Patterned surface alignment to create complex three-dimensional nematic and chiral nematic liquid crystal structures

Combining liquid crystals (LCs) with well-designed anchoring patterns at the substrates offers tremendous potential for the development of functional electro-optic devices or stimuli-responsive actuators. Photo-alignment techniques nowadays allow an almost arbitrary control over the surface anchoring and this flexibility is used to design highly efficient flat optical LC components with different functionalities. Part of this research, dealing with nematic and chiral nematic LC between substrates with patterned azimuthal anchoring, is reviewed here. The focus is on understanding the self-assembly of complex structures, steered by an interplay between surface anchoring and LC elasticity. Additional insight into the LC bulk behaviour is obtained by comparing experimental results with numerical simulations of the director configuration. Periodic anchoring patterns with azimuthal rotation at the top and bottom substrate are studied, as well as ring-shaped alignment patterns with a 180 degrees or 360 degrees azimuthal rotation in a confined region in space. Different combinations of anchoring patterns at the top and bottom substrates are investigated and in addition to nematic liquid crystal (NLC), also short and long-pitch chiral nematic liquid crystal (CLC) is considered

Full-2D simulation of in-plane liquid crystal lasers

Lasing in liquid crystals has been demonstrated in numerous configurations and material systems. In most systems the laser light is emitted perpendicularly to the liquid crystal layer, but in the last few years also in-plane lasers have been demonstrated [1]. Such cheap in-plane tunable lasers could be combined in an opto-fluidic device, allowing to build fully integrated platforms for biological sensing applications. The accurate modelling of light generation in in-plane liquid crystal laser is difficult because the structure is two-dimensional and the optical properties are anisotropic. Moreover, 2D simulations of the liquid crystal orientation in such layers is necessary because the lying helix structure, which is often used for such lasers, exhibits defects. These defects appear because typical planar or homeotropic alignment is not compatible with the lying helix structure. Quite a lot of theoretical and numerical work has been carried out for perpendicularly emitting LC lasers. A one-dimensional plane wave expansion method was previously applied for the analysis of light emission from OLEDs. The extension to anisotropic materials and to simulation of lasing threshold makes it suitable for the simulation of LC lasing characteristics. Good agreement between simulations and experiments was found [2]. For the simulation of in-plane lasers we rely on finite-element calculations of the optical modes in periodic two-dimensional structures [3]. The optical modes in a lying-helix configuration are calculated including the band diagram. The band diagram reveals at which wavelength lasing can occur while the optical mode profile gives information about the electric field profile and the polarization state. Additionally the laser mode of the complete structure can also be calculated. The figure below gives an example of the field profile of the laser mode in a lying helix liquid crystal. The structure consists of a number of periods, terminated by an air layer at both sides

Optimization of liquid crystal devices based on weakly conductive layers for lensing and beam steering

Liquid crystals are mostly known for their use in displays, but over the past decade these materials have been applied in a number of other devices such as tunable lenses or beam steering devices. A common technique to realize a gradual electric field profile as is required to obtain a gradual refractive index profile in these applications is the use of weakly conductive materials. The weakly conductive layers are able to spread the voltage profile which is applied through well-conductive electrodes at the side of the weakly conductive layer. The simulation and design of such structures is not trivial because two or three dimensional quasi-static electric field profiles need to be calculated. This is due to the fact that the resistivity of the conductive layers and the dielectric properties of the liquid crystal are coupled. An exact solution requires solving a number of coupled differential equations. In this paper, we develop a model to simulate the RC-effects with an approximate model

Patterned photo-alignment and surface topography for chiral liquid crystal superstructures with unique electro-optic properties

The combination of long-pitch and short-pitch chiral liquid crystals (CLCs) with patterned surface anchoring or surface topography is investigated with the aim to develop new electro-optic components. Highly efficient large-angle 1D diffraction gratings and metastable 2D gratings with hysteresis switching are demonstrated as well as electro-optic components with a uniform lying helix-like structure at intermediate voltages

Fringe-field-induced out-of-plane reorientation in vertically aligned nematic spatial light modulators and its effect on light diffraction

Liquid crystal (LC)-based spatial light modulators (SLMs) have the ability to shape the wavefront of a light beam and are widely used in applications where phase or amplitude modulation is required. In this work we study the LC director configuration in vertically aligned nematic (VAN) SLMs, with a focus on 1D binary gratings with different driving voltages. By comparing experimental microscopy measurements with simulations, we demonstrate that the director can rotate out of the plane determined by the pretilt of the SLM. By twisting out of the pretilt plane, the formation of a reverse tilt zone in the LC director configuration is avoided. The twist effect is asymmetric and only occurs at the edges where the fringe-field of the high voltage pixel is inclined in the same direction as the pretilt. Due to the out-of-plane reorientation of the director at one side of the pixel, binary gratings show strongly asymmetric diffraction in the pretilt plane. The out-of-plane reorientation also induces changes in the polarisation state of the light beam and the competition between two different out-of-plane reorientation directions may lead to slow switching. It is therefore of utmost importance to consider this effect when using VAN SLMs in applications