Optical gratings formed in thin smectic films frustrated on a single crystalline substrate.
30 Jul 2004-Physical Review E (Phys Rev E Stat Nonlin Soft Matter Phys)-Vol. 70, Iss: 1, pp 011709-011709
TL;DR: The interpretation of the results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.
Abstract: Through the combination of three different, complementary techniques (optical microscopy, x-ray diffraction and atomic force microscopy), we reveal the deformations inside thin smectic films frustrated between two interfaces imposing antagonistic anchorings. We study the model system, 4-n-octyl-4'-cyanobiphenyl (8CB) between MoS2 and air, which is characterized by the competition between homeotropic anchoring at air and planar unidirectional anchoring on the substrate, with thicknesses varying around 0.3 microm. Optical microscopy and x-ray diffraction demonstrate the continuous topology of smectic layers between the interfaces, which are stacked into periodic flattened hemicylinders. These latter are one-dimensional (1D) focal conic domains which form an optical grating in the smectic film, of a period ranging from 1 to 2.5 microm. The interpretation of our results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.
Citations
More filters
01 Dec 2013
TL;DR: In this article, the authors present an overview on particles/LC systems, the size of the dispersed particles being larger than the typical LC length, and discuss the evolution of the relevant interactions between nanoparticles (NPs) dispersed in liquid crystals when their size decreases from micrometers to nanometers.
Abstract: Besides nanostructured materials, individual particles are key elements for nanosciences. The structuring properties of liquid crystals (LCs) are appealing to assemble them, to organize them on substrates or to design functional composites. We present here an overview on particles/LC systems, the size of the dispersed particles being larger than the typical LC length. We first summarize the large number of advances made these last 10 years concerning microparticles assemblies. We then discuss the evolution of the relevant interactions between nanoparticles (NPs) dispersed in LCs when their size decreases from micrometers to nanometers. Various NPs assemblies obtained, either in LC bulk, at interfaces or within LC distorted areas or topological defects are then reported and discussed. Finally, we consider the recent possibilities to use NPs as building elements of complex fluids. We discuss accordingly the LC phases, which can be obtained with pure inorganic NPs in concentrated solution, as well as the sel...
161 citations
TL;DR: The authors summarize the research and development on stimuli-controllable diffraction gratings and beam steering using LCs as the active optical materials and describes dynamic gratings fabricated by applying external field forces or surface treatments.
Abstract: The ability to control light direction with tailored precision via facile means is long-desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next-generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli-controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.
148 citations
TL;DR: In the presence of oriented smectic liquid crystal defects, hybrid systems of nanoparticle/liquid crystals form straight chains of nanoparticles of length longer than tens of micrometers and width equal to one single nanoparticle, highlighting the control of optical absorption by light polarization monitored by gold nanoparticle concentration.
Abstract: We used oily streaks defects of smectic films, oriented by the underlying substrates to create chains of nano-particles of diameter 5nm, gold nano-spheres and quantum dots. Combination of Scanning Electronic Microscopy, fluorescence microscopy and micro-specto-photometry evidenced formation of straight chains of hundreds of micrometers, parallel to the oily streaks, They are constituted of single nano-particles of separation ranging from some micrometers (c'est bien cela?) to 1.5nm when the concentration is increased by ten thousands, in agreement with the most stable sites of trapping located within defects cores, not larger than 5nm. A hierarchy of trapping efficiency is demonstrated in relation with the defect nature. When concentration is further increased, single chains are thickened, but still anisotropic, thanks to the presence of trapping sites around the defccts cores. The hybrid systems with gold nano-particles are consequently optically active and the LSPR can be tuned by light polarization, in a matter controllable by the concentration of GNPs.
139 citations
25 Jul 2008
TL;DR: In this paper, the authors deal with the applications of the density functional formalism to the study of inhomogeneous systems with hard core interactions, including a brief tutorial on the fundamentals of the method, and the exact free energy DF for one-dimensional hard rods obtained by Percus.
Abstract: This chapter deals with the applications of the density functional (DF) formalism to the study of inhomogeneous systems with hard core interactions It includes a brief tutorial on the fundamentals of the method, and the exact free energy DF for one-dimensional hard rods obtained by Percus The development of DF approximations for the free energy of hard spheres (HS) is presented through its milestones in the weighted density approximation (WDA) and the fundamental measure theory (FMT) The extensions of these approaches to HS mixtures include the FMT treatment of polydisperse systems and the approximations for mixtures with non-additive core radii The DF treatment of non-spherical hard core systems is presented within the generic context of the study of liquid crystals phases The chapter is directed to the potential users of these theoretical techniques, with clear explanations of the practical implementation details of the most successful approximations
120 citations
TL;DR: Recently, it has been reported that liquid crystal (LC) defects can be used to create highly periodic templates by controlling the surface anchoring and the elastic properties of LC molecules as discussed by the authors, which takes advantage of the ability to achieve fast stabilization of molecular ordering and structure due to the reversible and non-covalent interactions of the LC molecules.
