Flexible glass substrates with via holes for TFT backplanes

This paper looks at flexible glass substrates with via holes for TFT backplane

Fabrication and Actuation of Hierarchically-Patterned Polymer Substrates for Dynamic Surface and Optical Properties

Switchable optical materials, which possess reversible color and transparency change in response to external stimuli, are of wide interest for potential applications such as windows and skylights in architectural and vehicular settings or optical sensors for environmental monitoring. This thesis considers the tuning of optical properties by tailoring and actuating responsive materials. Specifically, we demonstrate the design and fabrication of tilted pillar arrays on wrinkled elastomeric polydimethylsiloxane (PDMS) as a reversibly switchable optical window. While the original PDMS film exhibits angle-dependent colorful reflection due to Bragg diffraction of light from the periodic pillar array, the tilted pillar film appears opaque due to random scattering. Upon re-stretching the film to the original pre-strain, the grating color is restored due to the straightened pillars and transmittance is recovered. Then, we develop a composite film, consisting of a thin layer of quasi-amorphous array of silica nanoparticles (NPs) embedded in bulk elastomeric PDMS, with initial high transparency and angle-independent coloring upon mechanical stretching. The color can be tuned by the silica NP size. The switch between transparency and colored states could be reversibly cycled at least 1000 times without losing the film’s structural and optical integrity. We then consider the micropatterning of nematic liquid crystal elastomers (NLCEs) as micro-actuator materials. Planar surface anchoring of liquid crystal (LC) monomers is achieved with a poly(2-hydroxyethyl methacrylate)-coated PDMS mold, leading to monodomains of vertically aligned LC monomers within the mold. After cross-linking, the resulting NLCE micropillars show a relatively large radial strain when heated above nematic to isotropic transition temperature, which can be recovered upon cooling. Finally, the understanding of liquid crystal surface anchoring under confined boundary conditions is applied to the self-assembly of gold nanorods (AuNRs) driven by LC defect structures and to dynamically tune the surface plasmon resonance (SPR) properties. By exploiting the confinement of the smectic liquid crystal, 4-octyl-4’-cyanobiphenyl (8CB), to patterned pillars treated with homeotropic surface anchoring, topological defects are formed at precise locations around each pillar and can be tuned by varying the aspect ratio of the pillars and the temperature of the system. As a result, the AuNR assemblies and SPR properties can be altered reversibly by heating and cooling between smectic, nematic and isotropic phases

Fast-response Liquid Crystal Displays

After about five decades of extensive material research and device development, followed by massive investment in manufacturing technology, thin-film-transistor liquid-crystaldisplay (TFT-LCD) has finally become the dominant flat panel display technology. Nowadays, LCD performances, such as viewing angle, contrast ratio, and resolution, have reached acceptable levels. The remaining major technical challenges are response time, light efficiency, and sunlight readability. Fast response time is desired to reduce motion blur and to enable field sequential color displays using red (R), green (G), and blue (B) LEDs (light emitting diodes) without noticeable color breakup. Sequential RGB colors would eliminate the commonly used spatial color filters which in turn enhances light efficiency and resolution density by ~ 3X. In this dissertation, several new approaches for achieving fast-response LCDs are explored. From material viewpoint, the most straightforward approach for achieving fast response time is to employ a thin cell gap with high birefringence and low viscosity liquid crystal (LC). We investigated the thin cell approach theoretically and experimentally. Voltage shielding effect and anchoring energy effect of alignment layers are found to play important roles on operating voltage and response time. Simulations are carried out to understand the underlying physics and confirm the experimental results quantitatively. Another approach to realize fast response time is to explore novel device configuration. Here, we proposed a dual fringing-field switching (DFFS) mode in which small LC domains are iv formed following the distribution of fringing fields. Therefore, it exhibits submillisecond response time without using thin cell or overdrive/undershoot voltage method. The response time of the DFFS mode is ~20X faster than a conventional vertical aligned LCD. In addition, high optical efficiency is achieved from the complementary top and bottom active LC domains. Two transmissive and one transflective LCDs using DFFS mode are conceived and their electrooptical properties investigated. A shortcoming of DFFS LCDs is their fabrication complexity. To keep the advantages of this fast-response mode while avoiding the requirement of double-TFTs and pixel registration, we modified the device structure to transflective LCD which uses a single TFT in each pixel and vertical aligned positive dielectric anisotropy LC. Two types of electrodes are considered: fringing-field switching (FFS) and in-plane switching (IPS). Besides fast response time and high transmittance, such a transflective LCD shows good sunlight readability. As nematic LC is gradually approaching to its limit in term of response time, polymerstabilized blue phase (PSBP) LCD is emerging. It has potential to become next-generation display because of following revolutionary features: submillisecond response time, no need for alignment layer, good dark state and symmetric viewing angle, and cell gap insensitivity if IPS electrode is employed. In this dissertation, we studied the material-property correlation of Kerr effect-induced birefringence in nano-structured PSBP LC composites. Furthermore, a new device configuration of BP LCD with corrugated electrodes is proposed to solve two critical technical issues: high driving voltage and relatively low transmittance. The on-state voltage can be reduced from ~35 Vrms to ~10 Vrms which will enable TFT addressing, and the transmittance is improved from ~65% to ~85%. This new device configuration will accelerate the emergence of v PSBP LCD. Wide view is another important requirement for a high-end display. Several new LCD configurations with negative A-plate and biaxial plate as phase compensation films are proposed to achieve wide view and broadband operation. The underlying working principles are studied and detailed display performances are included in this dissertation

