Session N30: Liquid Crystals: Experiments

Alignment of Liquid Crystals by Nano- and Micro -Scale Topographic Patterns Made by Nanoimprint Lithography

Topographic patterns prepared using nanoimprint lithography have great potential in the alignment of liquid crystals (LCs) because their preparation is a parallel and low cost process. Nano- and micro- scale topographic patterns are made by stamping a mold on liquid material, which is then cured by UV light illumination. Such topographically patterned plates are used as one of the window of LC cells. Depolarized transmission light microscopy shows that nematic liquid crystals are aligned uniformly along the lines on the linear patterns and aligned either homeotropically or with bistable planar states in patterns of squares, depending on their scale.   

Absence of uniform nematic phase for thin 8CB films

Certain thermotropic liquid crystal films exhibit a strange phenomenon in which two different thicknesses coexist side-by-side on solid surfaces. For 5CB these two film thicknesses appear to correspond to two different phases: nematic and isotropic. The coexistence persists for a temperature range which depends on the initial thickness of the film. A similar phenomenon is present in films that have a smecticA phase such as 8CB. For these films just below the nematic to isotropic transition temperature, we observe a coexistence region very similar to that observed previously for 5CB. We also report new experiments which show that for 8CB films on silicon, near the smecticA to nematic transition temperature, there is a coexistence region that is strikingly different from the one observed near the nematic to isotropic transition. Furthermore, there is a new phenomenon for 8CB films thinner than a certain critical thickness d*: as the temperature increases or decreases the film goes from smecticA to isotropic and back with no intermediate uniform nematic phase.   

Electrically controlled negative refraction in a uniaxial nematic liquid crystal

We demonstrate the phenomenon of electrically controlled negative refraction at the interface between an isotropic material and a uniaxial nematic liquid crystal, in which the optic axis makes a large angle (40-60 degrees) with the interface and is switched by a modest (few volts) electric field. Depending on the applied voltage, the refracted beam is either on the opposite side of the interface normal as compared to the incident beam (positive refraction) or on the same side (negative refraction) .   

Effect of the Functionalization Compound on the Magnetic Nanoparticle -- Smectic-A Liquid Crystal Interaction: A Phenomenological Model

We present the details of a phenomenological model that explains how the functionalization or surface compound affects the way that a magnetic nanoparticle interacts with a liquid crystal molecule. This model considers the surface interaction between the functionalization compound and the liquid crystal, and the relative size of the liquid crystal compared to the size of the nanoparticle. These two properties can aid or hinder in the effects of the nanoparticle on the orientation of the liquid crystal, specifically on the magnetic field effects of the nanoparticle in the reorientation of the liquid crystal. Comparisons with experimental data will be presented.   

Broadening the Smectic C* Sub-Phases by Chiral and Achiral Doping

Materials exhibiting the liquid crystal smectic (Sm) C* sub-phases have been a topic of great interest for several years and recent work has shown that by mixing chiral and achiral dopants with these materials it is possible to broaden the SmC* subphases significantly, in particular, the intermediate phases (ferrielectric) which typically have a very narrow temp. range [1]. Mixtures of smectics with chiral or achiral dopants were studied to investigate phase stability in the SmC* sub-phases. As dopant conc. is increased, both phase width and the temp. range over which the transition occurs is broadened. These effects have been measured via calorimetry (DSC), and optical microscopy. Electro-optical characteristics of mixtures as a function of dopant conc. were also studied to investigate the effects of phase and transition broadening on material properties as it is not yet clear what role the addition of chiral dopants will play. By controlling phase widths, we hope to expand the possibilities, through carefully formulated mixtures, of generating commercially interesting materials in these phases. \newline \newline [1] S. Jaradat et al. J. MATER. CHEM. \textbf{16}, 3753, (2006)   

Use of the chiral smectic A liquid crystal electroclinic effect for sensitive measurement of enantiomeric excess

We present here a procedure which is capable of detecting an enantiomeric excess (ee) as low as $10^{-4}$\% using the electroclinic effect. The electroclinic effect is a field induced effect on the optic axis of the SmA phase of chiral molecules in which the tilt angle $\theta$ is linear with the electric field E. Thus, varying the voltage across a cell with planar alignment of a sample of unknown ee in SmA phase induces varying orientation of the director. This in turn induces varying intensity of a laser beam passing through the sample. The signal from the sensor detecting this beam is fed to a lock-in amplifier for low-noise measurement of the beam's intensity variation, from which we can extract even a small ee of the sample.   