Abstract: Recently, it has been reported that liquid crystal (LC) defects can be used to create highly periodic templates by controlling the surface anchoring and the elastic properties of LC molecules. The self-assembled defect ordering of the LC materials takes advantage of the ability to achieve fast stabilization of molecular ordering and structure due to the reversible and non-covalent interactions of the LC molecules. In this Featre Article, the defect structures of liquid crystalline materials will be demonstrated by the surface anchoring and elastic properties. A particular focus are the focal conic domains (FCDs) that are commonly observed in SmA liquid crystals and their lamellar lyotropic counterparts, which form periodic defect ordered structures. In addition, methodologies for creating lithographic templates from the defect order will be described. Finally, the review closes with a discussion of toric focal conic domain arrays that have been fabricated in this manner and used for various applications.
99 citations
References
More filters
TL;DR: In this paper, a review focusing on nematic liquid crystals is presented, where three main kinds of effects can be distinguished: the perturbation of the liquid crystalline structure close to the surface, the bulk liquid crystal structure is recovered with an orientation which is fixed by the surface and critical adsorption or wetting can occur at surfaces.
Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.
754 citations
TL;DR: In this article, an asymmetric surface reflection of x rays was found and it was experimentally determined that this anomalously reflected wave has almost the same wavelength as the incident wave.
Abstract: An asymmetric surface reflection of x rays was found. It was experimentally determined that this anomalously reflected wave has almost the same wavelength as the incident wave. The angle of this reflection changes only slightly with the incident angle and the crystal structure of the sample in the measured angular range. The glancing angle of this reflection increases with the wavelength of the incident x rays. According to measurements on evaporated surfaces of metallic elements, these reflection angles were found to have an important relation to the periodic system of the elements.
604 citations
TL;DR: Mise en evidence de the structure par des etudes de diffusion de rayons X haute resolution d'echantillons minces smectiques C prepares entre deux lames solides par refroidissement a partir de the phase smectique A.
Abstract: Mise en evidence de la structure par des etudes de diffusion de rayons X haute resolution d'echantillons minces smectiques C prepares entre deux lames solides par refroidissement a partir de la phase smectique A
446 citations
TL;DR: In this article, a method is developed for calculating the scattering of a beam of x rays striking the plane surface of a crystal at an angle below the critical angle for total external reflection.
Abstract: A method is developed for calculating the scattering of a beam of x rays striking the plane surface of a crystal at an angle below the critical angle for total external reflection The low penetration under this condition offers the possibility of determining the special structure of the surface layers, as has been pointed out by Marra, Eisenberger, and Cho A distorted-wave approach is developed whereby the crystal is first taken to be a homogeneous dielectric slab for the purpose of calculating the distorted wave The distorted wave is considered to illuminate the actual crystal, from which a scattering pattern can then be calculated The effects of absorption can be taken into account; in some cases absorption may offer the possibility of observing near-surface structures at angles of incidence larger than the critical angle Such illumination may also be useful for performing fluorescence analysis of the near-surface layers to determine their impurity content Synchrotron sources offer new opportunities for measurements of these kinds Thermal neutrons may also be used in place of x rays and offer unique opportunities for studying surface magnetization and, through inelastic scattering, surface phonons and, conceivably, surface magnons
410 citations
TL;DR: In this article, the splay and bend constants of K 11 and K 33 in two homologous series of nematic compounds, the alkyl- and alkoxy-cyanobiphenyls, were obtained in two laboratories using different methods and the extent to which they agree gives confidence that neither set is significantly affected by systematic error.
Abstract: We present results for the splay and bend constants K 11 and K 33 , in two homologous series of nematic compounds, the alkyl- and alkoxy-cyanobiphenyls and in PAA. They have been obtained in two laboratories using different methods, and the extent to which they agree gives confidence that neither set is significantly affected by systematic error Comparaison des mesures de deux laboratoires pour les constantes de Frank K 11 et K 33 de PAA et de deux series homologues d'alkyl- et alcoxy-cyanobiphenyles, par une methode electrique et par une methode magnetique
160 citations