Photorefractive Effect in Nematic Liquid Crystal Cell with Sandwiched Structure
- PDF / 71,191 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 39 Downloads / 203 Views
Photorefractive Effect in Nematic Liquid Crystal Cell with Sandwiched Structure Hyun-Wuk Kim and Jong-Duk Kim Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusong-gu, Daejeon 305-701, Korea. ABSTRACT Optical responses and orientational photorefractivities of nematic liquid crystal cells doped with fullerene were investigated with a sandwiched structure. The photorefractive effect was attributed to the reorientation and switching of liquid crystals using a photoalignment layer for vertical alignment. When nematic liquid crystals were injected at a nematic phase, a better cell texture could be obtained due to the capillary flow-induced orientation effect. Samples with this method showed the higher photorefractive effects than those injected at an isotropic phase. Therefore, the state of molecular ordering is important for the orientational photorefractive effects.
INTRODUCTION Extremely large change of refractative index in nematic liquid crystals (NLCs) induced by lasers and applied voltages has been attracted for creating holographic images [1]. Neither nonlinear optical (NLO) chromophores nor the applications of high voltages of several kVs are required to form diffraction gratings, which is significantly different from electro-optically induced photorefactive (PR) polymers. In addition, the fabrication of devices does not require much elaboration like inorganic and organic crystals. Although holographic images of NLCs disappear when an applied voltage is turn-off [2], the nematic PR LCs and displays can be utilized for a real-time holography. The PR effect of a layer-structured LC cell is higher than those of any other LC systems [3]. This may be attributed to the efficient functional arrangement: when voltages are applied, generated charges are transported to or through photoconductive layers along the photocurrent paths across a sample, and at the sides of bright sites in a photoconductive layers, the charge moved by diffusion and drift, are trapped at defect sites so that a space-charge field can be established in a photoconductive layers. The penetration field of the space-charge field in the photoconductive layers into a LC layer causes LC molecules to reorient along the field [4]. CC5.5.1
If LC molecules are not oriented, they can suffer light losses from undesirable scatterings due to the switching disorder of LCs under applied voltages, which can lower the PR effects in layer-structured LCs. Therefore, it is expected that the orientational state of LCs affects PR effects in a layer-structured LC system significantly. In this work, we present a new model of hybrid-structure [5] with one layer composed of a photoconducting polymer and the other layer of a photoalignment film by the UV irradiation to clarify the molecular reorientational PR effect, and show the orientation effect of a rod-like nematic LC phase injected between two photoconductive layers, instead of injecting an isotropic LC phase.
EXPERIMENTAL DETAILS Two NLC mixtures, E7 (Merck
Data Loading...
