PhD Thesis Presentation
Liquid Crystal Polarization Gratings and Their Applications

by Miss Li TAN

 :  31 Jul 2013 (Wed)
 :  10:30am
Venue  :  Room 5506, 5/F (Lifts 25-26), HKUST

Examination Committee
Prof Siu Wing CHENG, CSE/HKUST (Chairman)
Prof Hoi Sing KWOK, ECE/HKUST (Thesis Supervisor)
Prof Philip James Bos, Department of Chemical Physics, Kent State University (External Examiner)
Prof Michael M T LOY, PHYS/HKUST
Polarization gratings (PGs) are defined as spatially varying dichroic or birefringent elements whose anisotropy parameters vary across the device periodically. Because of its intriguing optical properties, a variety of applications have been proposed. This thesis investigates the fabrication, characterization, optimization and applications of liquid crystal (LC) PGs.

The defect free LC PGs with grating pitch as small as 2μm were successfully fabricated using polarization holography and photo-alignment techniques. Azo-dye SD-1 was applied as photo-alignment material. Complex type PGs with twist deformations were also fabricated through chiral doping into polymeric liquid crystals.

A fast and accurate method based on extended Jones matrix has been proposed to characterize the properties of various polarization gratings at oblique incidence. The propagation matrix was described by a spatially varying extended Jones matrix. The far field diffraction properties of the mth order of the grating were determined by vectorial Fourier coefficients of the transmitted field under Fraunhofer approximation. The experimental results showed very good agreement with the calculated data.

We further optimized the configurations of PGs according to different applications. We proposed to apply a PG to a Color Filter Liquid Crystal on Silicon (CF_LCOS) based pico-projector system, so that the un-polarized light can be potentially utilized and modulated directly. The overall system efficiency can be improved by 100%, maintaining the system contrast ratio and the compact system volume. A PG+CF_LCOS projection system using white light LED as light source was demonstrated.

We also develop a wide viewing angle (WVA) PG for beam steering applications. A simplified model was developed to characterize the polarization gratings at off-axis. The calculation speed was dramatically increased. Through the ultra-fast algorithm together with the parameter space approach method, we achieved a WVA PG with greatly suppressed 0th order leakage within a wide incident angle range.
*** ALL ARE WELCOME !! ***