1,872 research outputs found

    Modelling nematohydrodynamics in liquid crystal devices

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    We formulate a lattice Boltzmann algorithm which solves the hydrodynamic equations of motion for nematic liquid crystals. The applicability of the approach is demonstrated by presenting results for two liquid crystal devices where flow has an important role to play in the switching.Comment: 6 pages including 5 figure

    Topology and Bistability in liquid crystal devices

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    We study nematic liquid crystal configurations in a prototype bistable device - the Post Aligned Bistable Nematic (PABN) cell. Working within the Oseen-Frank continuum model, we describe the liquid crystal configuration by a unit-vector field, in a model version of the PABN cell. Firstly, we identify four distinct topologies in this geometry. We explicitly construct trial configurations with these topologies which are used as initial conditions for a numerical solver, based on the finite-element method. The morphologies and energetics of the corresponding numerical solutions qualitatively agree with experimental observations and suggest a topological mechanism for bistability in the PABN cell geometry

    Holographic liquid crystal devices

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    Liquid crystals have become natural candidates for use in electro-optic devices for their ability to change the orientation of the director with the application of an electric field, and exhibiting large range of refractive index. The aim of the work presented in this thesis is to fabricate liquid crystal optoelectronic devices such as electrically switchable liquid crystal diffraction gratings and polarization rotators by exploiting the holographic surface relief effect in photopolymer and by developing novel polymer dispersed liquid crystals (PDLCs). Alignment of liquid crystals is commercially achieved by creating grooves on a conducting layer such as polyimide or indium tin oxide (ITO) by rubbing. This process has disadvantages such as creation of static electricity and dust which are undesirable. An attractive alternative technique to rubbing is investigated. A photopolymer layer coated on a conducting ITO layer on a glass plate has the grooves inscribed holographically in it. An acrylamide based dry photopolymer developed in the Centre for Industrial and Engineering Optics, Technological University Dublin is used in this study. The dependence of photoinduced surface relief on the holographic recording parameters, chemical composition of photopolymer and on physical parameters of the photopolymer layer were studied. A model explaining the mechanism of surface relief grating formation is proposed. Electrically switchable diffraction gratings and polarization rotators were fabricated by filling these grooves with liquid crystals using the capillary filling technique. In the second approach, holographic switchable diffraction gratings were fabricated using a novel PDLC, which was also developed in the Centre for Industrial and Engineering Optics. PDLCs consist of microscopic liquid crystalline droplets embedded in a polymer matrix. Preliminary results for the recording parameters and the physical parameters of the PDLC layer needed to fabricate gratings are presented. The redistribution of LCs was observed by using techniques such as phase contrast microscopy and Raman spectroscopy. The electrically switchable diffraction gratings were characterized using linearly polarized light by measuring the dependence of the intensity in the first diffracted order on the applied electric field. The polarization rotator was characterized by studying the influence of the applied electric field on the twist angle and the variation of intensity in the zero and the first orders of diffraction. The capabilities of the photoinduced surface relief effect in the photopolymer and of a newly developed PDLC material for the fabrication of liquid crystal devices are demonstrated

    Nematic liquid crystal devices with sub-millisecond response time

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    Conventional nematic liquid crystal devices exhibit switching times that are in the order of several milliseconds. In this work we focus on two types of nematic liquid crystals that can overcome the limitations of conventional nematic liquid crystals and allow sub-millisecond switching times for both switching on and off: nano-pore polymer-liquid crystals and dual-frequency liquid crystals

    Fast Response Liquid Crystal Devices

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    Liquid crystal (LC) has been widely used for displays, spatial light modulators, variable optical attenuators (VOAs) and other tunable photonic devices. The response time of these devices is mainly determined by the employed liquid crystal material. How to obtain fast response for the LC devices is a fundamentally important and technically challenging task. In this dissertation, we investigate several methods to improve liquid crystal response time, for examples, using dual-frequency liquid crystals, polymer stabilized liquid crystals, and sheared polymer network liquid crystals. We discover a new class of material, denoted as sheared polymer network liquid crystal (SPNLC) which exhibits a submillisecond response time. First, dual-frequency liquid crystals and polymer network methods are demonstrated as examples for the variable optical attenuators. Variable optical attenuator (VOA) is a key component in optical communications. Especially, the sheared PNLC VOA shows the best result; its dynamic range reaches 43 dB while the response time is in the submillisecond range at 1550 nm wavelength, which is 50 times faster than the commercial LC-based VOA. Second, we report a new device called axially-symmetric sheared polymer network liquid crystals (AS-SPNLC) and use it as LC devices. An axially-symmetric sheared polymer network liquid crystal has several attractive features: 1) it is polarization independent, 2) it has gradient phase change, and 3) its response time is fast. It can be used for polarization converter and divergent LC lens. In addition, a new method for simultaneously measuring the phase retardation and optic axis of a compensation film is demonstrated using an axially-symmetric sheared polymer network liquid crystal. This simple technique can be used for simultaneously measuring the optic axis and phase retardations of both A- and C-plates. These compensation films have been used extensively in wide-view LCD industry. Therefore, this method will make an important impact to the LCD industry

