Surface stabilized topological solitons in nematic liquid crystals

Photo-alignment is a versatile tool to pattern the alignment at the confining substrates in a liquid crystal (LC) cell. Arbitrary alignment patterns can be created by using projection with a spatial light modulator (SLM) for the illumination. We demonstrate that a careful design of the alignment patterns allows the stabilization of topological solitons in nematic liquid crystal (NLC) cells, without the need for chirality or strong confinement. The created LC configurations are stabilized by the anchoring conditions imposed at the substrates. The photo-aligned background at both substrates is uniformly planar aligned, and ring-shaped regions with a 180° azimuthal rotation are patterned with an opposite sense of rotation at the top and bottom substrate. A disclination-free structure containing a closed ring of vertically oriented directors is formed when the patterned rings at the top and bottom substrate overlap. Thanks to the topological stability, a vertical director orientation in the bulk is observed even when the centra of both patterned rings are shifted over relatively large distances. The combination of numerical simulations with experimental measurements allows identification of the 3D director configuration in the bulk. A finite element (FE) Q-tensor simulation model is applied to find the equilibrium director configuration and optical simulations are used to confirm the correspondence with experimental microscopy measurements. The created LC configurations offer opportunities in the field of optical devices, light guiding and switching, particle trapping and studies of topological LC structures

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

Collimated backlight for liquid crystal displays

In this work, a collimated backlight design is presented. Traditionally backlights are either direct-lit in which an array of LEDs are directly behind the LC-panel, or edgelit, in which case there is a light guide behind the LC-panel and LEDs are placed at the side of this light guide. These designs have their advantages and disadvantages but in both cases they emit light under a wide angle

Liquid Crystal TV and OLED TV : issues and opportunities

There are currently two competing technologies on the TV market: liquid crystal TV and OLED TV. Each of them has its strengths and weaknesses, but at the same time, both technologies also see rapid developments that further improve their quality. In an OLED matrix the pixels are driven by a DC current and emit light proportional to the level of the current. In an LCD matrix the pixels are driven by an AC voltage and the polarization of incident light is modified, which, in combination with a polarizer determines the transmission. In both the OLED and the LCD TV red, green and blue color filters are used to render millions of colors by mixing filtered white light. In this presentation we will discuss differences between the two technologies, and address a number of important advances in the field of OLEDs (multilayer stacks, temperature dependency, quantum dot OLEDs) and LCDs (LEDarray illumination, photoluminescent quantum dots, brightness enhancement films, local dimming). In addition a few contribution of the research group will be highlighted [1-4]

Photoaligned liquid crystal devices with switchable hexagonal diffraction patterns

Highly efficient optical diffraction can be realized with the help of micrometer-thin liquid crystal (LC) layers with a periodic modulation of the director orientation. Electrical tunability is easily accessible due to the strong stimuli-responsiveness in the LC phase. By using well-designed photoalignment patterns at the surfaces, we experimentally stabilize two dimensional periodic LC configurations with switchable hexagonal diffraction patterns. The alignment direction follows a one-dimensional periodic rotation at both substrates, but with a 60° or 120° rotation between both grating vectors. The resulting LC configuration is studied with the help of polarizing optical microscopy images and the diffraction properties are measured as a function of the voltage. The intricate bulk director configuration is revealed with the help of finite element Q-tensor simulations. Twist conflicts induced by the surface anchoring are resolved by introducing regions with an out-of-plane tilt in the bulk. This avoids the need for singular disclinations in the structures and gives rise to voltage induced tuning without hysteretic behavior

Controlled wiring of disclination lines between patterned photoalignment layers in nematic liquid crystal

On demand creation of topological defect configurations is a relevant topic for theoretical studies but also enables new developments in the field of photonics, stimuli-responsive actuators and colloidal assemblies. Liquid crystals (LCs) provide a versatile and technologically relevant material system to study topological defects, and photoalignment enables complex surface patterning that includes the formation of defects. This work is based on periodic defect grids of +1/2 and -1/2 defects, imposed at the top and bottom surface of a LC cell. The periods at the two substrates are slightly different and the azimuthal orientation of the defects is varied, so that different types of disclination interconnections are formed in the bulk of the LC. Some disclination lines connect defects at opposing substrates, while others connect defects on the same substrate. The experimentally generated configurations are simulated with the help of finite element Q-tensor simulations in order to get a detailed understanding of the defect formation. The optical appearance in polymerizing optical microscopy images is compared to optical simulations to verify the validity of the director configurations

Photoaligned Chiral Liquid Crystal Grating with Hysteresis Switching

Metastable topological states are known to arise when chiral liquid crystal (CLC) is infiltrated in a geometry with strong confinement. This study demonstrates an alternative approach to obtain multistable states, by combining patterned surface anchoring with CLC in a less confined geometry. Long pitch CLC is introduced between two 1D periodically rotating anchoring patterns at the top and bottom substrate with perpendicular rotation direction. Voltage-induced switching is observed by polarizing optical microscopy and diffraction measurements. For some voltages two different stable topological states appear and electrical switching between these two states occurs via a hysteresis loop. The chirality in the LC material induces the formation of a new topological state at low voltages, which is not observed in nonchiral LC cells with the same anchoring configuration. Hysteresis switching between different optical states may find applications in low-power devices with internal memory

Two-Dimensional Liquid-Crystal Photoalignment by Multiple Illumination Steps

Photoalignment can define the average liquid-crystal (LC) orientation when a photosensitive material is illuminated with linearly polarized light. For azo-based materials, the alignment is perpendicular to the direction of the linear polarization for sufficiently high illumination doses. The defined orientation can be overwritten by a second illumination step with a different polarization. In general, the resulting alignment is a function of the detailed illumination procedure. In this work, a method is presented to model the LC director alignment as a function of an illumination procedure consisting of three steps in which the first and third steps have spatially rotating linear polarization, while the second step uses circularly polarized light. The model is used to simulate the optical transmission between crossed polarizers in order to verify experimental observations of a device with a two-dimensional orientation pattern. Because photoalignment can realize complex alignment patterns with high resolution for diffractive optical components, it is attracting increasing interest in the field of augmented reality displays