Topological Defect Structure and Annihilation in High-Polarization Freely Suspended Films

The texture of freely suspended liquid crystal SmC* films of a high polarization material C7 [4-(3-methyl-2-chloropentanoyloxy)-4'-heptyloxybiphenyl] is studied using polarized light microscopy. In particular, we focus on c-director defects with topological strength --1 found in the chessboard texture. Due to the competition between the elastic energy and the electrostatic energy of polarization splay, the c-director field near the defect core consists of four domains with homogenous orientation of the c-director inside each domain. The boundaries between domains are sharp and the c-director orientation jumps by 90 degrees at each boundary. We will present experimental and theoretical studies of the structure of these polarization-stabilized discontinuities. We will also present studies of the annihilation dynamics of +1 and --1 pairs of defects on this high-P material film and compare them to those on low-P material films. [1] E. Demikhov, Europhys. Lett. \textbf{25} (4), 259 (1994). [2] E. Demikhov and H. Stegemeyer, Liq. Cry. \textbf{18}, 37 (1995). [3] Ch. Bahr and G. Heppke, Phys. Chem. \textbf{91}, 925 (1987). [4] D. R. Link, N. Chattham, J.E. Maclennan, and N.A. Clark, Phys. Rev. E \textbf{71}, 021704 (2005). This work is supported by NSF MRSEC Grant DMR0213918.   

Interactions between Structurally Chiral Islands on Freely-Suspended Smectic C films

Islands on a freely-suspended Smectic C film, circular regions of greater thickness than the surrounding film area, have strong tangential boundary conditions of the c-director (the projection of the molecular long axis onto the plane of the film) at their edges. These islands form dipolar structures, with an s = +1 topological defect inside and an s = -1 defect nearby on the background film. Unlike in 2D nematics, the c- director field on Smectic C islands does not have reflection symmetry, and we see both left and right-handed islands on the film. Islands with the same handedness form chain-like structures with topological dipoles pointing in the same direction and along the chain (these have been reported in the literature). Here we describe the interaction between left and right-handed islands, where the topological dipoles point in opposite directions and form a quadrupolar structure. The two - 1 defects are half way between the islands and offset from the line joining them.   

A New Bistable SmA Display Mode

In the traditional SmA display, crossed polarizers are absent and one can switch a light transparent state to an opaque light scattering state by using laser addressing or electric addressing. Such displays are bright, but of only moderate contrast ratio. Here, we present a new operation mode for a SmA display using two sets of electrodes, with one to induce homeotropic orientation and the other having an in-plane structure to induce planar orientation. This switching with crossed polarizer and analyzer enables a high contrast, bistable electro-optical effect. This SmA display mode exhibits a high contrast ratio (2500:1) for non-striped ITO pattern cells, prefect bistability, and reasonably fast switching (a few ms). These characteristics may enable potential applications on e-paper.   

The frequency-dependent electrooptic response of the electroclinic effect in deVries SmA materials

It is well established that electroclinic switching in standard SmA* materials relates to a reorientation of the molecules in a plane normal to the layers, and thus there is no corresponding change in birefringence due to reorientation about a cone, as is the case in the SmC* phase. When the electrooptic response is then analyzed via lock-in amplifier, the signal at the driving frequency is strong, while the second harmonic response, is non-existent [1]. Using this method we have investigated deVries materials W530 and TSiKN65, and show that there is a frequency-dependent second order response -- implying an electroclinic switching that corresponds to a change in birefringence, suggesting a reorientation of the molecule about a cone. We will present our findings and a model for the type of electroclinic switching that occurs in these two materials. Work supported by NSF MRSEC Grant DMR-0213918 and The Swedish Foundation for Strategic Research 2002/0388. [1] W. Kuczynski, et. al., \textit{Ferroelectics}, \underline {244}, [491]/191, (2000)   

