid crystal components and of phenylpyrimidine derivatives as chiral dopants. The materials developed can be used for all basic electro-optical FLC modes such as surface stabilised FLC (SSFLC), deformed helix ferroelectrics (DHF) and electrically suppressed helix (ESH). The mixtures developed allow to reduce the FLC cells chromatic retardance variation due to the weaker birefringence dispersion as compared with the known FLC materials to date.

Investigation of the Thickness Mode in Surface Stabilized Ferroelectric Liquid Crystal Cells

Dielectric properties of surface stabilized ferroelectric liquid crystal (SSFLC) cells have been investigated both experimentally and theoretically. The thickness mode is found to consist of two relaxation processes. A decomposition of an analytical solution of the complex dielectric permittivity epsilon(omega) in terms of these processes is given. The experimental results show agreement with theory and lead to the conclusion that the thickness mode can be separated into two processes. It has been found that the ratio of the dielectric strengths for the two processes depends on the cell thickness, and that the ratio of their frequencies is approximately equal to 10. The lower frequency process is assigned to the relaxation within the bulk and the higher frequency process is assigned to the fluctuations of molecules at the two surfaces of the cell. The latter process is seen only in thicker cells for the reason that anchoring at surfaces predominantly controls the behaviour of thinner cells.

Chaotic transients in surface-stabilized smectic C* cells induced by magnetic field

ABSTRACTRecent theoretical developments regarding the understanding of weakly chaotic transients in ferroelectric liquid crystals (FLCs), induced by electric field, are studied in terms of the interaction with magnetic field. Our research is related with the nonlinear dynamical system represented by a thin film of surface-stabilized FLC in smectic C* phase, and subjected by the swinging magnetic field. The computation of the Lyapunov exponents from the dynamic equation for the director field reveals that the director dynamics exhibits limit cycle, hyperchaotic attractor and strange attractor behavior in the dissipative nonlinear media. The transients between director’s phase space trajectories can be handled by the magnetic field parameters. The fundamental understanding of the director dynamics may have a valuable contribution to the applications of thin liquid crystal films.

Biaxial potential of surface-stabilized ferroelectric liquid crystals.

A biaxial surface potential Φ_{s} of smectic-C^{*} surface-stabilized ferroelectric liquid crystals (SSFLCs) is introduced in this paper to explain the experimentally observed electric-field dependence of polarization P[over ̃]_{cell}(E), in particular the shape of the static hysteresis loops. Our potential consists of three independent parts. The first nonpolar part Φ_{n} describes the deviation of the prime director n (which is the most probable orientation of the long molecular axes) from the easy alignment axis R, which is located in the boundary surface plane. It is introduced in the same manner as the uniaxial Rapini potential. The second part Φ_{p} of the potential is a polar term associated with the presence of the polar axis in a FLC. The third part Φ_{m} relates to the inherent FLC biaxiality, which has not been taken into consideration previously. The Φ_{m} part takes into account the deviations of the secondary director m(which is the most probable orientation of the short molecular axes) from the normal to the boundary surface. The overall surface potential Φ_{s}, which is a sum of Φ_{n},Φ_{p}, and Φ_{m}, allows one to model the conditions when either one, two, or three minima of the SSFLC cell free energy are realized depending on the biaxiality extent. A monodomain or polydomain structure, as well as the bistability or monostability of SSFLC cells, depends on the number of free-energy minima, as confirmed experimentally. In this paper, we analyze the biaxiality impact on the FLC alignment. We also answer the question of whether the bistable or monostable structure can be formed in an SSFLC cell. Our approach is essentially based on a consideration of the biaxial surface potential, while the uniaxial surface potential cannot adequately describe the experimental observations in the FLC.

Application of thermodynamic model to study the phase transition properties of surface stabilized ferroelectric liquid crystals

In this work, we have considered the application of thermodynamic model, as developed by us, to investigate, under electric field, the variation of relative temperature(T−Tc), where Tc represents Sm C*−Sm A* transition temperature, and spontaneous polarization with tilt angle of Sm C* phase near Sm A*−Sm C* phase transition in surface stabilized ferroelectric liquid crystals (SSFLCs). Based on the concept of Landau-de-Gennes formalism, the free-energy density of the system has been written as an expansion series in powers of relevant order parameters and their coupling terms. Adopting usual method, we have evaluated the values of spontaneous polarization (Ps) and relative temperature (T−Tc) as a function of tilt angle for seven SSFLCs materials. In case of materials exhibiting second-order transition a wide temperature variation in the tilt angle has clearly been observed. For materials showings first-order transition, the transition enthalpy has been found to be directly related to the Landu coefficient (b) associated with cubic power of orientational order parameter. We have found a close agreement between theory and experiment.

Electrically switchable Fresnel lenses in polymer-stabilized ferroelectric liquid crystals

In this study, we demonstrate the fabrication of an electrically switchable Fresnel lens based on surface-stabilized ferroelectric liquid crystals (SSFLCs) with polymer networks. The Fresnel lens was fabricated by injecting a monomer-doped ferroelectric liquid crystal into an extremely thin cell and exposing the cell to ultraviolet light through a Fresnel-zone-plate mask. The fabricated Fresnel lens consisted of the SSFLC and polymer-stabilized SSFLC structures. The focusing effect can be switched on by applying an appropriate voltage, and the characteristics of FLCs enable a rapid response on the order of milliseconds.

