One- and two-dimensional liquid crystal structures for lasing applications

Liquid crystal (LC) lasers have gained a lot of research interest in the last decade. Especially out-of-plane emitting chiral nematic liquid crystal (CLC) lasers have been studied extensively. These regular CLC lasers have a one-dimensional (1D) structure and the active cavity length is inherently limited. By using CLCs in two-and three-dimensional structures, the flexibility and applicability of the laser structures can be strongly enhanced. In this paper we focus on 2D in-plane emitting CLC lasers with a lying helix structure. We elaborate further on different techniques to obtain the lying helix structure and we analyze the lasing properties and compare these to regular 1D out-of-plane emitting CLC and NLC lasers. Both differences in emission spectrum, laser threshold, slope efficiency and maximal output energy are discussed

Optimization of liquid crystal structures for real time holography applications

In this paper we present results of experiments designed to increase our understanding of the photorefractive effect occurring during processes of dynamic hologram generation in Hybrid Photorefractive Liquid Crystal Structures (HPLCS). We also propose equivalent mathematical model which can be used to optimize those structures in order to obtain the highest diffraction efficiency in possibly shortest time. (C) 2011 Optical Society of Americ

Ring-shaped liquid crystal structures through patterned planar photo-alignment

Patterned liquid crystal (LC) configurations find widespread applications in functional devices such as lenses, gratings, displays and soft-robots. In combination with external stimuli such as an applied electric field, photo-alignment at the surfaces offers an attractive way to stabilize different LC structures in the bulk of a device. Herein, a planar LC cell is developed using a photo-alignment layer at the bottom substrate and a rubbed nylon film at the top substrate. Patterned planar photo-alignment is achieved by modulating the linear polarization with a spatial light modulator (SLM) and projecting the pattern onto the bottom substrate. A ring pattern is written into the photo-alignment layer with a continuous rotation between an inner radius and an outer radius. In the other regions the alignment is parallel to the rubbing direction at the top substrate. Four different LC configurations are observed: structure A in which a ring-shaped region is formed with an out of plane (vertical) orientation perpendicular to the substrate, structure B which has a single disclination loop and a 180 degrees twist at the inner region of the photo-patterned ring (r < r(in)), structure C which has no discontinuities but a 360 degrees twist in the inner region of the photo-patterned ring (r < r(in)) and structure D with 2 disclination loops. The LC director configuration for all 4 structures is simulated through finite element (FE) Q-tensor simulations and the optical transmission for each structure is simulated using a generalized beam propagation method

Cyclic polymer liquid crystal structures.

Recent advances in liquid crystal research have included the synthesis of polymeric materials which contain liquid crystalline moieties. The work presented here concerns the study of the structure-property relationships of a particular group of liquid crystalline polymers in which the polymer backbone is cyclic, with the mesogenic moieties attached as side-chains. We have observed mesogenic phases above room temperature for materials comprising cyclic poly(dimethylsiloxane) backbones with mesogenic moieties attached by alkyl spacer units. Dielectric relaxations have been observed in the mesophases of these materials and the activation energies and extent of broadening of the relaxations have been related to the physicalstructure of the molecules. Theoretical studies have been undertaken by the use of the Metropolis Monte Carlo technique and a mean field calculation. Two models have been studied by the Monte Carlo technique in the NVT ensemble. In the first, each complete molecule was represented by a disc-like interactions potential and a tendency for the molecules to align in columns was revealed at low temperature and high density. In the second model, each mesogenic unit was represented separately, with the cyclic polymerrepresented as a constraint on the relative motions of the attached mesogens. A variety of liquid crystalline phases, from discotic nematic to calamitic nematic, were observed at low temperature as the coupling between the side-chains and the backbones was adjusted. In the mean field model energy terms were included for ring-ring interactions, mesogen-mesogen interactions and the coupling between the mesogenic moieties and the backbones. The uniaxial solution of this model also showed a shift from calamitic nematic to discoticnematic phases as the strength of the coupling was increased. Comparisons of the results of the models and the physical measurements are presented and suggestions for future work are proposed

Vector beams generated by microlasers based on topological liquid-crystal structures

Structured light with designable intensity, polarization and phase fields is today of high relevance, with application ranging from imaging, metrology, optical trapping, ultracold atoms, classical and quantum communications and memory. Specifically, vortex and vector beams can be generated directly in the laser cavity, however, a controllable, geometrically simple and easy to manufacture laser microcavity that generates structured light on demand, especially tailored polarization, is still an open challenge. Here we show that tunable laser vector beams can be generated from self-assembled liquid-crystal (LC) micro-structures with topological defects inside a thin Fabry-P\'erot microcavity. The LC superstructure provides complex three dimensional birefringent refractive index profiles with order parameter singularities. The topology of the LC structures is transferred into the topology of the light polarization. The oriented fluorescent dye emission dipoles enable the selection of optical modes with a particular polarization, as enabled by the birefringence profile in the laser cavity. The proposed lasers have no principal limitation for realizing structured light with arbitrarily tailored intensity and polarization fields

Orientation of liquid crystalline materials by using carbon nanotubes

The solution of some problems, where the initial black field is necessary for the regime of light transmission through the electrooptical organic nematic liquid crystal structures has been considered via a homeotropic alignment of liquid crystal molecules on the substrate covered by carbon nanotubes. The results of this investigation can be used to develop optical elements for displays with vertical orientations of nematic liquid crystal molecules (for example, for MVA-display technology)

Electric-field modulation of liquid crystal structures in contact with structured surfactant monolayers

We present experiments in which we use an electric field to switch between different configurations in the cellular patterns induced in a confined nematic liquid crystal by the contact with a surfactant monolayer that features lateral order and surface defects. By using different combinations of far-field alignment and mesogen dielectric anisotropy, we unravel the nature and stability of point defects and disclinations resulting from the hybrid boundary conditions