Inge Nys

Liquid crystals (LCs) require precise alignment for advanced photonic applications, with photoalignment offering a powerful route for complex spatial patterning. While recent studies demonstrate that Brilliant Yellow (BY) photoalignment layers can induce out‐of‐plane (homeotropic) anchoring via a two‐step illumination process, these approaches typically necessitate thick BY films that limit device transparency. In this work, we present an optimized two‐step laser illumination technique utilizing a spatial light modulator (SLM) and local exposure to circularly polarized laser light to precisely pattern alternating planar and homeotropic domains. Crucially, we introduce the application of an AC electric field across the LC layer during the second illumination step. This field reorients the LC, strongly minimizing the phase retardation of the propagating laser beam and significantly reducing the required exposure dose to induce homeotropic alignment. Utilizing this technique, we relax the need for thick films, successfully demonstrating versatile micro‐patterns–such as gratings and checkerboards–using a highly transparent alignment layer. Furthermore, we demonstrate that this approach enables improved in‐plane rewritability of the planar aligned regions. Ultimately, this methodology provides a highly controllable and transparent platform for developing next‐generation spatially patterned LC devices.

Inkjet-printed droplets on photoaligned substrate is analyzed via polarization optical microscope, a zoomed-in view shows a stable, grating-guided fingerprint textures.

The recently discovered ferroelectric nematic liquid crystals, combining fluidity with spontaneous polarization, offer unique opportunities for engineering polar domain structures for (nonlinear) photonic applications. A central challenge lies in achieving precise control over the self‐assembly of their polar order in confined geometries. This control is demonstrated through photoalignment patterning, which creates periodic arrays of half‐integer twist disclination lines. In RM734, the nematic to ferroelectric nematic (N _F ) phase transition induces spontaneous structural reconfiguration with disclination line displacement. Because of the broken inversion symmetry in the N _F phase, the resulting half‐integer disclinations template periodic localized polarization reversal mediated through surface‐pinned defects or polarization reversal walls. Polarized optical microscopy combined with optical simulations reveals the self‐assembled bulk director configuration in both phases. Contrasting experiments with and without ionic dopants emphasize the important role of electrostatic self‐energy in determining the self‐assembled configuration. Different N _F materials and geometric parameters influence structural formation, leading to diverse structures including polar 2‐twist disclination lines separating domains with opposite twist handedness. This research provides new insights into ferroelectric nematic self‐assembly for next‐generation (nonlinear) photonic devices.

A new LC phase, viz, ferroelectric nematic (NF) phase, has recently been realized experimentally in RM734 material, following the theoretical prediction in 1910 by Peter Debye and Max Born. Due to the breaking of inversion symmetry, the NF phase of RM734 exhibits polarization in the order of μC/cm2 , almost 1000 times greater than that of normal SmC*-phase-based ferroelectric liquid crystals (FLCs), representing a potentially significant discovery from both basic and technological perspectives. The polarized optical micrograph of the RM734 material in the NF phase reflects a chevron-modulated ferroelectric nematic state, where elastic anisotropy and confinement led to the formation of periodic chevron defects. The electro-optical study revealed that the response times of the RM734 material in the NF and nematic phases have been found to be ≈ 0.5 ms and 100 μs, respectively. The thermal parameters, including thermal diffusivity, thermal effusivity, and thermal conductivity, have been ascertained utilizing the photopyroelectric technique. The measured thermal diffusivity, thermal effusivity, and thermal conductivity at 160 °C were α _m = 2.28±0.54 × 10 ^-8 m ² /s, e _m =632±70 Ws ^1/2 m ^-2 K ^-1 , and κ _m =0.095± 0.022 Wm ^-1 K ^-1 , indicating relatively low values rendering in a weak thermal transport in the nematic phase. Moreover, in the NF phase, these thermal parameters are diminished by 10-30 %, resulting in significantly low thermal conductivity in the NF phase. The NF material demonstrated a significant potential as a futuristic material for quantum electronics due to its high polarization and low thermal conductivity.

With nematic liquid crystal (LC) topological defects can be created that are important for the generation of laser beams with orbital angular momentum. Arrays of defects have been realized through photoalignment, by projecting images of a spatial light modulator or digital mirror device. However, such pixel‐based approaches seriously limit the achievable density of defects. Here, we propose a scalable method based on two‐step interference illumination to achieve a much higher density. In the first step, a rotating director pattern is generated by the interference of two circularly polarized beams. In the second step, amplitude modulation by two‐beam interference is used to rewrite the pattern and introduce the defects. By adjusting the illumination doses and angles of incidence in both steps, 2D defect patterns are obtained, with spacing down to 1.25 micrometer. The resulting disclination lines are analyzed, and the proposed structure is supported by numerical simulations. For the first time, a high defect density defect grid is obtained through a scalable two‐step photoalignment procedure. This approach enables optical components with large‐angle diffraction and bridges the gap between LC topological optics and metasurfaces.

