Thin-film Liquid Crystal Research Articles

Liquid Crystal Films with Defects

We investigate the structure of nematic and smectic C liquid crystal thin films whose textures include topologically induced defects. In the case of a chiral smectic C film the defects interact with the film’s spontaneous polarization causing thickened islands to form. Using Landau models we characterize the number, location, and degree of the induced defects. Smectic C films have tilted molecules and have a vector description. In this case we prove that minimal energy states have defects with degree 1. Tangential nematic fields are characterized by tensor fields and we prove that minimizing states for these have defects of degree .

Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon

Anchoring, i.e. the orientation and ordering of organic molecules at a surface and its persistence away from the interface, is a key phenomenon for liquid crystal (LC) physics and technology, as well as for organic transistors and photovoltaic cells where it strongly affects transport. Recent exciting experiments have started to explore anchoring at the nanoscale, but seemingly conflicting views are emerging, suggesting either a surface order very slowly decreasing with distance, or the expected exponential decay for adhesion energies. The current theoretical description of anchoring is, however, essentially empirical, and methods for predicting the molecular structuring and orientation are conspicuously lacking. Here we establish such predictive tools by using atomistic molecular dynamics simulations and obtain the order and molecular organization of 12 and 24 nm-thick nematic and isotropic films of the popular LC 4-n-pentyl-4-cyano biphenyl (5CB) at its interfaces with an atomically flat, H-terminated, (001) crystalline silicon surface and vacuum. We show that the film adopts a hybrid configuration, with a change of the 5CB preferred orientation from planar uniform at the silicon to perpendicular at the free surface. This variation is nearly discontinuous for the thin film, and continuous with twist and bend for the thicker one. Our findings provide for the first time a full description of the interfacial silicon-LC structure at the nanoscale level.

Electro-optic, thermo-optic and dielectric responses of multiwalled carbon nanotube doped ferroelectric liquid crystal thin films

Multiwalled carbon nanotubes (MWCNTs) doped ferroelectric liquid crystal (FLC) (KCFLC10R) thin films in different (0.00, 0.01 and 0.03 wt./wt.%) ratios have been prepared and studied. In doped sample, fastening of switching time about ~ 80% has been observed. In addition to that, an increase in spontaneous polarization and decrease in rotational viscosity in doped samples than pure FLC sample was also noticed. Improvement in electro-optic responses in doped cells is due to the ion trapping which results an enhancement in effective electric field applied to the liquid crystal cells. Dielectric measurements revealed a decrease in permittivity in doped sample cell as compared to pure FLC cell. Conductivity behavior with MWCNTs concentration has been investigated. MWCNTs doping has good response in terms of increased contrast ratio and low threshold voltage over pure FLC cell.

Distributed feedback leaky laser emission from dye doped gel-glass dispersed liquid crystal thin film patterned by soft lithography

Gel-glass dispersed liquid crystal (GDLC) thin films doped with organic laser dye Rhodamine 6G (R6G) were prepared via a sol–gel procedure of tetraethyl orthosilicate (TEOS) and ethyl triethoxisilane (ETES). As characterized by scanning electronic microscope (SEM), surface-relief structures were successfully patterned on lower refractive index GDLC thin films by soft lithographic technology, which support distributed feedback (DFB) laser emission based on leaky mode propagation. The performance of the DFB laser emission was investigated and the spectral narrowing of the emitted radiation and the fine structure pattern were found to be controlled by the doping concentration of liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB). We also showed the synchronous excitation of a DFB lasing with random lasing mediated by light scattering inside the same GDLC leaky waveguide.

Tunable distributed feedback lasing from leaky waveguides based on gel-glass dispersed liquid crystal thin films

Different amount of nematic liquid crystal (LC) 4-Cyano-4′-pentylbiphenyl(5CB) was encapsulated in sol–gel derived organically modified silica(ORMOSIL) matrices to fabricate dye doped gel-glass dispersed liquid crystal (GDLC) thin films by co-hydrolyzation/condensation of ethyl triethoxisilane (ETES) and tetraethyl orthosilicate (TEOS) precursors. Distributed feedback (DFB) leaky waveguide lasers from GDLC thin films using a dynamic grating were demonstrated. Tunable laser action from 558 to 570 nm was obtained in the sample with lowest LC concentration of 0.375 mol%. The DFB laser performance of GDLC thin films with various LC concentrations was investigated using the same pumping geometry, and it was shown that the encapsulation of 5CB also led to the red shift of lasing peak.

Modeling flows of confined nematic liquid crystals

The flow of nematic liquid crystals in tightly confined systems was simulated using a molecular theory and an unsymmetric radial basis function collocation approach. When a nematic liquid crystal is subjected to a cavity flow, we find that moderate flows facilitate the relaxation of the system to the stable defect configuration observed in the absence of flow. Under more extreme flow conditions, e.g., an Ericksen number Er=20, flows can alter the steady-state defect structure observed in the cavity. The proposed numerical method was also used to examine defect annihilation in a thin liquid crystal film. The flows that arise from shear stresses within the system result in a higher velocity for s = +1∕2 defect than for the defect of opposing charge. This higher velocity can be attributed to reactive stresses within the deformed liquid crystal, which result in a net flow that favors the motion of one defect. These two examples serve to illustrate the usefulness of radial basis functions methods in the context of liquid crystal dynamics both at and beyond equilibrium.

