This PDF file contains the front matter associated with SPIE Proceedings Volume 11092, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Many published papers bear flaw information caused by misinterpreting the pseudo-dielectric relaxation in liquid crystal (LC) confined in thin cells. The pseudo-dielectric relaxation stems from the cell geometries or parameters, which should not be misrecognized as an inherent material property of the LC. In this work, we explored the pseudo-dielectric relaxation and demonstrated an unconventional tuning means to electrically manipulate the reflective color of a typical cholesteric LC cell based on the heating due to the pseudo-dielectric relaxation. A tuning range in the full-color spectrum can be easily achieved by a limited temperature change of merely a few degrees Celsius.

Volume Bragg gratings (VBGs) have many applications including filters, wavelength multiplexing devices, and seethrough displays. As a kind of VBGs, polarization volume gratings (PVGs) based on liquid crystal polymer show the advantages of nearly 100% efficiency, large deflection angle and unique polarization selectivity. Previous studies of transmissive and reflective PVGs are based on a planar architecture. In this work, we introduce slanted configuration, i.e. slanted cholesteric liquid crystals. The optical properties of these two different-type PVGs are investigated and compared. Specifically, we emphasize on the diffraction efficiency and polarization state of the diffracted light. Through comparing the experimental results with simulations, the existence of slanted PVGs is validated. We further report a stretchable, flexible, and rollable PVG film with high diffraction efficiency. Previously reported PVGs are of high diffraction efficiency but with fixed diffraction angles. By transferring PVGs onto a flexible and stretchable substrate, the obtained PVG films exhibit high diffraction efficiency, tunable periodicity, and excellent flexibility. The PVG films offer tunable diffraction angles and Bragg reflection bands by mechanical stretching. Stretch-release cycles test is also performed to ensure the mechanical robustness and reliability. This PVG film is especially useful for laser beam steering and augmented reality (AR) waveguide coupler.

Topological defects arise when the symmetry of order in liquid crystal is broken during phase transitions or under specified boundary conditions. Fascinating applications, such as, optical vortex and optical memories, will be realized only if the type and position of defects can be precisely controlled. Topological defects were generated in nematic liquid crystal cell with homeotropic surface alignment when voltage is applied. The defects were precisely placed on designed positions using electric field. The topological charge and shape of the defect were assigned by the delicately designed pattern of the pixelated electrodes. The point defects with topological charges of +1 (hedgehog) and -1 (hyperbolic hedgehog) were arranged in square or hexagonal lattices as two dimensional crystals. Topological rules in the stable defect array were discovered. The topological charge of a bulk defect was equal to the Euler number of the pixel in twodimensional space. The number of the hyperbolic hedgehogs was determined by the folds of rotational symmetry of the defect array. The transmission and scattering of light through the defect were characterized, and the visual effect of large array of defects were demonstrated. Scattering, diffraction, and lensing effects from the defect array renders hazy, watery, and glittering images, which is artistic like impressionist paintings.

In this paper, we aim to show that liquid crystal films (LCs) with well-defined molecular orientations are an exceptional platform for flat optical devices based on the Pancharatnam-Berry (PB) phase. Especially, the development of plasmonic photopatterning technique in recent years has made it easy to align liquid crystal molecules in to designer orientation patterns with both high spatial resolution and high throughput and thus enables large scale manufacturing liquid crystal optical devices with low costs. Here we present liquid crystal laser beam shapers and microlenses as two examples to illustrate the design principles and the fabrication processes for liquid crystal flat optical elements. In comparison with flat optical devices made of plasmonic or dielectric metasurfaces, liquid crystal flat optical elements are advantageous due to the high optical efficiencies and low fabrication costs.

Active nematics are out-of-equilibrium liquid crystal fluids composed of rod-like subunits, which can generate large-scale, self-driven flows. In this emerging field of active nematics, new methods are needed to investigate and potentially control phase structure and dynamics. The use of complex engineered surfaces using microfabrication is an excellent way to control local orientation directors, taking advantage of the interplay between surface curvatures and topological defects. Epoxy-based lithography represents a simple and appealing approach, using low cost, minimal materials and a time efficient process. In this manuscript, we discuss methods for optimized fabrication protocols using negative and positive tone epoxy-based photoresists to create microfluidic devices for active matter. Arrays of curved objects and submerged topographies can be used to generate a variety of liquid crystal defect configurations not typically observed on unconfined planar surfaces.

