Browsing by Author "WEI, JG"

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CONFIGURATION TRANSITION OF A CLOSED-CYLINDER NEMATIC LIQUID-CRYSTAL IN AN EXTERNAL ELECTRIC-FIELD
(1994-03-15)
CHEN, SH
;
WEI, JG
;
交大名義發表
;
光電工程學系
;
National Chiao Tung University
This study reports on the director-field configurations of a closed-cylinder nematic liquid crystal with negative dielectric anisotropy in an external electric field. Raising the applied voltage causes the director orientation with a volume point defect to be transformed from a normal to a twisted configuration. The voltage at which the configuration is transformed depends on the aspect ratio of the cylinder.
ELECTRIC-FIELD-INDUCED DIRECTOR-FIELD TRANSITION OF NEMATIC LIQUID-CRYSTAL IN A CLOSED CONICAL CAVITY
(1994-11-01)
WEI, JG
;
CHEN, SH
;
交大名義發表
;
光電工程學系
;
National Chiao Tung University
Continuum theory and numerical method are utilized in this study to report the dependence of director-field transition induced by electric field on the elastic constants of nematic liquid crystal (NLC) with negative dielectric anisotropy in a closed conical cavity with perpendicular boundary conditions of NLC. Normal umbilic structure (NU), twisted umbilic structure (TU), normal hyperbolic point structure with a disclination ring (NH), twisted hyperbolic point structure with a disclination ring (TH), and normal Poincare structure (NP) are found to be stable structures in electric-field-induced transitions. Moreover, five typical sequences of transition, T-NU-->TU-->TH, T-NU-->TU-->NH-->TH, T-NU-->TU-->NH, T-NU-->NH, and T-NU-->NP-->NH are found in the range of applied voltage less than 10 V. The transition sequences and the transition voltages are strongly dependent on the ratios of elastic constants K-11/K-33 and K-22/K-33.
OPTICAL-PATTERN SIMULATIONS FOR CLOSED CYLINDERS OF LIQUID-CRYSTALS
(1993-05-01)
LIANG, BJ
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WEI, JG
;
CHEN, SH
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光電工程學系
;
Department of Photonics
Optical patterns for the polarizing microscope textures of closed cylinders of liquid crystals (CCLCs) are studied by the Jones vector formulation. The simulated director configuration diagram of a CCLC with normal orientation at the walls contains four kinds of stable director configurations. The resulting texture under some approximations depends only on phase shifts. The patterns are characterized by dark brushes that coincide with the directions of the crossed polarizers. Additionally, there are concentric circle fringes. For the ring defect structure, the centre region of the cylinder exhibits a black extinction. However, this is not distinguishable as between the hyperbolic and radial structures for both the point and the ring defect patterns. Comparing the observed patterns with the simulated ones, we find that the dark cross brushes and the centre black extinction of simulated patterns are similar to those that are observed. However, the concentric circle fringes of the observed pattern are not in accordance with the simulated pattern.
STABILITY OF AN UMBILICAL DIRECTOR FIELD OF NEMATIC LIQUID-CRYSTAL IN A CLOSED CONICAL CAVITY
(1994-05-01)
WEI, JG
;
CHEN, SH
;
交大名義發表
;
光電工程學系
;
National Chiao Tung University
This letter studies the stability of a single umbilical director field of nematic liquid crystal (NLC) in a closed cavity with external electric field. To form the single umbilic structure, we filled NLC with negative dielectric anisotropy in a closed conical cavity and made molecules align perpendicular to the cavity wall. Electric field was applied along the symmetry axis of the cavity. Using the continuum theory, we show that an umbilical director field will transform to a director field of a hyperbolic singular point with disclination ring instead of a Poincare structure at a critical voltage.
UNIQUE BEHAVIOR OF THICKNESS DEPENDENCE IN THE NONLINEAR WAVE-MIXING PROCESS WITH A NEMATIC THIN-FILM
(1993-04-15)
KUO, CL
;
WEI, JG
;
CHEN, SH
;
交大名義發表
;
光電工程學系
The dependence of the optimal electric biasing field and the diffraction efficiency on the thickness of the nematic thin film through the degenerate four-wave-mixing process is investigated. For relatively low optical pumps, the optimal bias monotonically increases with thickness, while the corresponding diffraction efficiency increases at first and then decays for a thickness even less than 150 mum, which is well within the phase-matching regime. For stronger pumps, the optimal bias can be doubly valued, and the curve of optimal bias versus thickness shows a closed loop. This behavior is unique to liquid crystals. The main mechanism is due to the twist effect of molecular orientation in addition to the scattering loss. Both the numerical and experimental results show these peculiar phenomena.