Modeling and simulations of the spreading and destabilization of nematic droplets

A series of experiments [C. Poulard and A. M. Cazabat, “Spontaneous spreading of nematic liquid crystals,” Langmuir 21, 6270 (2005)] on spreading droplets of nematic liquid crystal (NLC) reveals a surprisingly rich variety of behaviors. Small droplets can either be arrested in their spreading, spread stably, destabilize without spreading (corrugated surface), or spread with a fingering instability and corrugated free surface. In this work, we discuss the problem of NLC drops spreading in a simplified two-dimensional (2D) geometry. The model that we present is based on a long-wavelength approach for NLCs by Ben Amar and Cummings [“Fingering instabilities in driven thin nematic films,” Phys. Fluids 13, 1160 (2001); L. J. Cummings, “Evolution of a thin film of nematic liquid crystal with anisotropic surface energy,” Eur. J. Appl. Math. 15, 651 (2004)]. The improvements in the model here permit fully nonlinear time-dependent simulations. These simulations, for the appropriate choice of parameter values, exhibit 2D versions of most of the phenomena mentioned above.

Textures in thin films of nematic liquid crystals induced by strongly focusing a circularly polarized laser

We investigate the rearrangement of the director in thin films of nematic liquid crystals caused by tightly focusing circularly polarized laser beams. We find either target or spiral patterns, depending on the topology of the director configuration at the position of the beam focus. The induced rearrangements of the director are governed by the viscosity of the media, the handedness of circular polarization, and the irradiation power of the laser. Experimental observations are interpreted using a model derived from nematic continuum theory.

Doping and photo-induced current in discotic liquid crystals thin films: Long-time and temperature effects

Here we address the problem of time and temperature instability of the electrical current related to parasitic doping effects in discotic liquid crystals (DLCs). We investigated the electrical transport in thin films of a phthalocyanine-based DLC in-dark and upon laser irradiation during long thermal cycles. Measuring the electrical transport between two electrodes we observed that in-dark the current is mostly due to doping levels while, upon laser irradiation, the photocurrent reflects the diffusion length of the photogenerated charge carriers and it is limited by the coherence length of a column. We prove here that the two contributions are completely independent. Our result experimentally supports the theoretical studies performed at the molecular level on discotic liquid crystals.

Homeotropic Alignment and Director Structures in Thin Films of Triphenylamine-Based Discotic Liquid Crystals Controlled by Supporting Nanostructured Substrates and Surface Confinement

We explore the effects of nanoscale morphology of supporting solid substrates on alignment, defects, and director structures exhibited by thin films of triphenylamine-based discotic liquid crystals. Fluorescence confocal polarizing microscopy and intrinsic polarized fluorescence properties of studied molecules are used to visualize three-dimensional director fields in the liquid crystal films. We demonstrate that, by controlling surface anchoring on supporting or confining solid substrates such as those of carbon nanotube electrodes on glass plates, both uniform homeotropic and in-plane (edge-on) alignment and nonuniform structures with developable domains can be achieved for the same discotic liquid crystal material.

Influence of mesoscopic structures on single molecule dynamics in thin smectic liquid crystal films

Mesoscopic structures in liquids have an impact on the diffusion dynamics of the constituting molecules. Smectic 8CB liquid crystals on silicon wafers show the formation of mesoscopic structures on the μm scale at a film thickness of 200 nm. Depending on the kind of substrate (thermally grown or native SiOx), we observed the formation of focal conic domains (FCDs) and a new type of terraced holes, respectively. Dynamics are described via single perylene diimide tracer molecule tracking of translational diffusion and in the case of FCDs by a combination of translation and rotation detected viafluorescence correlation spectroscopy. Tailoring perylene diimide molecules such that the optical transition dipole moment follows the liquid crystal director allows mapping out FCDs and investigating the dynamics within a single FCD.

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Evaluation of Alignment of Nematic Liquid Crystal Thin Film by Spectroscopic Ellipsometry

Generally, macroscopic molecular orientation state in liquid crystal layer can be described by Ossen and Frank's continuum theory. However, the validity of applying the continuum theory to nematic liquid crystal thin film (NLCF) has not been confirmed yet. In this study, we experimentally examined NLCF by employing spectroscopic ellipsometer. As a result, theoretical and experimental result represent a good agreement when NLCF thickness is from 60 to 250 nm. Furthermore, the influence of surface roughness on alignment film surface and glass substrate was also examined.

