Examination Committee

Prof Weiping LI, MATH/HKUST (Chairperson)
Prof Hoi Sing KWOK, ECE/HKUST (Thesis Supervisor)
Prof Tsung-Hsien LIN, Department of Photonics, National Sun Yat-Sen University (External Examiner)
Prof Vladimir CHIGRINOV, ECE/HKUST
Prof Patrick YUE, ECE/HKUST
Prof Jiannong WANG, PHYS/HKUST

Abstract

Nowadays, flat panel displays (FPDs), such as computer monitors, mobile phones and televisions, are ubiquitous in our daily lives, presenting us information and entertainment. The liquid crystal display (LCD) and organic light-emitting diode (OLED) display are the mainstream display technologies due to their high-quality performance. The polarizer is an indispensable element in LCDs, and has a great influence on their light efficiency and contrast ratio. The employment of a polarizer is also necessary in OLED displays to improve the contrast ratio and readability under sunlight. Conventional thin film polarizers have a thickness of around 25 μm and are based on stretched polyvinyl alcohol (PVA) doped with iodine. The shrinkage force resulting from the stretching process is a serious concern for their application in flexible displays. For the development of ultra-thin and flexible displays, polarizers with an ultra-thin profile and compatibility with flexible displays are demanded.

Photoalignment is a simple and low-cost way to produce dichroic polarizers, and it shows advantages for flexible displays because the produced polarizers are ultra-thin, are able to be integrated inside LC cells and are free from the shrinkage issue. The main problem that has hindered the practical application of dichroic polarizers produced by photoalignment is the insufficient dichroic ratio.  We propose two approaches to realize high-performance dichroic polarizers based on photoalignment methods. Firstly, the photoalignment kinetics of AD-1 thin films deposited by spin coating are studied, and the effect of relative humidity on the photoalignment process and final achieved dichroic ratio is investigated on a hydrophobic azobenzene molecule (AD-1) for the first time. An average dichroic ratio of up to 100 from 450 nm to 550 nm is realized after optimization, which is the highest value achieved among all photoaligment-based approaches. Secondly, a novel photoalignment method is proposed, and polarizers with a high average dichroic ratio of 55 from 400 nm to 500 nm are fabricated by vacuum evaporation. Moreover, the energy consumed by this method is low, which is more economical for practical manufacturing. Furthermore, the polarizers produced by both methods are only several hundreds of nanometers thick, which are urgently demanded for ultra-thin and flexible displays. 

Additionally, broadband reflective polarizers with a thickness less than one micrometer are designed to improve the light efficiency of LCDs. We propose to generate giant form birefringence based on a subwavelength silicon grating. The optical characteristics of the proposed polarizer are investigated by the finite-difference time-domain (FDTD) method, and show high transmission, a large extinction ratio and a wide acceptable angle. The proposed polarizer can not only reduce the thickness of LCDs, but also improves the efficiency by 50% via combination with a backlight recycling technique.