Observation of symmetry breaking in photoalignment-induced periodic 3D LC structures

Nersesyan, Varsenik; Nys, Inge; Acker, Frederik Van; Wang, Chun-Ta; Beeckman, Jeroen; Neyts, Kristiaan

doi:10.1016/j.molliq.2020.112864

Cited by 11 publications

(15 citation statements)

References 34 publications

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“…A similar alignment configuration has been investigated before, but only for crossed assembly of both substrates (ψ = 90°) [ 22 , 23 , 24 , 25 , 26 , 27 ] or anti-parallel assembly (ψ = 180°) [ 28 , 29 ]. Somewhat different from the other alignment configurations, ψ = 180° gives rise to a 1D configuration.…”

Section: Introductionmentioning

confidence: 99%

Photoaligned Liquid Crystal Devices with Switchable Hexagonal Diffraction Patterns

2022

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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.

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Section: Introductionmentioning

confidence: 99%

Photoaligned Liquid Crystal Devices with Switchable Hexagonal Diffraction Patterns

2022

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“…In a standard LC cell, the top and bottom substrate can easily be patterned with different (periodic or non-periodic) alignment patterns. This ability was exploited in recent years to create an artificial web of disclination lines (in non-equilibrium conditions) [21], and to stabilize 2D diffractive structures with intricate bulk director configurations demonstrating symmetry breaking and regions with pronounced non-planar director orientation in the bulk [22][23][24][25]. In general, patterned planar photo-alignment offers valuable possibilities to study the formation of (twist) disclination lines, their interaction and their annihilation when the alignment patterns are well designed [21][22][23][24][25][26][27][28][29][30].…”

Section: Introductionmentioning

confidence: 99%

“…This ability was exploited in recent years to create an artificial web of disclination lines (in non-equilibrium conditions) [21], and to stabilize 2D diffractive structures with intricate bulk director configurations demonstrating symmetry breaking and regions with pronounced non-planar director orientation in the bulk [22][23][24][25]. In general, patterned planar photo-alignment offers valuable possibilities to study the formation of (twist) disclination lines, their interaction and their annihilation when the alignment patterns are well designed [21][22][23][24][25][26][27][28][29][30]. When a different azimuthal angle is imposed at corresponding locations at the top and bottom substrate, a position dependent twist in the bulk can occur.…”

Section: Introductionmentioning

confidence: 99%

“…When a different azimuthal angle is imposed at corresponding locations at the top and bottom substrate, a position dependent twist in the bulk can occur. However, for some combinations of alignment patterns at the substrates, the topology at the interfaces requires either the creation of disclinations in the bulk or regions with vertical director orientation [15,[23][24][25][26][27][28][29][30]. We recently demonstrated for different alignment configurations that surface induced twist conflicts are often elegantly resolved, without the need for singular disclination lines, by introducing a region with vertical director orientation in the bulk of the device [15,[23][24][25].…”

Section: Introductionmentioning

confidence: 99%

“…However, for some combinations of alignment patterns at the substrates, the topology at the interfaces requires either the creation of disclinations in the bulk or regions with vertical director orientation [15,[23][24][25][26][27][28][29][30]. We recently demonstrated for different alignment configurations that surface induced twist conflicts are often elegantly resolved, without the need for singular disclination lines, by introducing a region with vertical director orientation in the bulk of the device [15,[23][24][25]. Honma et al described the same phenomenon as the formation of so-called "thick disclinations" (upon combination of two thin disclinations, without the presence of singularities in the final configuration) and Ouchi et al discussed this phenomenon as the stabilization of nonsingular wall loops [26,28].…”

Section: Introductionmentioning

confidence: 99%

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Surface Stabilized Topological Solitons in Nematic Liquid Crystals

2020

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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.

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Photoaligned Chiral Liquid Crystal Grating with Hysteresis Switching

Nys

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Neyts

2022

Advanced Optical Materials

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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...

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Observation of symmetry breaking in photoalignment-induced periodic 3D LC structures

Cited by 11 publications

References 34 publications

Photoaligned Liquid Crystal Devices with Switchable Hexagonal Diffraction Patterns

Photoaligned Liquid Crystal Devices with Switchable Hexagonal Diffraction Patterns

Surface Stabilized Topological Solitons in Nematic Liquid Crystals

Photoaligned Chiral Liquid Crystal Grating with Hysteresis Switching

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