In this work a new mechanism of photoalignment based on intermolecular bonding was developed and implemented to explain a large number of experimental data. The thesis consists of two main parts: the physics part 1, where investigation of photoalignment materials and technology are presented and new photoalignment mechanism based on hydrogen bonding is formulated; and the application part 2, where liquid crystal photoalignment is implemented for fabrication of new photonic and display devices.
In Chapter 1. Basic information on types, structures and applications of hydrogen bonds is reviewed. Materials forming the ‘linear’ main chain structures are considered as the promising candidates for liquid crystal alignment. The rod-like azo-dye photoalignment material, SD1, with terminal groups forming intermolecular bonds, is investigated by FTIR and UV-VIS spectroscopy. Photoisomerization in DMF solutions and its absence in solid film, as well as molecular orientation in photoaligned films can be explained by formation of double hydrogen bonds in-between identical dye molecules leading to a H-bonded chain structure.
In Chapter 2. Experimental studies of photoalignment materials in thin layers are presented. The negative order parameter is observed in the thin films of photoalignment materials. The orientation and reorientation dynamics of molecular order parameters during light exposure are studied by the absorption ellipsometry technique. It reveals the presence of biaxial states, which can be induced by an external impact of light exposure. This work presents a new possible photoalignment mechanism based on hydrogen bonds, which is in good agreement with the experimental data. The formation of alignment direction, phenomena of spontaneous increase of the order parameter after light exposure and sensitivity to water are explained through photo- and thermodissociation and hydration of intermolecular H-bonds followed by thermodynamic relaxation in the mean field potential.
In Chapter 3. Experimental investigations on liquid crystal photoalignment inside the LC cell are presented. Wavelength and intensity dependences of LC photoalignment are investigated. New approach to create novel photoalignment materials with tunable alignment properties by forming solid-solutions with the pure basic dye is proposed. It is based on molecular level of understanding of the H-bond photoalignment. The bi-component compounds have characteristic reorientation dynamics of alignment direction, which is tuned from fast to slow by proper weight mixing.
We developed an Optical Rewritable (ORW) photoalignment technology, which is the result of investigation and modifications of standard azo-dye photoalignment process. This offers reversible liquid crystal alignment using polarized light of 450nm light source. It is compatible with conventional photolithography processes and saves one masking step on creation of patterned LC alignment. When, first alignment direction is formed uniform; and then second alignment direction is rewritten through a photomask. N alignment directions of LC photoalignment are possible with N-1 mask, as the ORW photoalignment is the memory-less effect.
In Chapter 4. We demonstrate the large potential of this ORW photoalignment technology in LC photonics to fabricate LC diffractive gratings and other integrated LC optical elements; apply photoalignment to silicon photonics to control LC alignment in photonic devices with LC cladding. We present both our theoretical calculations and experimental results.
In Chapter 5. We utilize the ORW photoalignment technique for display application and create a new type of optical rewritable electronic paper. This is light printable rewritable matter with a polarization dependent gray scale. Bare plastic or polarizers are used as substrates and no conductor is required. Double-sided light-printable ORW e-paper is shown to display a stereoscopic 3D image. A continuous grey image maintains proper performance even when the device is bent. The simple construction provides both durability and low cost.