    Polarization-independent Liquid Crystal Devices

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    Liquid crystal (LC) devices can be operated as amplitude modulators and phase modulators. LC amplitude modulation is commonly used in liquid crystal display (LCD) while phase-only modulation is useful for laser beam steering, tunable grating, prism, lens, and other photonic devices. Most LC devices are polarization dependent and require at least one polarizer. As a result, the optical efficiency is low. To enhance display brightness, a power hungry backlight has to be used leading to a high power consumption and short battery life. In a LC phase modulator, the polarization dependent property complicates the laser beam steering system. It is highly desirable to develop new operating mechanisms that are independent of the incident light polarization. In this dissertation, we have developed eight polarization-independent liquid crystal operation principles: three of them are aimed for displays and the other five are for phase modulators. For amplitude modulations, a new polymer-dispersed liquid crystal (PDLC) and two new dye-doped LC gels are polarizer-free by combining light scattering with dye-absorption effects. In phase modulation, we explore five device concepts: PDLC and Polymer-Stabilized Cholesteric Texture (PSCT), homeotropic LC gels, thin polymer film separated double-layered structure, and double-layered LC gels. In the low voltage regime, both PDLC and PSCT have a strong light scattering. However, as the voltage exceeds a certain level, the phase modulation is scattering-free and is independent of polarization. The homeotropic LC gels do not require any biased voltage and the response time is still fast. Although the remaining phase in these devices is small, they are still useful for micro-photonic device applications. To increase the phase change, thin polymer film separated double-layered structure is a solution. The orthogonal arrangement of top and bottom LC directors results in polarization independence. However, the response time is slow. Similarly, double-layered LC gels are not only polarization independent but also fast response due to the established polymer network

    Importance of alignment layers in blue phase liquid crystal devices

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    In this paper we present how alignment layers affect Blue Phase Liquid Crystals and how we can use this effect to our advantage. We argue that contrary to the prevailing perception alignment layers can be of vital importance to blue phase liquid crystal based devices

    Liquid crystal devices in adaptive optics

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    Large aperture astronomical telescopes have a resolution that is limited by the effects of the Earth's atmosphere. The atmosphere causes incoming wavefronts to become aberrated, to correct for this adaptive optics is employed. This technique attempts to measure the incident wavefront and correct it, restoring the original image. Conventional techniques use mirrors that are deformed with piezo-electric crystals, this thesis uses an alternative technique. Two different types of liquid crystal spatial light modulators are used as the corrective elements. The advantages and disadvantages of both are assessed in an attempt to find which system is the best for astronomical adaptive optics

    Grating Aligned Ferroelectric Liquid Crystal Devices

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    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

    Electrical Modeling of Tristate Antiferroelectric Liquid Crystal Devices

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    Proocedings of: 3rd International Workshop on Liquid Crystal for Photonics (LCP2010). Elche (Alicante), Spain. September 8-10, 2010Copyright 2011 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.A new electrical equivalent circuit (EEC) has been proposed to model antiferroelectric liquid crystal devices. This circuit includes a constant phase element to take into account the ferroelectric part of the dielectric response in these devices. Electrical characterization of samples has been carried out using a specific experimental protocol based on impedance spectroscopy. The parameters of waveforms used in impedance measurements have been optimized. The procedure to obtain the components of the EEC has also been explained. Finally, the EEC has been validated by comparing experimental and simulated impedance results. A reasonable agreement between both of them has been obtained in a wide frequency range for all selection voltagesCiencia e Innovaci√≥n of Spain (Grant No. TEC2009‚Äď13991-C02‚Äď01) and Comunidad de Madrid (Grant No. S2009/ESP-1781).Publicad
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