Photoaligned Chiral Liquid Crystal Grating with Hysteresis Switching

Abstract

ubiquitous in nature (beetle exoskeletons, DNA, etc.) and show many intriguing material properties. CLC is a versatile soft matter system to study chiral superstructures and can be successfully applied in electro-optic devices including color filters, reflective displays, tunable lasers, beam steering devices, etc. [3a,6] The self-assembly of CLC in a periodic helical structure leads to the formation of a photonic bandgap for circularly polarized light and is often exploited in these devices.The presence of chirality in the LC material often gives rise to the existence of multiple metastable states, depending on the anchoring properties, the amount of chirality, and the cell pretreatment. [7,8] Even in cells with uniform planar anchoring, regions with a different number of chiral periods can coexist. In general, different cooling rates, electricfield treatments or optical excitations can give rise to another chiral bulk configuration. Textural transitions in the CLC state often give rise to hysteresis behavior in the electro-optic switching. In some cases this is unwanted and textural transitions are suppressed (e.g., by polymer stabilization) to obtain fast and hysteresis free optical switching. [7a] However, the formation of different metastable topological structures, relying on the competition between surface anchoring, elasticity, and chirality, is sometimes also actively sought for. It is well-known that confinement-unwound CLC systems offer an interesting playground for the formation of cholesteric fingers, bubbles and elastic quasiparticles. [8] Dynamic tuning and reconfiguring of these (metastable) topological states, by low-frequency or optical-frequency electric fields, has been demonstrated and light control by this soft-photonics platform was investigated. [9] The use of uniform alignment layers, as obtained by mechanical rubbing at the confining substrates, has been well studied but limits the number of director configurations that can be achieved. During the last decades a lot of attention was given to photoalignment techniques that allow spatially nonuniform alignment with high resolution and offer the flexibility to stabilize complex LC configurations. At the inner surface of the two confining substrates a photosensitive material can be deposited that adopts a preferential orientation, determined by the polarization direction of incident blue or UV light. Patterning of the alignment direction has been demonstrated with the help of interference illumination, direct-write illumination, plasmonic patterning, illumination structured by a spatial light modulator (SLM), etc. [10] Different alignment patterns at the top and bottom substrate can be combined, by assembling the cell after 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 introduc...

30‐1: Invited Paper: Chiral liquid crystal layers with patterned photoalignment for thin diffractive optical components

“…In the absence of a voltage, the liquid crystal self-organizes into a complex 3D director pattern, with regions where the director is perpendicular to the substrates. When a voltage is applied to this device, the 3D structure and the transmission diffraction pattern show a hysteresis behavior, with switching between a low voltage state and a high voltage state [12]. Figure 2 shows polarization microscopy images of the periodic structure at different voltages.…”

“…That means there is a hysteresis switching between two states. Both states have a continuous 3D director profile without disclination lines [12]. Reproduced from [12].…”

28.2: Chiral Liquid Crystal Flat Optics Devices for Optical Components and Augmented Reality Displays

“…The variation of the azimuthal angle can be differently at the top and bottom substrates, and in this way a CLC structure with a complex periodic pattern can be obtained. In this case we used a CLC with a larger intrinsic pitch (5 m) which showed a switching characteristic with a reliable hysteresis loop between a high voltage state and a low voltage state [6] als illustrated in Fig. 2.…”

Highly dispersive liquid crystal diffraction gratings with continuously varying periodicity

“…By enclosing LC material between substrates, it is possible to tune the director configuration in the enclosed LC material both via surface anchoring or via external stimuli such as electric fields. Tunable LC-based optical elements are in use for applications such as displays [1], filters [1,2], lenses [3,1,4], sensors [1,5], lasers [1,6], smart windows [7,8,9,10] and augmented reality glasses [11,12].…”