Ming-Shann Tsai

Optically switchable gratings based on polymer-dispersed liquid crystal films doped with a guest–host dye

In this work, we add a small amount of a guest–host dye G-206 in the polymer-dispersed liquid crystal mixtures to fabricate holographic gratings. Following formation, the grating is irradiated by a single Ar+ laser beam while probed by a He–Ne laser. Experimental results indicate that the first-order diffracted intensity of the probe beam increases significantly. Such an optically switchable diffraction effect is attributed to thermal expansion. The G-206 dye absorbs the Ar+ laser, subsequently resulting in thermal expansion. The liquid crystal (LC) droplets are then squeezed so that the LC molecules within the droplets are reoriented collectively. Thus, the refractive index difference between the LC-rich and polymer-rich stripes Δn increases.

Dynamical Studies of Gratings Formed in Polymer- Dispersed Liquid Crystal Films

The use of polymer-dispersed liquid crystal films to fabricate gratings was demonstrated. The written gratings are permanent, but are electrically switchable. In this article, we report the results obtained from the dynamical studies of gratings formed in polymer-dispersed liquid crystal films having various liquid crystal-polymer mixing ratios. The results showed that during the initial phases of formation, a thermal grating effect dominated. Later, it was offset or quenched by the photopolymerization effect which eventually dominated and determined the characteristics of the final grating.

Polymer Network Formed in Liquid Crystals: Polymer-Network-Induced Birefringence in Liquid Crystals

A monomer with reactive double bonds was mixed with a liquid crystal and polymerized under UV irradiation in various phases of the liquid crystal. The polymer networks formed are found to be anisotropic and fiber-like, if the monomer is polymerized in an oriented liquid crystal state. The orientational order of the polymer network is shown to depend on that of the liquid crystal. After polymerization, the polymer network, in turn, is found to affect the orientation of the liquid crystal significantly. It induces birefringence in nematics at temperatures just above the nematic-isotropic phase transition.

Polymer-dispersed liquid crystal films for storing optical holographic images

We have developed an economic liquid crystal-polymer dispersion material that can be used for storing optical holographic images. In this paper, we demonstrate the use of thin samples of this material for such an application. The obtained results show that the written gratings-holograms are permanent, but are electrically switchable. Furthermore, the diffraction efficiency is high and does not change significantly after many tests over a period of months. Thus, we believe this material is promising for practical uses.

Multidirectional rubbed liquid-crystal cells

The alignment characteristics of the homogeneous liquid-crystal (LC) cell rubbed multidirectionally were examined. LC molecules align along an axis between two different rubbing directions. The rubbing strength, cell thickness, and ambient temperature markedly influence the final orientation of the LC molecules. The orientation of the LC molecules and the transmission of a multidirectionally rubbed cell can be controlled according to the ambient temperature. A model that assumes substrate with various groove densities along various rubbing directions is presented. The elastic constant and the viscosity of the LC molecules are found to be the key factors that influence the orientation of the LC molecules.

Dynamics of Laser-Induced Holographic Gratings in Dye-Doped Liquid Crystal Films

Dynamic holography and polarimetric methods are used to study torques on liquid crystals exerted by photo excited dyes doped in planar nematic liquid crystal (NLC) films. The dye used is methyl red (MR). The dynamical reorientation of liquid crystal directors induced by photo-excited dyes is examined. The results indicate that the dynamics include the initial transient reorientation, followed by the transient three-dimensional rotation, and finally the stable LC alignment parallel to the optical field. A model is presented to explain these results. Quantitative analyses of the grating dynamics yield the dye-diffusion coefficients in the directions parallel and perpendicular to the LC director.

Scattering Light Interference from Liquid Crystal Polymer Dispersion Films

The Quetelet-type ring pattern is observed in liquid crystal polymer dispersion (LCPD) films. The clusters of the polymer network and liquid crystal (LC) domains with different director axes in the LCPD films serve as scatterers. Cells with unidirectional and multidirectional rubbings are fabricated. Experimental results show that the polarization of incident light, the applied voltage and the ambient temperature significantly affect the ring intensities. However, the contribution of the LC domains is not evident until the voltage is applied. Finally, rubbing the cells in multiple directions reveals that measurement of the Quetelet-type ring intensity can be used to readily identify the orientation of the liquid crystals. This finding also reveals that the LCs in an LCPD mixture are aligned closer to the final rubbing direction than are pure LCs in a multidirectional rubbed cell. A simple model was proposed to explain the observations.

Studies of Holographic Gratings Formed in Polymer-Dispersed Liquid Crystal Films and Their Dynamical Behavior

Abstract We have developed an economic liquid crystal-polymer dispersion material that can be used for storing optical holographic images, The obtained results show that the written grating-holograms are permanent, but are electrically switchable. Further, we studied the dynamical behavior and the temperature-dependence of the formed gratings. The gratings morphologies were also investigated using scanning electron microscopy (SEM). The results showed the thermal grating was competing with the grating due to photopolymerization. The former dominated in the initial period, Later, it was offset or quenched by the photopolymerization effect which eventually dominated and determined the characteristics of the final gratings whose diffraction efficiencies were found to have a good correlation with their SEM images.

Dynamic formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields

The application of a low-frequency oscillating magnetic field perpendicular to a magnetic fluid thin film leads to the separation of a phase that is concentrated in particles from a dilute phase. The concentrated phase forms cylindrical columns that construct two-dimensional lattices. The ordered structure of magnetic fluid thin films is the basis for potential optical applications. We investigate the dynamical ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields.

Dynamic studies of polymerization in liquid crystal-polymer mixtures

This work studied the dynamic behavior of polymerization in films of liquid crystal (LC)-polymer mixtures by using the pump-probe technique. Chopped Ar+ laser pulses were used as pump beams, and probed by a cw He—Ne laser. The experimental results indicated that the transmission of the probe beam was modulated in response to the chopping frequency of the pump beam. The modulation was such that transmission during the on-time of the pump beam was initially smaller than, then equal to, and finally larger than that in the off-time. Analyzing the transparency difference between the films with and without the Ar+ laser pulse allows us to recognize the three stages of polymerization process, initiation, propagation, and termination. Moreover, both the LC content in the sample and the chopping on/off ratio of the pump beam significantly affected the turning point occurred and the modulation amplitude of the probe beam. A model is proposed to explain these observed results.