Relaxation Dynamics of Ferroelectric Liquid Crystals in Pulsed Electric Field

In this contribution we report a theoretical study of relaxation processes in surface-stabilized ferroelectric liquid crystals with spontaneous polarization. The influence of pulsed electric field on the behavior of ferroelectric liquid crystal in the SmC* phase, which is placed in a thin cell with strong anchoring of SmC* molecules with the boundary substrate, is studied. In the vicinity of the substrate interface, temporal dependence of the azimuthal motion of the director induced by electric field is obtained. The response to the external distortion of ferroelectric liquid crystal confined between two microstructured substrates is the occurrence of periodic temporal formation of solitons connected with the distortion of the director field n in the sample bulk. The interplay between microstructured substrates and director distribution of the ferroelectric SmC* phase is explained by the Frenkel–Kontorova model for a chain of atoms, but adapted for the continuum problem.

Electro–optic response in thin smectic C* film with chevron structures**Project supported by the Russian Foundation for Basic Research (RFBR) (Grant Nos. 16-32-00043 and 14-02-97026).

The effects in electrostatic models of chevron surface-stabilized ferroelectric liquid crystals are investigated through numerical modeling. To study smectic C* director distribution within the cell, we consider two nonlinear approaches: the chevron interface does not interplay with the electric field; the electric field interplays with the chevron interface. The obtained results of the director field distribution are compared with the earlier linearized studies. We find that whether or not the electric field interplays with the chevron interface, the electro–optic response requires a generalized approach for its description. The threshold electric field, which is necessary for switching between two stable director states in the chevron cell is evaluated. This study suggests that, in many cases of practical interest, electro–optic response to the electric field and the threshold electric field can be precisely estimated. We argue that, beside being numerically efficient, our approach provides a convenient and a novel standpoint for looking at the electro–optic response problem.

Threshold properties of nanodoped surface stabilized ferroelectric liquid crystals under electric and magnetic fields

Surface stabilized ferroelectric liquid crystal (SSFLC) doped with ferroelectric nanoparticles shows interesting polarization and orientational behavior in external fields. In this work we study the behavior of a nanodoped SSFLC subjected simultaneously to electric and magnetic fields. The threshold fields and maximum deviation of the polarization vectors for a pure and nanodoped system are calculated and the strong influence of spontaneous polarization on magnetic threshold field is revealed as well as on electric field. We assume that the smectic layers consist of uniform planes with a fixed orientation and the system is free from dislocation of constant layer thickness due to nanoparticles. Comparing the obtained results for a nanodoped one with the transition behavior in a pure cell shows the significant influences of nano dispersions on ordering behavior even at low concentrations. The results exhibit the dependence of magnetic threshold field on the presence of ferroelectric nanoparticles and the spontaneous polarization as well as some other material and external parameters. The boundaries for the stability of the uniform configuration of a pure and nanodoped SSFLC system are determined. It is also found that for a SSFLC structure, beyond a critical thickness between the boundary plates of the cell, the Fredericks threshold fields would not be sensitive to the cell thickness.

Fredericks Transition in Pure and Nanodoped Surface Stabilized Ferroelectric Liquid Crystals

Recently, ferroelectric liquid crystals (FLCs) doped with nanoparticles have attracted a wide interest not only from a scientific but also from a practical point of view and there is continuously growing interest in the effects caused by doping the ferroelectric nanoparticles. Furthermore, the presence of external fields exhibits a very interesting transition, reported by recent experiments. In this work, we investigate the response of a surface stabilized ferroelectric liquid crystal (SSFLC) doped with ferroelectric nanoparticles to an applied electric field. We assume that the smectic layers consist of uniform planes with a fixed orientation and the system is free from dislocation of constant layer thickness due to nanoparticles. We obtain the threshold field of orientational transition and the maximum deviation of the polarization vectors for a pure and a doped SSFLC medium. Then, we discuss the Fredericks transtion of the system and formation of inhomogeneous texture. It is found that the ferroelectric nanoparticles have significant influences in ordering behavior of a SSFLC medium and the threshold fields are critically changed by doping nanoparticles in the SSFLC, which is fundamental to operation of many liquid crystal devices.

Polarization switching detection method using a ferroelectric liquid crystal for dichroic atomic vapor laser lock frequency stabilization techniques.

We present a concept of the polarization switching detection method implemented for frequency-stabilized lasers, called the polarization switching dichroic atomic vapor laser lock (PSDAVLL) technique. It is a combination of the well-known dichroic atomic vapor laser lock method for laser frequency stabilization with a synchronous detection system based on the surface-stabilized ferroelectric liquid crystal (SSFLC).The SSFLC is a polarization switch and quarter wave-plate component. This technique provides a 9.6 dB better dynamic range ratio (DNR) than the well-known two-photodiode detection configuration known as the balanced polarimeter. This paper describes the proposed method used practically in the VCSEL laser frequency stabilization system. The applied PSDAVLL method has allowed us to obtain a frequency stability of 2.7×10⁻⁹ and a reproducibility of 1.2×10⁻⁸, with a DNR of detected signals of around 81 dB. It has been shown that PSDAVLL might be successfully used as a method for spectra-stable laser sources.

Electrical properties of ferroelectric liquid crystals during thermal phase transition

The ferroelectric properties of ferroelectric liquid crystals (FLCs) during thermal phase transition in the surface-stabilized ferroelectric liquid crystal (SSFLC) cells were examined. The coercive electric field (Ec) and remanent polarization (Pr) changed with increasing temperature. These temperature-dependent variations of Ec and Pr were in agreement with the differential scanning calorimetry results. Moreover, the fatigue characteristic of the SSFLC cells was examined. No polarized fatigue was observed after 1 × 106 cycles of polarization switching, because FLCs may have the self-repair property for structural and interfacial defects.