S. P. Palto

Two-dimensional ferroelectric films

Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films,, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250u2009Å in diameter, in perovskite films 100u2009Å thick and in crystalline ferroelectric polymers as thin as 25u2009Å (refs 7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10u2009Å (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

Location of charge transfer states in α-sexithienyl determined by the electroabsorption technique

Abstract Absorption and electroabsorption measurements were carried out at 300 and 4 K on evaporated thin films of α-sexithienyl, which is a chemically well-defined prototype of conjugated polymers (i.e. polythiophene). The results allowed for the determination of the energy for the optical charge pairs formation in this material. The polarizabilities of the excited Frenkel exciton states have also been estimated. The singlet exciton binding energy is in the order of 0.4 eV.

Cholesteric helix: topological problem, photonics and electro-optics

Using numerical simulations and the results of our experiments we discuss the behaviour of a defect-free cholesteric liquid crystal in rather a strong electric field. The smooth helix unwinding predicted by de Gennes (Solid State Commun. 1968, 6, 163,) occurs under the thermodynamic equilibrium conditions and can easily proceed with the assistance of multiple defects. The defect-free helix in the field strictly perpendicular to the helical axis cannot be unwound for topological reasons. The field deforms the helix without a change of its period and induces higher harmonics of the helical structure. Due to the latter, the shape of the Bragg photonic stop-band changes and a second-order photonic band is observed in the optical transmission. Moreover, novel laser effects are predicted by modelling for the dye-doped, distorted cholesteric structure. The same field-induced anharmonicity results in a strong change of the polarisation state of the beam passing the helical structure along its axis. Due to fast relaxation of the higher harmonics, the switching time of the devices using the corresponding electro-optical effect (called in-plane switching) can be improved significantly. This is shown both experimentally and by numerical calculations.

Simulation of electrooptical effects and dynamics of ferroelectric liquid crystals

The behavior of ferroelectric liquid crystals in an external electric field is simulated numerically. The equations that describe the dynamics of the director of a liquid crystal are derived within the continuum theory of elasticity with due regard for compressibility of smectic layers, finite anchoring energy, and dielectric properties of orienting coatings and external elements of a real electric circuit. These equations make the basis for simulation of the electrooptics of ferroelectric liquid crystals. The specific features and mechanisms of the surface-stabilized bistability and hysteresis-free electrooptical switching (the V-shape effect) are discussed.

Optical second harmonic generation probe of two-dimensional ferroelectricity

Optical second-harmonic generation (SHG) is used as a noninvasive probe of two-dimensional (2D) ferroelectricity in Langmuir-Blodgett (LB) films of the copolymer vinylidene fluoride with trifluoroethylene. The surface 2D ferroelectric-paraelectric phase transition in the topmost layer of the LB films and a thickness-independent (almost 2D) transition in the bulk of these films are observed in temperature studies of SHG.

Hysteresis-free electro-optical switching in conductive ferroelectric liquid crystals: experiments and modelling

The hysteresis-free electro-optical switching, or so-called V-shaped, regime has been studied in a commercial ferroelectric liquid crystal (FLC) mixture having a smectic C* phase with a very small value of spontaneous polarization. The FLC was introduced into commercial EHC cells with thin aligning layers. In such cells V-shaped switching could be observed only at very low frequencies, less than 1u2009Hz. However, when the same material is strongly doped with a conductive impurity, its conductivity markedly increases and hysteresis-free switching is observed over a wide range of applied frequencies and voltages. Experimental results are in good agreement with computer modelling carried out as part of this work. The modelling takes into account all the important parameters of smectic C* FLC: non-polar anchoring conditions, possible bookshelf and chevron structures, the capacitance of the aligning layers and the conductivity of a FLC. The last two factors appear to be the most crucial for hysteresis-free switching in the smectic C* phase.

Modeling Electrooptical Effects in Ferroelectric Liquid Crystals. 1. Basic Equations and Experimental Tests

Electrooptical effects in ferroelectric liquid crystal cells have been simulated and the calculation results have been compared with experiments. Two sets of equations have been solved numerically: (1) the dynamic equations for the director coupled with the electric current equation; (2) the Maxwell equations which allow the calculations of the optical transmission T for different polarization, wavelength and coherency of light incident onto a cell. The anisotropy of elastic, viscous and dielectric properties and arbitrary anchoring conditions have been taken into account. The experimental data on the director switching and the electroclinic effect in the Smectic C∗ phase agree quantitatively with the modeling, and novel features of these effects are discussed.

Lyotropic liquid crystal guest–host material and anisotropic thin films for optical applications

We propose a new approach for the production of thin film optical functional materials. The method is based on molecular design whereby two different types of lyotropic liquid crystals (LC), lyotropic LC based on columnar supramolecules and water-soluble rod-like polymer molecules are mixed. The resulting lyotropic guest–host system allows production of optical retardation films with tunable optical anisotropy controlled by composition of the guest–host system. Coatable retarders can be used in modern liquid crystal displays and TVs for optical compensation and enhancement of the LCDs performance.

Quasi-in-plane leaky modes in lasing cholesteric liquid crystal cells

Using a planar cell consisting of a prism and a flat glass, we have carried out precise measurements of the angles at which quasi-in-plane leaky (QIPL) laser modes propagate within a thin layer of a cholesteric liquid crystal (CLC) doped with a laser dye. Both the prism and the glass have refraction indices higher than relevant CLC indices, therefore, the waveguide effect in CLC is excluded. For this type of almost thresholdless lasing neither mirrors nor distributed feedback is necessary. The modes are generated in the amplifying CLC layer due to strong Fresnel reflections from the glass boundaries at propagation angles very close to 90° with respect to the cell normal. In the experiment, two modes polarized differently (s- and p-) have been found outgoing from the prism. Using equivalency between optical properties of a CLC and optically negative nematic liquid crystal at the propagation angles close to 90°, the analytical approach known for a uniform nematic liquid crystal was used for calculations of ...

Two-dimensional ferroelectricity and second harmonic generation in PVDF Langmuir-Blodgett films

Abstract Two-dimensional (2D) ferroelectricity and 2D ferroelectric–paraelectric phase transitions are experimentally studied, for the first time, by means of optical second harmonic generation (SHG) in ultrathin Langmuir–Blodgett (LB) films of vinylidene fluoride with trifluoroethylene. The temperature dependence of the SHG intensity confirms the existence of two types of 2D ferroelectric–paraelectric phase transition: the first one in the topmost layer of LB films and the other, almost thickness independent, in the bulk of ferroelectric LB films.