Effect of Polarization on the Interactions of SmC* Islands on Freely-Suspended Films

We generate islands, circular regions of greater thickness than the surrounding film area, on freely-suspended SmC* films by applying air jets to the film. The c-director field (the projection of the molecular long axis onto the plane of the film) is strongly tangential at the edges of the islands, leading to the formation of s = +1 topological defects inside the islands and s = -1 defects nearby on the background film. Islands interact via these topological defects, with a short- range repulsion and a long-range dipolar attraction governing their stability and leading to their organization in chain-like structures with an equilibrium island separation. We use optical tweezers to measure the force between a pair of islands as a function of their separation. As we vary the enantiomeric excess and hence the polarization of the liquid crystal, the force scales as the polarization squared and the shape of the force curve changes. Simulations based on a simple model of film elasticity and polarization explain our experimental results fairly well.   

Combination of separate smectic-C-alpha phases in binary mixtures

Applying resonant x-ray diffraction, we showed that the two previously considered separate smectic-$C^*_{\alpha}$ phases were combined into a single phase in a series of binary mixtures. The pitch evolved continuously and smoothly, contradicting a previously successful theoretical model. New phase sequences found in our studies can be explained by a newer model. The comparison indicates long-range interactions exist in the smectics.   

Evidence of Broken Reciprocity in Cholesteric Liquid Crystals

Reciprocity of scattering of a plane incident wave is predicated on bounded scattering media with symmetric and linear permittivity, conductivity and permeability. In chiral media, such as cholesteric liquid crystals, the dielectric tensor is asymmetric due the presence of odd powers of the wave vector resulting from nonlocality and broken inversion symmetry. Evidence of non-reciprocity has been found in optically active crystals by Bennet [1] and in stacks of cholesteric and nematic liquid crystal cells [2]. Here we present transmittance and reflectance data for cholesteric cells with different pitches having overlapping but distinct reflection bands. We relate our results to simple analytic descriptions of the materials properties and of propagating modes and assess them in light of the requirements for reciprocity. 1. P.J. Bennett, S. Dhanjal, Yu. P. Svirko and N. I. Zheludev, \textit{Opt. Lett}. \textbf{21}, 1955 (1996) 2. J. Hwang; M.H. Song; B. Park; S. Nishimura; T. Toyooka; J.W. Wu; Y. Takanishi; K. Ishikawa; H. Takezoe, \textit{Nat. Mat.} \textbf{4}, 383 (2005).   

Lasing Thresholds of Obliquely Pumped Cholesteric Liquid Crystal Lasers

Cholesteric liquid crystals (CLCs) in the helical cholesteric phase are 1D photonic band gap materials. Mirrorless lasing has been achieved in dye doped CLC systems. An area of great interest is the reduction of the lasing threshold for optically pumped CLCs. The effects of sample thickness and dye concentrations have been investigated and optimized for CLC systems [1]. However, the pump beam geometry has remained unchanged in most experiments. Numerical calculations of the DFB lasing threshold as function of the angle of incidence have been recently carried out for helical cholesterics [2]. The results predict that the lasing threshold is reduced at oblique incidence for critical angles, and the minimum threshold occurs at the first critical angle closest to the band edge. We have carried out measurements of the lasing thresholds of a variety of cholesteric samples as function of the angle of incidence. The samples were pumped by both ns and ps sources. We present our experimental results, and compare these with predictions of theory. [1]. W. Cao et al, \textit{Mol. Cryst and Liq. Cryst.}, \textbf{429}, 101 (2005). [2] V.A. Belyakov, \textit{Mol. Cryst and Liq. Cryst.}, \textbf{453}, 43 (2006).