Browsing by Author "Tsung, Jieh-Wen"

Now showing 1 - 2 of 2

Shaping and positioning of topological defects in nematic liquid crystal cells equipped with patterned electrodes
(2020-01-01)
Tsung, Jieh-Wen
;
Chen, Bo-Yao
;
Wang, Ya-Zi
;
電子物理學系
;
Department of Electrophysics
Generation of topological defect that has a particular type (topological charge) and specific shape (radial, spiral, or circular) on a designed position is a crucial step in producing q plates and defect-mediated self-assembled micro structures. Shaping and positioning of topological defects can be achieved by applying delicately designed electric field on nematic liquid crystal (NLC). The guidelines for the electrode design are provided in this paper. The following are the three frequently asked questions: 1) What kinds of defects can exist in NLC? Points, lines, or walls? 2) How to generate a defect with a particular topological charge by setting proper boundary condition? 3) How to control the orientation of liquid crystal to have a defect with desired vorticity? The three questions can be answered by the three theories in physics: 1) Symmetry breaking in ordered material, 2) Euler characteristic and vectors in closed confinement, and 3) Minimization of free energy of NLC in electric field. Topological defects in spherical and toroidal confinements were generated in NLC cell and examined under polarized microscope. Radial and circular hedgehog defects were successfully created by fishbone- and coil-shaped patterned electrodes, respectively. By applying the three physical theories in NLC cell design, topological defects were successfully arranged in square and hexagonal arrays. They are large, stable, and re-configurable, and the applications in optics are demonstrated.
Topological defect array in nematic liquid crystal
(2019-01-01)
Tsung, Jieh-Wen
;
電子物理學系
;
Department of Electrophysics
Topological defects arise when the symmetry of order in liquid crystal is broken during phase transitions or under specified boundary conditions. Fascinating applications, such as, optical vortex and optical memories, will be realized only if the type and position of defects can be precisely controlled. Topological defects were generated in nematic liquid crystal cell with homeotropic surface alignment when voltage is applied. The defects were precisely placed on designed positions using electric field. The topological charge and shape of the defect were assigned by the delicately designed pattern of the pixelated electrodes. The point defects with topological charges of +1 (hedgehog) and -1 (hyperbolic hedgehog) were arranged in square or hexagonal lattices as two dimensional crystals. Topological rules in the stable defect array were discovered. The topological charge of a bulk defect was equal to the Euler number of the pixel in two-dimensional space. The number of the hyperbolic hedgehogs was determined by the folds of rotational symmetry of the defect array. The transmission and scattering of light through the defect were characterized, and the visual effect of large array of defects were demonstrated. Scattering, diffraction, and lensing effects from the defect array renders hazy, watery, and glittering images, which is artistic like impressionist paintings.