Switchable focus using a polymeric lenticular microlens array and a polarization rotator

We demonstrate a flat polymeric lenticular microlens array using a mixture of rod-like diacrylate monomer and positive dielectric anisotropy nematic liquid crystal (LC). To create gradient refractive index profile in one microlens, we generate fringing fields from a planar top electrode and two striped bottom electrodes. After UV stabilization, the film is optically anisotropic and can stand alone. We then laminate this film on a 90 degrees twisted-nematic LC cell, which works as a dynamic polarization rotator. The static polymeric lenticular lens exhibits focusing effect only to the extraordinary ray, but no optical effect to the ordinary ray. Such an integrated lens system offers several advantages, such as low voltage, fast response time, and temperature insensitivity, and can be used for switchable 2D/3D displays

Grating Aligned Ferroelectric Liquid Crystal Devices

This thesis is concerned with the vertical grating alignment of ferroelectric liquid crystals (FLCs). FLCs exhibit fast electro-optic response times compared to traditional nematic devices, and so are of particular interest for use in micro-displays and liquid crystal on silicon (LCoS) spatial light modulators. Unfortunately such materials are highly susceptible to shock induced ow. This work introduces the VGA-FLC device geometry: a vertical grating aligned ferroelectric liquid crystal display. The vertical alignment gives preferential alignment to the smectic layers, and the amplitude and pitch of the grating ensure stable alignment of the c-director of the FLC. The combined effect is shown to result in a shock-stable FLC geometry. The device is addressed with in-plane electric fields, and is shown to obtain fast optical response times. The theory and physics of the device is explored, and further experiments are suggested that can be performed for device optimisation

Vloeibaar-kristaltoepassingen met in-het-vlak draaiende director

Vloeibare kristallen zijn een veel gebruikt materiaal in tal van optische toepassingen door een aantal unieke eigenschappen. Ze zijn optisch dubbelbrekend en kunnen door hun elektrisch anisotropie op eenvoudige wijze geschakeld worden met behulp van elektrische of magnetische velden. De meest bekende toepassing is het vloeibaar-kristalbeeldscherm of Liquid Crystal Display (LCD). Het onderzoek van Chris Desimpel is begonnen als een studie van de optische eigenschappen van LCD’s met als belangrijkste aandachtspunt de in-het-vlak draaiende of in-plane switching (ips) mode. In de ips-mode wordt de gemiddelde richting van de moleculen in het vloeibaar kristal gedraaid in een vlak parallel met de substraten. Doorheen het doctoraat komt zowel modellering, optica als technologie van vloeibare kristallen aan bod. Na een inleiding tot vloeibare kristallen wordt aandacht besteed aan verschillende optische algoritmes die gebruikt worden om de transmissie doorheen vloeibare kristallen te berekenen. Vervolgens gaat het doctoraat dieper in op de oppervlakteverankering van vloeibare kristallen, een belangrijk aspect bij toepassingen. Gebruik makend van de ips-mode is uiteindelijk een nieuw vloeibaar-kristalcomponent ontwikkeld met een aantal opvallende eigenschappen