The growing demand for holography and its wide-ranging applications necessitate advancements to make this technology more accessible and adaptable to modern technological requirements. Nematic liquid crystals (NLCs) enable holographic projection by exploiting the geometric phase of light through carefully designed phase patterns. However, NLC-based holography faces challenges, particularly in terms of projection quality and the presence of zero-order leakage—a bright spot that overlaps the reconstructed image. The efficiency of the reconstructed image is strictly related to the thickness of the NLC layer, which is challenging to control with high precision. In this work, we use chiral liquid crystal (CLC) to fabricate reflective holograms. To address the issue of leakage waves overlapping the reconstructed image, we propose an interference-based fabrication approach. This approach combines the phase pattern of the hologram with a diffraction grating by interfering a plane wave with a tailored wavefront precisely matching the hologram’s phase pattern. As a result, the image is reconstructed away from the zero-order reflection, and the leakage overlapping with the reconstructed image is reduced significantly.

Multilayer liquid crystal devices can offer enhanced optical functionalities for augmented reality and photonic applications, but fabrication remains severely limited by solvent incompatibility between photoalignment materials and underlying polymerized layers. Conventional photoalignment agents use aggressive solvents like N,N-dimethylformamide that damage polymerized substrates, necessitating protective interlayers. This study demonstrates a water-soluble photoalignment approach using AbA-2522 that eliminates these fabrication barriers. The water-soluble alignment material enables direct multilayer processing without layer damage while maintaining alignment quality equivalent to conventional materials. We successfully fabricate compact transmissive devices integrating liquid crystal polarization gratings with quarter-wave plates, achieving a first-order diffraction efficiency of 65.4% for 9 μm period gratings for linearly polarized incident light (λ = 457 nm). The multilayer structure exhibits highly selective polarization-dependent diffraction with efficiency ratios exceeding 10:1 between preferred and suppressed orders, eliminating external polarization control elements. Polarized optical microscopy confirms excellent alignment uniformity, while the fabrication process offers environmental benefits and reduced complexity. This approach establishes a practical pathway for advanced multilayer photonic devices critical for next-generation augmented reality systems and photonic integration, addressing fundamental challenges that have limited multilayer liquid crystal device development.

In liquid crystal (LC) cells, the surface patterning directs the self‐assembly of the uniaxial building blocks in the bulk, enabling the design of stimuli‐response optical devices with various functionalities. The combination of different anchoring patterns at both substrates can lead to surface induced frustration, preventing a purely planar and defect‐free configuration. In cells with crossed assembly of rotating anchoring patterns, elastic deformations allow to obtain a defect‐free bulk configuration, but an electrical stimulus can induce disclination lines. The disclination network is preserved without applied voltage. Depending on the electric field treatment and geometrical parameters, different multi‐stable states with and without disclinations are obtained. This is demonstrated with the help of dual‐frequency LCs, for which the frequency dependent dielectric properties allow repeatable switching between multi‐stable states. Topological protection and the associated energy barrier between different states explains the observed metastability. The obtained configurations are retrieved with Q‐tensor simulations and the validity is confirmed by matching optical simulations with experimentally obtained microscopy images. The realized multi‐stable topological states interact differently with light, resulting in distinct optical properties. Optimization allows to switch between a highly transparent state and an opaque state, opening up opportunities for smart windows with low energy consumption.

Blue phase liquid crystals (BPLCs) are chiral self-assembled three-dimensional (3D) periodic structures which have attracted a lot of attention due to their electro-optical properties, relevant for tunable soft photonic crystals and fast-response displays. However, to realize this application potential, controlling the BPLC alignment at the surfaces is crucial, and one way to obtain the desired alignment is by photoalignment patterning. In this article, monodomain BPLC samples with controlled orientation are achieved by imposing different alignment patterns that have a periodicity that is compatible with the size of the BPLC unit cell, using two-step photoalignment with polarized ultraviolet (UV) light. Experiments are complemented by numerical simulations to design striped surface alignment patterns, which induce specific director orientations on the boundary layer of the confined BPLC. By designing the patterns and matching the periodicity to a specific BP material, we can control the orientation of the blue phase unit cell lattice in the sample, including the azimuthal angle. The orientation is measured by the Kossel patterns and matches the optimal configuration predicted by stability analysis using Landau-de Gennes free energy modeling. The detailed structure and reduced symmetry of the BP near the surface are investigated, and the corresponding (meta)stable structures are demonstrated. Overall, we demonstrate that two-step photoalignment patterning is a reliable, relatively simple, and reconfigurable method to achieve a high-quality monodomain BP with controlled and tunable crystalline orientation.

A novel design and approach for the realization of liquid crystal optical waveguides using photopatterning as a manufacturing method is presented. The developed method is used to demonstrate straight waveguides of different widths and curved waveguides with different radius of curvature. The waveguides are terminated with +1 or −1 surface defects through which the light incouples/outcouples. The effect of the straight waveguide thickness and the radius of curvature of the curved waveguides on the attenuation of the light in the waveguide is experimentally explored. The experiments are supported with numerical simulations of light propagation through both types of waveguides and outcoupling out of the straight waveguides. Furthermore, the effect of the size of the −1 defect terminations on the outcoupling efficiency is explored using simulations. Based on the presented design and manufacturing approach, liquid crystal optical waveguides of various shapes can be realized accurately with low‐cost.