A optically controlled cholesteric grating is proposed. The dye-doped cholesteric grating (DDCLC grating) can be rotated by changing the polarization of the pump-beam. The experiment results reveal that the orientation of the cholesteric grating can be rotated in a specific range decided by the alignment direction of the two substrates. Moreover, the hybrid alignment condition for the cholesteric grating is also investigated. When the ratio of d/p is larger than 0.5,the cholesteric grating is observable without applying voltage in hybrid aligned cholesteric cell, and the cholesteric gratings with hybrid alignment can have a 360 rotation angle, thus achieve the two dimension beam steering cholesteric.

This study investigates the cholesteric grating based on a hybrid alignment. The experiment results reveal that the ordered stripe of the hybridly aligned cholesteric grating (H-fingerprint) only exists for the d/p ratio between 1.5 and 3, and the stripe orientation of the H-fingerprint is difficult to control due to the variation of the d/p ratio. Optical properties of the H-fingerprint are also investigated. The results show that the 1st order diffraction efficiency of the H-fingerprint is higher than the diffraction efficiency 2nd order, and its two switching times are about 1500ms and 500ms for the off-time and the on-time, respectively. The H-fingerprint with an anti-parallel rubbing process and the thermal effect are also investigated. The results reveal that the anti-parallel rubbing and the thermal effect can effectively control the stripe orientation of the H-fingerprint. The results greatly increase the possibility of applications for the H-fingerprint.

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Uniform lying helix(ULH) is a cholesteric LC structure with helical axix lying on the substrate. This structure has a periodic refractive index profile characteristics, so that it can be used as a phase grating. The ULH period can be tuned with different electric fields which provide with various applications. However, ULH is not a steady state in general, once the electric field shutdown, ULH texture will transform to planar texture. Generally, the ULH exist only under a few conditions: well homogeneous alignment and cell the ratio of gap to pitch smaller than 2 , in case the ratio more than 2, the ULH won¡¦t align well. We use two methods to make ULH align well at large cell gap pitch ratio; cooling down from isotropic temperature under electric field and putting mechanical stress on the sample, therefore we can get a well alignment and more stable ULH. We discuss the influence of different boundary conditions to the stability of ULH, and we discover that when d/p large than 10, the stability of ULH improved significantly. Then we study the influence in different alignment anchoring, we find stronger anchoring energy makes ULH transform to planar easily. So we use photo alignment to get weaker anchoring energy, and the ULH using photo alignment stable more than 24 hours. Finally we use a large d/p ULH as tunable phase grating, the pitch length can be tuned about 380nm with low power consumption, and it still have grating function when the field shutdown . Keyword: cholesteric liquid crystal¡Buniform lying helix¡Bgrating

This research explores the concept of mimicking structural colour in nature as an alternative to traditional textile coloration techniques. In particular, the research focuses on certain species of buttery and beetle. Structural colours originate from the physical interaction of light with nanoscale structures. Firstly, this study explores the use of thin, multilayer films to aid designing and producing bi-component interference fibres, exhibiting structural colour similar to that of the Morpho buttery. In the textiles industry, a bicomponent fibre called the Morphotex® fibre has been produced. This fibre replicates the structure observed on the surface of the wings of the Morpho buttery, responsible for achieving the distinctive iridescent blue. The project aims to replicate and extend on previously implemented biomimetic structures on textiles. Secondly, this project investigates ways in which Cholesteric Liquid Crystals(CLC) can be printed onto a range of textile substrates using k-bar coating and inkjet printing methods, to exhibit structural colour similar to that of the Chrysina Gloriosa beetle. CLCs produce a wide colour gamut and provide angular colour effects that would be a welcome addition to the 'toolbox' of a textile designer. In this study, solvent based ink formulations containing CLCs are applied to pre-treated textile substrates, using a piezoelectric inkjet printer. Different ink formulations, with varying concentrations of nematic liquid crystal and chiral dopant, are investigated to create a range of coloured films. This research determines whether fibre content, fabric structure, thread density, film thickness and surface treatments have an impact on the colour perceived by the observer.

Polymer stabilized cholesteric liquid crystals (PSCLCs) provide many advantages over other electro-optical materials. The unique helical structure of the cholesteric liquid crystal (CLC) creates a natural gradient for light interacting across each CLC domain layer. Not only does the CLC helical structure greatly increase the bandwidth tuning and broadening range, it also allows CLCs to act as a polarizer, notch filter, reflector, and optical rotator all in one material. However, while many novel PSCLC materials have been created, little is understood about how complex initial system interactions affect final electro-optical (e-o) properties.1,2 In this work, the principal variables affecting PSCLC blue shift electro-optical behavior have been determined through structural analysis and measurement of electro-optical properties. Typical PSCLC materials must meet both formulation and photopolymerization processing requirements to display blue shift e-o properties. Threshold photoinitiator concentrations (0.5-1.5 wt%) and morpholine containing group concentrations (0.25-1.0 wt%) were both shown to be primary factors, along with sufficient UV exposure time (10-30 min) and light intensity (500 mW/cm2, 365 nm), for PSCLC blue shift bandwidth tuning/broadening to occur. Morpholine was initially identified as a component of photoinitators Irgacure 369 and 907 and was proven to increase PSCLC ion density altering LC-polymer network interactions with several proposed theories included later in this work. The use of an appropriate morpholine containing LC monomer to directly incorporate morpholine into the LC-polymer network was shown to greatly improve PSCLC sample stability. Through the results of this research we successfully induced blue shift e-o behavior in a previous red shift only PSCLC using only 30% of the UV exposure that a model PSCLC blue shift sample required while extending the blue shift broadening range over threefold (from 75 nm to 250 nm). The fundamental understanding and design of PSCLC systems described herein serves as a starting point for engineering PSCLC materials with specific and desirable electro-optical properties.