Planar, anisotropic liquid crystal (LC) optics, along with metasurfaces, have shown to be the predominant meth- ods of producing a geometric (or Pancharatnam-Berry) phase hologram (GPH). One of the simplest GPHs, the traditional continuous polarization grating (PG), implements a continuous linear phase ramp. This PG has received significant attention due to its polarization-selective nature and 100% diffraction efficiency. However, when this linear phase is sampled with a 0-π alternating phase profile, theoretical reasoning predicts polarization- independent qualities. In order to distinguish this grating from continuous PGs, we call this a binary polarization grating (bin-PG). Traditional PGs, with a continuously varying nematic director profile, are simple to manufacture with many holographic methods. However, no bin-PG fabricated with patterned LCs have yet been reported. In this work, we experimentally study bin-PGs formed using a photo-aligned LC polymer network. Particular attention is brought to the problematic rotational ambiguity of LC at the phase step. To prevent disclination lines, a rotation biasing pixel of varying size is implemented at the phase transition boundary. We measure the diffraction efficiencies, the input polarization response, and the impact of the non-zero transition region. At the smallest transition pixel size (0.625μm) an average +1-order efficiency of 36% was measured with an input-polarization sensitivity of only ±1.7%.

Liquid crystal elasticity forms distortions and topological defects that allow for the controlled self-assembly of inclusions within liquid crystal media. Many groups have developed this idea for dispersion, alignment, and manipulation of a collection of individual particles to create plasmonic polarizers and others have studied colloidal crystallization processes within LC hosts. Extending this idea to consider more complex superstructures where a variety of forces can produce effects on larger lengthscales can expand the space of materials design and answer first principles questions about complex fluid dynamics. I will discuss results on colloidal inclusions at liquid-liquid and liquid- air interfaces where surface tension and capillary forces augment elasticity to provide the necessary tools for such hierarchical self-assembly.

Small, surface functionalized, Fe-doped LiNbO₃ (FeLN) particles were experimentally investigated. Planar thin-cuts (0.5 – 1 mm thick) of FeLN crystals have recently been investigated as field generating substrates in LC test devices. Here, realignment of the liquid crystal (LC) director can be locally driven with photovoltaic fields, generated by the substrates (surface-fields, generated via the bulk photovoltaic effect). Such samples can be exposed with tightly focused light beams and show unexpectedly high non-linear optical responses, light induced defect formation; some samples could even reversibly store patters inscribed by the use of light. Here, it is investigated how the use of field-generating crystals be pushed further. Small particles are still suitable to control LC realignments – at least to a certain amount. Small particles of Fe-doped LiNbO₃ and for reference also BaTiO₃ were integrated in test devices. The particles were prepared in a top-down approach, that is larger specimen of ferroelectric crystals were first crushed with mortar and pestle to obtain a powder. From these powders, dispersions of particles were fabricated by high energy ball-milling. A small amount of the powder together with an organic solvent (continuum medium) was placed in the milling jars of a planetary ball mill. A small amount of a surfactant was added – both use of a conventional surfactant and in-house synthesized, photochemically active species was investigated. The dispersions obtained were used to coat the surfaces of LC test cells and for doping of polymer films. The samples obtained show many interesting and sometimes unexpected properties.

We present photonic devices based on a nematic liquid crystals (NLC) infiltrated in polydimethylsiloxane (PDMS) channels, named LC:PDMS waveguides, for flexible photonic integrated circuits. A simulation study of the NLC ordering and possible defects under an electric field between coplanar gold electrodes has been carried out by a Monte Carlo approach. The minimization of the free energy in the NLC core waveguides computed by means of finite elements allows to derive the refractive index profile, which is implemented in a BPM algorithm to design a large variety of switchable and tuneable photonic devices. In particular we report the design of two photonic devices based on LC:PDMS waveguide technology: a 2x2 optical switch made of a zero-gap electro-optical controlled directional coupler and a multimodal interferometer (MMI) acting as an optical multi/demultiplexer. The electro-optical controlled directional coupler is able to switch light from one waveguide to a second one with an extinction ratio of 16 dB by applying a voltage of just 1.62 V to coplanar gold electrodes deposited on PDMS by electroplating technique. Light remains in the same waveguide with an extinction ratio of about 18 dB with a voltage of 1.76 V. An MMI has been also designed to demultiplex wavelengths at 980 nm and 1550 nm in two output waveguides with an extinction ratio better than 11 dB by applying about 7 V. Main advantage of these devices is the low driving voltage due to the combination of electro-optic effect of the NLC with an optimized design.

Blue phase liquid crystals (BPLC) are self-assembled 3D photonic crystals with high dynamic tunability. We review our recent progress on dynamic tuning of BPLC’s photonic band gap, crystal symmetry and orientation by electric field and light. With negative dielectric anisotropy, electrically induced switching between [110]- and [200]-oriented BPI states can be achieved through a transient flow state induced by electro-hydrodynamic instabilities. In dielectrically positive polymer-stabilized BPLCs, applying a DC electric field will induce a stretch of the lattice along the field direction, resulting in a bandgap red-shift of over 200 nm. Such a field-induced stretch can also induce chirped lattice spacing along the field axis in samples fabricated under a different photopolymerization condition, leading to an effective expansion of the photonic bandgap. With chiral azo molecular switch doped in polymer-free BPLCs, reversible lattice dilation and phase transition from simple cubic to body-centered cubic symmetry can be induced by light. These techniques bring about many new possibilities of blue-phase photonic crystals for photonic applications.