Large, Tunable Liquid Crystal Pretilt Achieved by Enhanced Out-of-Plane Reorientation of Azodye Thin Films

Abstract

In-plane, or azimuthal, photo-reorientation of azodye films using polarized exposure makes them promising alignment layers for a host of liquid crystal (LC) applications beyond displays including beam steering, q-plates, liquid crystal elastomer origami, and control of active matter. Out-of-plane, or polar, reorientation of azodye films, which dictates the liquid crystal pretilt, has received far less attention. Spatial control over the full polar and azimuthal orientation enables the generation of complex patterns that have broad interests and applications. In this paper, we describe an enhanced out-of-plane reorientation in Brilliant Yellow films utilizing a two-step exposure and demonstrate a liquid crystal pretilt angle that is tunable over a range of 0–33° with the associated anchoring strength of the alignment layer being unaffected by the inclusion of a pretilt. We report an order of magnitude increase in both amplitude and tunability of the pretilt angle in terms of previous results for single photoalignment films. This is a significant result for liquid crystal applications because it offers a simple, scalable, single-component solution with the potential to provide three-dimensional (3-D) patternability of the LC director at the surface.

Spatial Photo‐Patterning of Nematic Liquid Crystal Pretilt and its Application in Fabricating Flat Gradient‐Index Lenses

“…The collapse of the data sets in Figure 1d reveals that for this range of fluxes, the variation of β depends only on the energy dose received. [ 28 ] Notably, the transmitted intensities in Figure 1c also follow the expected variation with θ, implying that the azimuthal alignment of $$\hat{n}$$ is preserved as β varies with the exposure to unpolarized light.…”

“…Varying the incident angle of unpolarized light with respect to the sample plane of an LC cell, photosensitive azo‐dye Brilliant Yellow (BY) (see Supporting Information for chemical structure) has been shown to re‐orient out of plane, generating an exposure‐dependent pretilt in $$\hat{n}(\mathbf {r})$$ over a limited range of angles. [ 28 ] The out‐of‐plane alignment arises from the azo‐molecules undergoing trans – cis ‐ trans isomerization cycles, ultimately acquiring an out‐of‐plane angle as they tend to align parallel to the light propagation direction. Filling the sample with LC broadens the range of achievable pretilt angles.…”

Synthesis and Study of Copolymers Bearing Two Coumarin Side Groups in Monomer Unit: Toward Photoalignment of Liquid Crystals

“…[46,47] Even though the photoalignment technique is getting increasingly closer to practical implementation, there is still a need for developing new efficient photoalignment materials to address the challenges remained, which are to obtain a uniform and controllable alignment of LCs with favorable pretilt angle, [50,51] sufficient anchoring energy as well as excellent resistance to heat and light. [52] Generally, polymers with photosensitive species incorporated into their structures are used for the photoalignment surface layer. There are mainly three groups of photosensitive species that have been investigated for photoalignment surface layers of LCs, which undergo either photoisomerization [53][54][55][56] (e.g., azobenzene) or photodimerization [35,57,58] (e.g., coumarin, cinnamate) or photodegradation (e.g., polyimides).…”

“…[ 46,47 ] Even though the photoalignment technique is getting increasingly closer to practical implementation, there is still a need for developing new efficient photoalignment materials to address the challenges remained, which are to obtain a uniform and controllable alignment of LCs with favorable pretilt angle, [ 50,51 ] sufficient anchoring energy as well as excellent resistance to heat and light. [ 52 ]…”