High Efficiency Wideband Aperture-Coupled Stacked Patch Antennas Assembled Using Millimeter Thick Micromachined Polymer Structures

Micromachined stacked patch antenna devices with high efficiency and wideband characteristics are reported. Polymer based fabrication and assembly processes have been developed in order to produce the stacked suspended antenna devices. Millimeter thick micromachined SU8 based polymer rings are used to create air gaps between the patches and the microwave substrate for optimized high efficiency operation. Thin film liquid crystal polymer (LCP) and polyimide substrates are used to support the radiating and parasitic patch elements. The polymer rings also form cavities to protect the patches and substrate from moisture and dust. The antenna structures are fabricated in layers and then assembled to obtain 3D devices. The antenna devices have been designed using an electromagnetic simulation package. The aperture coupled devices are impedance matched for wideband operation. RF measurements show wideband operation of the devices and the results are in good agreement with that of simulation. Typical gain and bandwidths are 7.8 dBi and 39% for a microstrip fed antenna device while they are 7.6 dBi and 44% for a CPW fed device. The predicted efficiency from the results of simulation is above 97% for the antenna devices.

Measurement of liquid crystal film thickness using interferometry

Thickness measurements of thin films having thickness less than 1 µm are difficult to obtain by an interferometer. These difficulties arise from the overlap of the fringes from the upper and lower surfaces of the thin films. This paper presents a new methodology that mediates the consequences of this overlap and then implements it with thickness measurements of liquid crystal (LC) thin films. It takes into consideration the properties of light propagation within these films in order to rectify the images obtained from the interferometer. It assumes that the lower fringe pattern is much stronger that the upper one and hence the latter may be ignored. This occurs in situations where thin films are coated on substrates of significantly higher reflectivity, as happens when an LC thin film is coated on a polished iron substrate. The thickness and topography of LC thin films were experimentally measured with this methodology and were compared with measurements taken by an atomic force microscope.

Thin Liquid Crystal Films on Liquids in the Nematic Range of Temperatures

Hybrid nematic films deposited on liquid substrates reveal a complex behavior, which is not fully understood. Here, the behavior of the n-cyanobiphenyl series on water and glycerol has been studied in a wide temperature range, including the vicinity of the nematic-isotropic (NI) transition. Wettability, allowed film thicknesses, and line tension of nematic domains have been investigated. The study provides a coherent picture of hybrid nematic films, allowing us to account for lower thickness threshold, structure of the film edge, and line tension of domains in the whole nematic range of temperatures.

Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic–isotropic phase transition temperature ( T NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.

Electric-Field Induced Transitions in a Cholesteric Liquid-Crystal Film with Negative Dielectric Anisotropy

A combination of analytical, numerical, and qualitative methods is used to study competing equilibrium orientational configurations in a liquid-crystal thin film. The material is a cholesteric liquid crystal and has a negative dielectric anisotropy. The system has strong homeotropic anchoring of the liquid-crystal director on the confining substrates and is subject to a voltage applied across the film thickness. A free-energy functional embodies the competing influences of the boundary conditions, the intrinsic chirality of the material, and the electric field. Attention is restricted to director fields that are functions only of the distance across the cell gap. A detailed phase and bifurcation analysis of the two equilibrium configurations of this type is presented;the control parameters are the ratio of the cell gap to the intrinsic pitch of the cholesteric and the applied voltage. The study was motivated by potential technological applications. The phase diagram contains both first-order and second-order transition lines, the former terminating at an isolated point and the latter at a triple point. The voltage-dependent nature of the total twist of the director across the cell is revealed and explained, and an effective upper bound is obtained on the ratio of cell gap to pitch for the system to support a simple voltage-driven second-order transition between the two 1-D equilibrium states.

All-optical switching in an optofluidic polydimethylsiloxane: Liquid crystal grating defined by cast-molding

We report an optofluidic photoswitchable grating, based on a polydimethylsiloxane periodic structure on a glass substrate, separated by a thin liquid crystal film. The polydimethylsiloxane microstructure was realized via high resolution replica molding and was employed to both confine and align a photosensitive nematic liquid crystal. In the absence of any surface treatment, the liquid crystal exhibited homeotropic alignment. By inducing planar alignment on the glass substrate, a hybrid orientation of the liquid crystal was achieved, inducing polarization sensitive transmission. The photosensitivity of the liquid crystal enabled the all-optical control of the grating transmission and 20% diffraction efficiency was measured.

Optical monitoring of anchoring change in vertically aligned thin liquid crystal film for chemical and biological sensor

A significant advance in sensitivity of liquid-crystal (LC)-based chemical and biological sensors can be achieved by actively monitoring anchoring energy change. We simulate the deformation of a LC director with different anchoring energies using the finite element method and the optical properties of the LC film using the finite-difference time-domain method. Polarizing micrographs are collected and compared with simulated textures. Measurement of optical transmission is used to monitor the anchoring change. Experimental and simulation results both demonstrate the optical method can effectively monitor the surface anchoring change due to the presence of targeted analytes.