In this study, we demonstrated a flexible cholesteric liquid crystal display using polymerization induced phase separation (PIPS) on a single substrate. Cholesteric liquid-crystal displays (ChLCDs) provide with color reflection owing to their periodic helical layer, without any optic device like polarizer or color filter. We discussed the influence of surface treatment and process conditions on PIPS, and improve the reflectivity of single substrate Ch-LC display from 9.05% to 19.27%.

In this thesis, the nonlinear properties of dye-doped liquid crystal films measured via Z-scan technique are presented, and we¡¦ve also compared the features of nonlinear effect between nematic and cholesteric liquid crystals, because the purpose was to obtain a strong nonlinear effect which is polarization independent. The nonlinear refraction of the samples would induce self-focusing/self-defocusing effect, therefore the Z-scan technique was utilized for the measurements in this study. The degrees of self-focusing/self-defocusing effect can be used to calculate the optical Kerr constants n2 related to the variation of refractive indices. Moreover, this technique is uncomplicated and powerful for nonlinear effect observations, because it only requires a single laser beam and a simple setup to measure both nonlinear refractive indices and nonlinear absorption coefficients. In the first section of the study, we had investigated the nonlinear effect of azo-dye-doped liquid crystal (ADDLC) films by Z-scan technique. Owing to the birefringence provided by homogeneous-alignment liquid crystal films and the absorption difference between the long axis and the short axis of the azo dye, the self-defocusing effect in nematic phase was larger than that in isotropic state as the polarization of incident light was parallel to the long axis of aligned liquid crystals. While we set the incident polarization perpendicular to the long axis of liquid crystals, a self-focusing effect was observed and the in nematic phase was found smaller than that in isotropic state. As a result, it can be analyzed that the nonlinear effect of homogeneous aligned ADDLC films is intensely polarization-dependent. In the second section, in order to acquire a polarization-independent nonlinear effect, we fabricated the azo-dye-doped cholesteric liquid crystal (ADDCLC) films which were arranged in planar texture with the reflection bands managed to avoid the III wavelength of incident light. We had studied the nonlinear effects in different concentrations of the azo dye and also measured the influence of incident polarizations. We discover that when the concentration of dyes was raised, the n2 increased as well; the nonlinear indices were almost the same in various polarized light. The incident light suffered an average refractive index in an ADDCLC film, and azo dye arranged in helices resulted in identical absorption to different polarized light. In summary, employing ADDCLCs with planar texture in nonlinear optical applications, we could achieve not only strong nonlinear effects but also high polarization independence in liquid crystals.

Blue phases have been known to exist in chiral liquid crystals between the cholesteric and isotropic phases. A blue phase as a self-assembled three-dimensional cubic structure with lattice periods of several hundred nanometers exhibits not only selective Bragg reflections of light in the visible wavelength but optically isotropy owning to its highly symmetric molecular structure. Locally, blue phases still exhibit local anisotropic physical properties because of anisotropic structure of the nematic liquid crystal molecules, which make it possible to be easily controlled by an external field. This dissertation studies the effects in blue phases under various external fields, including electrical field, optical field, and temperature. Firstly, we investigated the bistable effect under the influence of an electric field and transition mechanism between various lattice orientations in the negative liquid crystal blue phase. The blue phase exists over a wide temperature range ~16oC, and three lattices (110), (112) and (200) of BPI are confirmed with Kossel diagrams. The red platelet (110) lattice and blue platelet (200) lattice can be stabilized and switched to each other by particular pulse voltages. We also studied the behavior that an electric field induced planar state and electro-hydrodynamatic effect in the blue phase. Additionally, the reflected color of the (200) lattice can be adjusted from 455nm to 545 nm by temperature induced lattice distortions and provided with reversibility. Secondly, we presented an optically switchable band gap of a 3D photonic crystal that is based on an azobenzene-doped liquid crystal blue phase. Two kinds of azobenzene, M12C and 4MAB, were utilized to switch photonic band gap of blue phases and to change the phase transition temperature of blue phase, respectively. For M12C- doped liquid crystal blue phase, the trans-cis photoisomerization of M12C induced by irradiation using 473nm light caused the deformation of the cubic unit cell of the blue phase and a shift in the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with 532nm light. The crystalline structure was verified using a Kossel diffraction diagram. Moreover, we also demonstrated an optically addressable blue phase display, based on Bragg reflection from the photonic band gap. For 4MAB- doped liquid crystal blue phase, the trans-cis photoisomerization of 4MAB destabilizes cubic unit cell of the blue phase and reduces the phase transition temperature. We observed the phase sequences of the 4MAB-doped blue phase as a function of the time of UV irradiation. Various distinct phases can be switched to another specific phase by controlling irradiated time and temperature of the sample. Therefore, the corresponding bandgap can be switched on and off between blue phase and isotropic phase, or varied from 3D to 1D between blue phase and cholesteric phase. Finally, we investigated the thermal hysteresis in the phase transition between the cholesteric liquid crystal and the blue phase of liquid crystal. The thermal hysteresis of such a chiral doped nematic liquid crystal occurs over 6oC. Both the CLC phase and the blue phase can stably exist at room temperature and be switched to each other using temperature-controlled processes. Further, we demonstrated two sets of bistable conditions using various surface treatments. In a homogeneous aligned sample, two stable states, CLC with a planar alignment and blue phase with a uniform lattice distribution, reflect light of wavelengths 480-510nm and 630nm, respectively, as determined by the corresponding Bragg¡¦s reflection conditions. In the untreated sample, the CLC phase with a focal conic texture can scatter light and the blue phase with a non-uniform lattice distribution provides high isotropic optical transparency.

No description available.