Small inclusions in smectic C* free standing films

The creation and the coalescence of inclusions has recently been observed in smectic C* freely suspended films at the temperature near the smectic C*-cholesteric phase transition. A small finite anchoring energy permits to describe small inclusions by analytical approximate solutions of dipolar or quadrupolar type. Using the proposed solutions our model enables us to discuss the coalescence of smallest inclusions and their observed growth. This work was supported by the research project AV0Z1-010-914 and by Grant No. 202/02/0840 of the Grant Agency of the Czech Republic.     

Colloidal inclusions in smectic films with spontaneous bend

Inclusions in free-standing smectic films are simple model systems for two-dimensional anisotropic dispersions. From theory and experiment, different topologies of elastic distortions of the embedding liquid crystal are known. Quadrupolar and different dipolar defect configurations in the vicinity of the inclusion are possible, and these configurations determine the type of interactions between the inclusions. The quadrupolar configuration is often energetically preferred. We show, however, that dipolar director configurations around inclusions can be energetically favourable over quadrupolar arrangements in chiral smectics, as a consequence of a spontaneous-bend term in the elastic-energy formulation. As the inclusion size influences the selection of the deformation types, the corresponding spontaneous-bend constant can be estimated for the strong anchoring limit if the c -director fields around inclusions of different diameters are taken into account.     

Dimer structures formed in smectic films by inclusions with parallel and antiparallel topological dipole moments

Self-organization of cholesteric and nematic droplets in smectic free-standing films is investigated. Strong tangential anchoring at the droplet boundary leads to nucleation of a topological defect and formation of a topological dipole from the droplet and the defect. The interaction between droplets results in assembly of the droplets in dimers and line chains. Topological dipoles of the droplet-defect pairs can be oriented in dimers with polar (ferroelectric) and nonpolar (antiferroelectric) ordering. In this paper we found novel dimer structures formed by droplets with different handedness of the c-director field near the droplets. Depending on the relative magnitude of the droplet dipoles the resulting structure may be dipolar or quadrupolar. Formation of antiferroelectric dimers is discussed on the basis of electromagnetic analogy.     

Corona patterns around inclusions in freely suspended smectic films

We discuss the structure and physical origin of corona patterns observed around solid or liquid spherical inclusions in freely suspended smectic films. Such patterns are observed when droplets or solid beads of micrometer size are sprayed onto the films. They are found in the smectic C phase and in the smectic A phase above such a smectic C phase, but disappear, for example, at the transition into a lower-temperature smectic B phase. We show that these structures are equivalent to splay domains found in the meniscus of freely suspended films, originating from surface-induced spontaneous splay.     

Two-dimensional ordering of inclusions in smectic C phase

The organization of inclusions nucleated in free standing films of smectic C (SmC) liquid crystal is investigated using polarized light microscopy. Anchoring on the inclusion boundaries induces distortions of the in-plane orientational order of the SmC phase, which drive the elastic interactions between inclusions. Such interactions show a quadrupolar character. At low concentration, the inclusions self-organize in linear or branched chain structures, while at high concentrations two-dimensional patterns appear in the film.     

Edge apodization of crystal-like structures

It is suggested that the selectivity of crystal-like structures (semiconductor superlattices, photonic crystals, and phononic crystals) be raised by apodization of their edges. The transmission and reflection coefficients illustrating the efficiency of this approach are presented.     

Phase apodization of crystal-like structures

It is suggested that the selectivity of crystal-like structures (semiconductor superlattices, photonic crystals, and phononic crystals) be raised by phase apodization. The efficiency of the method is demonstrated with a number of parameters.     

Impedance characteristics of crystal-like structures

Simulation of crystal-like structures (CSs) based on the impedance model is considered. It is shown that CSs peculiarly transform the wave impedance, inducing unusual properties of the medium. The impedance characteristics for various CSs are considered.     

Studies of thin smectic C films by X-ray reflectivity

The smectic order in thin and ultra thin films (150–600Å) of the chiral ferroelectric liquid crystal mixture ZLI-3654 is studied using the X-ray reflectivity technique. The spin cast films on various substrates (float glass, Si wafer, polymer coated glass, etc.) order spontaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describe well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We demonstrate, for the first time, that is possible to extract the molecular tilt angle, , in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature dependence of the tilt angle in the smectic C^* phase are almost independent of the film thickness (down to 200 Å) and are similar to those in the bulk.     

Behavior of inclusions with different value and orientation of topological dipoles in ferroelectric smectic films

Cholesteric droplets in ferroelectric free-standing films with tunable anchoring on the droplet boundary are investigated. A droplet and satellite topological defect(s) form a topological dipole. We obtained droplets with different angles α between two radial lines from the droplet center to −1/2 topological defects. Droplets with parallel dipoles form linear chains in which the interparticle distances decrease with increasing the defect angle α. For the first time, the dependence of the interparticle distance on the angle between topological defects was measured. We can adjust the magnitude and orientation of topological dipoles formed by the droplets. For the first time, the droplets with antiparallel topological dipoles were prepared in a smectic film. Interaction of the droplets with parallel and antiparallel dipoles differs drastically. Formation of antiparallel dipoles leads to a decomposition of the droplet pairs and chains of droplets. Our observations may be used to change the magnitude, anisotropy of the interparticle interaction, and structures of inclusions in liquid crystal media. The article is published in the original.     

