Final Examination of Ms. Linghui Rao for the Degree of Doctor of Philosophy in Optics
Thursday, June 28, 2012 2:00 PM to 4:00 PM
Dissertation Title: Low Voltage Blue Phase Liquid Crystal Displays
CREOL Room 102
Polymer-stabilized blue phase liquid crystal displays (BPLCDs) based on Kerr effect is emerging as a potential next-generation display technology. In comparison to conventional nematic devices, the polymer-stabilized BPLCDs exhibit following attractive features: (1) submillisecond response time, (2) no need for molecular alignment layers, (3) optically isotropic dark state when sandwiched between crossed polarizers, and (4) transmittance is insensitive to cell gap when the in-plane electrodes are employed.
However, aside from these great potentials, some tough technical issues remain to be addressed. The major challenges are: 1) the operation operating voltage is still too high (~50 Volts vs. 5 Volts for conventional nematic LCDs), and the transmittance is relatively low (~65% vs. 85% for nematic LCDs), 2) hysteresis and residual birefringence are still noticeable, 3) the mesogenic temperature range is still not wide enough for practical applications (-40 o
C to 80 o
C), and 4) the ionic impurities in these polymer-stabilized nano-structured LC composites could degrade the voltage holding ratio, which causes image sticking.
In this dissertation, the BPLC materials are studied and new BPLC device structures are designed to optimize display performances. From material aspect, the electro-optic properties of blue phase liquid crystals are studied based on Kerr effect. Temperature effects on polymer-stabilized blue phase or optically isotropic liquid crystal displays are investigated through the measurement of voltage dependent transmittance under different temperatures. The physical models for the temperature dependency of Kerr constant, induced birefringence and response time in BPLCs are first proposed and experimentally validated. In addition, we have demonstrated a polymer-stabilized BPLC mixture with a large Kerr constant K~13.7 nm/V2 at 20 o
C and ?=633 nm. These models set useful guidelines for optimizing material performances.
From devices side, the basic operation principle of blue phase LCD is introduced. A numerical model is developed to simulate the electro-optic properties of blue phase LCDs based on in-plane-switching (IPS) structure. Detailed electrode dimension effect, distribution of induced birefringence, cell gap effect, correlation between operation voltage and Kerr constant, and wavelength dispersion are investigated. Viewing angle is another important parameter. We have optimized the device configurations according to the device physics studied. With proper new device designs, the operating voltage is decreased dramatically from around 50 Volts to below 10 Volts with a reasonably high transmittance (~70%) which enables the BPLCDs to be addressed by amorphous silicon thin-film transistors (TFTs). Moreover, weak wavelength dispersion, small color shift, and low hysteresis BPLCDs are achieved after their root causes being unveiled. Optimization of device configurations plays a critical role to the widespread applications of BPLCDs.
In addition to displays, blue phase liquid crystals can also be used for photonic applications, such as light modulator, phase grating, adaptive lens and photonic crystals. We will introduce the application of blue phase liquid crystal as a modulator to realize a viewing angle controllable display. The viewing angle can be tuned continuously and precisely with a fast response time. The detailed design and performance are also presented in this dissertation.
B.S.: 2007, Huazhong University of Science and Technology
M.S.: 2010, University of Central Florida
Committee in Charge:
Shin-Tson Wu (Chair)
Boris Y. Zeldovich
Eric W. Van Stryland
Thomas X. Wu
Approved for distribution by Prof. Shin-Tson Wu, Committee Chair, on 06/12/2012
The public is welcome to attend.
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