Final Examination of Ms. Yan Li for the Degree of Doctor of Philosophy in Optics

Friday, June 29, 2012 3:00 PM to 5:00 PM
CREOL Room 102

Dissertation Title: High-Efficiency Blue Phase Liquid Crystal Displays

Abstract:
Blue phase liquid crystal (BPLC) has a delicate lattice structure existing between chiral nematic and isotropic phases. The stable temperature range is only about 2 degrees. But due to short coherent length, these self-assembled nano-structured BPLC has a fast response time. In the past three decades, the application of BPLC is rather limited because of its narrow temperature range. In 2002, Kikuchi et al. developed a polymer stabilization method to extend the blue-phase temperature range to more than 60 K. This opens a new gateway for display and photonic applications.

In this dissertation, we investigate the material properties of polymer-stabilized BPLCs. According the Gerber’s model, the Kerr constant of a BPLC is linearly proportional to the dielectric anisotropy of the LC host. Therefore, in the frequency domain, the relaxation of Kerr constant follows the same trend as the dielectric relaxation of the host LC. We have carried out experiments to validate the theoretical predictions, and proposed a model called extended Cole-Cole model to describe the relaxation of Kerr constant. On the other hand, because of the linear relationship, Kerr constant should have the same sign as the dielectric anisotropy of the LC host. That is, a positive or negative Kerr constant results from positive (?e>0) or negative host LCs (?e>0), respectively. BPLCs with positive Kerr constant have been studied extensively, but there has been no study on negative ?e BPLCs. So we have prepared a BPLC mixture using a negative ?e LC host and investigated its electro-optic properties. We proved that indeed the induced birefringence and Kerr constant are of negative sign.
Due to the fast response time of BPLC, color sequential display is made possible without color breakup. By removing the spatial color filters, the optical efficiency and resolution density are both tripled. With other advantages such as alignment free and wide viewing angle, polymer-stabilized BPLC is emerging as a promising candidate for next-generation displays.

However, the optical efficiency of the BPLC cell is relatively low and the operating voltage is quite high using conventional in-plane-switching electrodes. We have proposed several device structures for improving the optical efficiency of transmissive BPLC cells. Significant improvement in transmittance is achieved by using enhanced protrusion electrodes, and a 100% transmittance is achievable using complementary enhanced protrusion electrode structure.

For a conventional transmissive blue phase LCD, although it has superb performances indoor, but when exposed at strong sunlight the displayed images could be washed out, leading to a degraded contrast ratio and readability. To overcome sunlight readability problem, a common approach is to adaptively boost the backlight intensity, but the tradeoff is on the increased power consumption. Here, we have proposed a transflective blue phase LCD where backlight is turned on during dark ambient and ambient light is used to illuminate the displayed images under bright ambient. Therefore, a good contrast ratio is preserved even for a strong ambient. We have proposed two transflective blue phase LCD structures, both of which have single cell gap, single gamma driving, reasonably wide view angle, low power consumption, and high optical efficiency.

Among all the 3D technologies, integral image is an attractive approach due to its high efficiency and real image depth. However, the optimum observation distance should be adjusted as the displayed image depth changes. This requires a fast focal length change of adaptive lens array. BPLC adaptive lens is a good candidate because of its intrinsic fast response time. We have proposed several BPLC lens structures. In addition to fast response time, these lens structures are polarization independent and exhibit a parabolic phase profile.

To meet the low power consumption requirement set by Energy Star, high optical efficiency is among the top lists of next-generation LCDs. In this dissertation, we have demonstrated some new device structures for improving the optical efficiency of a polymer-stabilized BPLC transmissive display and proposed sunlight readable transflective blue-phase LCDs by utilizing ambient light to reduce the power consumption. Moreover, we have proposed several blue-phase LC adaptive lenses for high efficiency 3D displays.
 
Major: Optics

Educational Career:
B.S.: 2005, Zhejiang University, China
M.S.: 2007, Zhejiang University, China

Committee in Charge:
Dr. Shin-Tson Wu (Chair)
Dr. Bahaa E. A. Saleh
Dr. Boris Y. Zeldovich
Dr. Thomas X. Wu

Approved for distribution by Dr. Shin-Tson Wu, Committee Chair, on 06/18/2012

The public is welcome to attend.

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