Zhizhong Zhuang

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.

Achromatic linear polarization rotator using twisted nematic liquid crystals

In this letter, we propose a simple configuration that is capable of rotating a linear polarization state of light by a certain angle for a wide range of wavelengths. The device consists of three liquid-crystal cells: two homogeneous cells and one twisted nematic (TN) cell. It is well known that a thick TN cell can rotate the linear polarization state of light by following the twisted structure. However, for a thin TN cell, achromatic polarization rotation is not possible. By the use of the Poincare sphere model of the TN structure, we demonstrate that if one thin homogeneous cell is placed before and another one is placed after a thin TN structure, then a linear polarization state can be transformed close to the eigenmodes of TN. Therefore, this structure can be used to achieve the achromatic polarization rotation. This letter provides a detailed discussion of the theoretical analysis and a demonstration of the achromatic linear polarization rotator by the use of a 1.9 μm TN cell for the wavelength range...

Bistable twisted nematic liquid-crystal optical switch

In this letter, we describe a bistable twisted nematic (BTN) liquid-crystal device optimized for use in a fiber optical system. The device configuration is optimized so that the states of polarization for the two bistable states are linear and orthogonal to each other for a linearly polarized input light. This optimization is accomplished by the use of the Poincare sphere method. Using this method, we obtained the analytical forms of the optimization conditions. Several different optimized conditions are obtained. In order to experimentally explore the use of the BTN structure in optical latching switches, one optimized structure (11.25° and −348.75° twists, dΔn/λ=0.496) is studied in the infrared region by the measurement of the output state of polarization. The experimental results agree well with our theoretical predictions. This study also suggests that transmissive BTN displays with high contrast are possible by the use of these optimized conditions with nonparallel and noncross polarizer configurations.

Optimized configuration for reflective bistable twisted nematic displays

In this letter, we experimentally verify the operation of a single-polarizer reflective bistable twisted nematic (BTN) liquid-crystal display using an optimized geometry. Based on the polarization optimization using the Poincare sphere representation, several optimized operating modes, which can simultaneously optimize both bistable states, are possible. One of these optimized configurations, the 67.38° and −292.62° twisted bistable states with 310.69 nm birefringence and a 24.25° incident polarization angle, is studied in detail. The evolution of the state of polarization of light passing through this reflective BTN structure is numerically studied, and the electro-optic property of the reflective BTN structure is experimentally demonstrated. Our results show that, using these optimum configurations, a high-contrast display is possible.

Parameter optimization for a reflective bistable twisted nematic display by use of the Poincaré sphere method

Although reflective bistable twisted nematic (RBTN) displays have potential in low-power-consumption applications, to achieve the optimum conditions for both bistable states simultaneously remains a challenge. We use a geometrical method based on the Poincaré sphere representation to obtain the optimum conditions that can simultaneously satisfy both bistable states for a RBTN structure. With this method, the optimum conditions can be obtained analytically and the operation modes can be clearly visualized and better understood.

Behavior of cholesteric liquid crystals in a Fabry–Perot cavity

We investigate the behavior of cholesteric liquid crystals (CLCs) inside a Fabry-Perot (FP) cavity. Although FP cavities filled with various liquid crystals have been extensively studied, to our knowledge the behavior of CLC-based FP cavities has not been reported. In CLC the twisted structure can be changed because the pitch is a function of temperature. In a parallel-rubbed CLC FP cavity the balance between strong surface anchoring and elastic energy yield a steplike resonance spectrum. This corresponds to the quantized effective pitch that the system assumes when both surface alignments are fixed. Experiment results for parallel-rubbed samples are presented and explained theoretically by use of Jones matrix calculations.

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.

Electrically controllable azimuth optical rotator

The transformation of the state of polarization (SOP) of light from one state to another can be graphically illustrated by a trajectory on the Poincare sphere (PS). We use this method to illustrate the control of the azimuth SOP rotation on the PS. Traditionally, azimuth rotation is achieved by the use of a Faraday rotator or by the mechanical movements of birefringent wave plates. In this letter, an electrically controllable azimuth optical rotator is introduced theoretically and verified experimentally. It consists of two quarter-wave plates and one liquid-crystal variable wave plate and allows polarization rotation of an arbitrary polarized light by an angle determined by the magnitude of the applied field. This electrical approach avoids the errors and inconvenience associated with the magnetic field and the mechanical movements of other methods.

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.

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.