Periodic Planar‐Homeotropic Anchoring Realized by Photoalignment for Stabilization of Chiral Superstructures

Abstract

polar and azimuthal anchoring at an interface allows us to obtain periodic stripes with planar and homeotropic anchoring with two-beam UV interference. The method is based on a mixture of homeotropic alignment material and azo-based photoalignment material illuminated with a periodic pattern of linearly polarized UV light. To the best of our knowledge, this is the first demonstration of controlling azimuthal versus polar anchoring by a maskless UV interference illumination method. In the past, reversible switching between uniform homeotropic and planar alignment was demonstrated by using cis-trans photoisomerization and patterned homeotropic and planar alignment was achieved by depositing self-assembled monolayers of molecules. [4] Chen et al. also established a photoalignment technique that makes use of a mixture of nanoparticles and azo-dyes in the LC. [5] In order to stabilize a lying helix structure, different methods were used to obtain a similar periodic anchoring: a polyimide layer with planar alignment was coated with a chromium complex surfactant for homeotropic anchoring and patterned by hard-contact photolithography. [3a] Alternatively, a mixture with photoreactive cholesteric monomer was spincoated on top of an alignment layer with large pretilt and photopolymerized. [3b] The photoalignment technique that we propose here is mask-less, allows high resolution and is based on the deposition of a single layer and therefore excels in versatility and ease of realization compared to previously reported methods. We expect that a further optimization of the technique can lead to full control of the azimuthal and polar alignment properties on a micrometer scale. This is not only interesting for the aforementioned applications but has very promising applications in geometrical phase optics and can lead to improved control over the mechanical response of liquid crystal elastomers. [6] Moreover, Qian and Sheng found that spatially varying alignment can lead to multiple-degenerate orientational states in the bulk and that the effective anchoring strength can be tuned by varying the alignment texturing. [7] Additionally, the rewritability of photoalignment materials might allow an in situ optical manipulation and reorientation of the LC structures. In combination with stimuli-responsive LCs this may lead to a wealth of new possibilities. Both light stimuli, thermal stimuli and electrical stimuli have proven to be very promising for different applications. [2] CLC in bulk spontaneously forms periodic helical structures and the distance over which the molecular director rotates Self-organization of chiral superstructures in liquid crystal (LC) materials is a powerful tool in developing functional devices based on soft matter. The control and manipulation of the helical axis orientation in chiral nematic liquid crystals are currently attracting significant attention, but are hindered by limited control over the anchoring at the interfaces. The development of new alignment techniques with local control over polar ...

Electrically turning periodic structures in cholesteric layer with conical–planar boundary conditions

“…For instance, aligned bubbles can be obtained using laser radiation 30 or substrates with a specific profile 31 , 32 . Such substrates contain orienting coating, which forms periodically located bands with homeotropic or planar CLC anchoring enforcing the formation of strip structure 33 , 34 . Preparation of the periodic planar anchoring on the substrate with the period similar to the equilibrium helix pitch results in the uniformly twisted structure, whose helicoid axis deviates from substrate plane by an intermediate angle between 0 and 90 35 , 36 .…”

Nematic and Cholesteric Liquid Crystal Structures in Cells with Tangential-Conical Boundary Conditions

“…CLCs can be used in various applications such as electro-optical devices with memory effect [2,3], quantum generation of light [4,5], switchable diffraction gratings [6][7][8][9][10], optical rotators [11], the formation of colloidal systems with a periodic distribution of particles [12], etc. The applications require various stable and metastable orientational structures depending on the boundary conditions, LC material parameters, ratio between LC layer thickness d and cholesteric pitch p, external factors [1,[13][14][15][16]. For example, the homeotropic director orientation is formed in the cells at normal CLC anchoring with substrates and confinement ratio d/p < K 33 /2K 22 , where K 22 , K 33 are the twist and bend elastic constants, respectively [17][18][19].…”

Voltage-controlled formation of short pitch chiral liquid crystal structures based on high-resolution surface topography

“…Photo-alignment makes use of (polarized) illumination to obtain a preferential LC orientation. Different illumination techniques (direct write, holography, plasmonic patterning) were optimized in recent years to obtain high-resolution, high-throughput and repeatable patterns with a low cost [8][9][10][11][12][13][14]. Photo-alignment is a very versatile method, but the spatial resolution is limited and the period of the alignment pattern should be above several hundred nanometer.…”