Seongwoo Suh

Polarization controller using nematic liquid crystals.

In this Letter we demonstrate a polarization controller capable of changing any state of polarization of light from one arbitrary state to another. The controller consists of a stack of three homogeneous nematic liquid-crystal cells. The polarization state is controlled by proper adjustment of the voltages applied across each of the cells. The mathematical algorithm and principles of this polarization controller are developed in the framework of the Stokes parameters, allowing easy visualization by use of a Poincaré sphere representation. The transformation functions are given for conversion of an arbitrary input state to any output state. Experiments are carried out to demonstrate arbitrary polarization transformation.

Precise determination of the cholesteric pitch of a chiral liquid crystal in a circularly aligned configuration

We propose a novel method of measuring the cholesteric pitch of a liquid crystal (LC) in a circularly aligned configuration. The LC cell is made with one homogeneously rubbed surface and one circularly rubbed surface so that it produces a disclination line passing through the singularity created by the circular rubbing. This disclination line is associated with two domains with different twist orientations. In such a geometry, the relationship between the angular deviation of the disclination line and the cholesteric pitch is analytically derived. The concept of a homogeneous-circular configuration is adopted to accurately measure the cholesteric pitch of LC in a broad range from 0.5 to 500 μm.

36.4: Propagation and Optimization of Stokes Parameters for Arbitrary Twisted Nematic Liquid Crystal

Twisted nematic liquid (TN) crystal structures have been used extensively to rotate the input polarization of the light by 90°. For optimum performance of these devices, optimization is necessary for exact rotation of linear polarization of the input light. Using Stokes parameter representation, we derive the generalized relationship, which allows the calculations of the optimum parameters necessary for arbitrary rotation of the linearly polarized light with arbitrary amounts of twist of the liquid crystal structure. We show that “Gooch and Tarry” relation is a specialized case of our generalized results.

Defect in the circular-circularly rubbed liquid crystal cell with off-center alignment

In this letter, we investigate the behavior of the defects in a circular-circularly rubbed liquid crystal cell with an off-center alignment. We show that the line defect forms a circle that passes through the rubbing centers of the two surfaces. The size and the position of the defect circle depend on the cell gap and the pitch of the materials. We propose a simple model, based on an analysis of the free energy, to explain this interesting phenomenon. This technique of defect making is useful to confine the defect to a particular position by controlling the cell parameters and the material properties. It can also be applied to the pitch measurement, the generation of the space-variant polarized light, and the study of the dynamic properties of the defect.

Reflective homeotropic mode in a twisted nematic liquid crystal

This letter reports on a novel liquid crystal (LC) display device capable of achromatic reflection in a twisted homeotropic configuration using a chiral nematic LC with negative dielectric anisotropy. Using the elastic continuum formalism, we arrived at simulated values for each parameter of interest, including external twist, cell gap, molecular chirality for the chiral pitch, and the surface anchoring energy. The experimental results correspond well with the calculated expectations, exhibiting EO properties that demonstrate excellent achromatic reflection, extinction, and a high contrast ratio.

Arbitrary-to-arbitrary polarization controller using nematic liquid crystals

In the study of lightwave systems, polarization related effects have become important considerations. This has led many researchers to focus on developing practical means of controlling the state of polarization of light. This paper demonstrates a polarization controller, consisting of a stack of three homogeneous nematic liquid crystal cells, that is capable of changing any state of polarization of light from one arbitrary state to another. by adjusting the voltages applied across each cell, the state of polarization can be controlled. The mathematical algorithm and the principles of this polarization controller are developed in the framework of the Stokes parameters, allowing an easy visualization and better understandings using Poincare sphere representation. In addition to providing the transformation functions for converting an arbitrary input polarization state to any output state, this paper describes the experiments that were carried out to illustrate and confirm the arbitrary polarization transformations.