Thesis Examination Committee
Prof Tianshou ZHAO, MAE/HKUST (Chairperson)
Prof Hoi Sing KWOK, ECE/HKUST (Thesis Supervisor)
Prof Vladimir CHIGRINOV, ECE/HKUST (Thesis Co-supervisor)
Prof Philip J BOS, Liquid Crystal Institute, Kent State University (External Examiner)
Prof Zhiyong FAN, ECE/HKUST
Prof Abhishek Kumar SRIVASTAVA, ECE/HKUST
Prof Ophelia K. C. TSUI, PHYS/HKUST
Patterned structures of polyimide have been seldom reported though it is widely used in display fabrication. Based on the inhomogeneous surface theory, varied pretilt angles are realized by nanostructured patterned polyimide. As an embodiment, a liquid crystal (LC) lens is achieved by designing the phase retardation profile to be parabolic. It utilizes a uniform cell gap and uniform electrodes, which simplifies the fabrication process to a great extent compared to other technologies. Within 10V, the focal length can be tuned in a wide range, while other LC lenses need dozens of volts to drive.
Polyimide is robust to temperature and moisture, but the rubbing process introduces debris, non-uniform alignment, and mechanical damages. Photoalignment has been acknowledged as the replacement for rubbed polyimide technology, due to its non-contact alignment process, high resolution and easily patterned properties.
However, to fabricate multi-domain alignment displays utilizing photo-aligned crosslinking materials, multi-exposure is needed. To simplify the exposure process to only one step, a multilayer thin film structure is demonstrated to act as an advanced photomask. The designed structure contains 3D LC director distribution, composing of a patterned polarization grating thin film and a patterned quarter-wave plate. Light properties including phase, polarization, and intensity can be spatially controlled.
It is worth noting that crosslinking materials are aligned by deep UV light, and the exposure dosage cannot exceed a certain value to avoid degradation of the alignment layer. Recently, we invented a composite for generating vertical alignment by blue light. The blue LED is much cheaper and more robust than deep UV light source modules. Moreover, the alignment layer will not be damaged by large exposure dosages. Compared to crosslinking materials, our composite is more suitable for the mass production. Last but not the least, patterned vertical alignment structures can also be easily achieved by our composite.