Formation of two-dimensional ordered magnetic nanolattices in opal structures

A method of forming a two-dimensional ordered superlattice of magnetic nanoparticles in close-packed opal structures of silica (SiO₂) spheres has been developed. Nickel nanopowder with an average particle size of about 70 nm is used as a source of magnetic particles. Atomic-force and magnetic-force microscopy studies show that all magnetic particles are located in the interstices of the opal lattice, while the magnetization vectors of neighboring nickel particles can have different magnitudes and directions.     

The mean-field type phase transition from smectic A to smectic C

The critical properties of the smectic A to smectic C phase transition are analyzed in the framework of the modified Ginzburg-Landau model-similar to that of Chu and McMillan.The transition shows mean-field type behaviour.     

Optical reflectivity study of synclinic and anticlinic structures in thin freely suspended smectic films

Optical reflectivity studies have been conducted on freely suspended films with synclinic and anticlinic structures. For the first time quantitative data were obtained on orientational ordering in films with anticlinic structure. In the same wide temperature range of more than 30 K we observed both transverse and longitudinal ferroelectricity in the films of the same thickness. Equilibrium transition temperatures between structures with transverse and longitudinal polarization were above the bulk transition temperature to the SmC phase. The molecule tilt was determined in superthin films with synclinic and anticlinic ordering. Received 29 October 1999 and Received in final form 17 February 2000     

Simulation and improvement of the selectivity of crystal-like structures

A model of equivalent reflectors proposed for crystal-like structure makes it possible to obtain the dispersion characteristic in explicit form. The similarity of the dispersion characteristics of waveguides and crystal-like structures is demonstrated. A fundamental solution is proposed for improving the selectivity of such structures.     

Elastic model of the primary stripe texture in free-standing chiral smectic C films

In paper E. Górecka et al.: Phys. Rev. Lett. 75 (1995) 4047 the modulations of the director inside smectic layers of chiral smectic C free-standing film were identified as a periodic system of domain walls on which the director orientation changes by . Domain walls were terminated in the film bulk. An improved model, in which domain walls are terminated by (±)-disclinations, is proposed. The model is an extension of the model of Meyer and Pershan [Solid State Commun. 13 (1973) 989] for the case of chiral smectic C films. The experimental dependence of the periodicity of the system on the film thickness is used to estimate the model parameters.     

Length scale dependence of chiral symmetry breaking in free-standing films of achiral smectic C

The textures of a smectic C phase in free-standing films of the Schiff base 5O.6 are studied as a function of the number of layers and temperature. Periodic stripes with alternating contrast are observed in SmA boundary layers. On further cooling a transition to the splay stripes occurs in the smectic A temperature interval. In the smectic C phase all the periodic textures disappear, whereas in films with one free boundary a periodic arrangement of splay stripes is found. Mechanisms of symmetry breaking are discussed. Pis'ma Zh. éksp. Teor. Fiz. 64, No. 1, 29–32 (10 July 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Anticlinic-synclinic transitions in superthin free-standing smectic films

Anticlinic-synclinic transition was studied in superthin smectic films using polarized light reflected microscopy. The measurements were made in a compound exhibiting the mC _FI1 ^* subphase in a narrow temperature interval between antiferroelectric C _A ^* and ferroelectric SmC* phases. In films, we observed series of transitions with numbers increasing with increasing film thickness. Surface ordering leads to increasing transition temperatures with decreasing film thickness and to change of orientation of the molecular tilt plane in layers. Succession of transitions results from competition between the surface and the bulk ordering. We found that line string defects may form in a film, their orientation and collective behavior resulting from elastic deformation of molecular ordering.     

Electroclinic effect in free-standing smectic elastomer films

The coupling of the mesogenic tilt in smectic liquid crystals to external electric fields in the layer plane (electroclinic effect) provides an opportunity to control the smectic layer thickness with electric fields. In ordered ferroelectric smectic elastomers it is possible to achieve a macroscopic electromechanical response. The effect is particularly pronounced near the smectic A-to-smectic C^* phase-transition temperature. In this work, the electrostriction of weakly cross-linked smectic elastomers is studied by means of optical interferometry of thin films. Its observed magnitude corresponds to an induced tilt of approximately 7.7°, yielding a layer contraction of 1%, in a 3 MV/m electric field. This value is in agreement with earlier measurements of tilt susceptibility in a structurally similar polymer, but is considerably smaller than previously reported data on elastomers with a comparable chemical structure. PACS 61.30.Vx; 83.80.Va; 77.65.Bn