A Molecular Dynamics Simulation Study of the Nonlinear Optical Response of Liquid Crystalline Systems
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A Molecular Dynamics Simulation Study of the Nonlinear Optical Response of Liquid Crystalline Systems* Kenji Kiyohara1,3, Koji Ohta2, and Yo Shimizu1 1 Special Division for Human Life Technology and 2Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan 3 Research Institute for Computational Sciences, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
ABSTRACT We investigated the third-order nonlinear optical response of a Gay-Berne system at the isotropic, nematic, and smectic phases by molecular dynamics simulation. The components of the optical response were calculated for the three different axes with respect to the director of the system, separately. In the nematic phase, in particular, we observed that the response function does not vanish at long times. This means that the orientation of the director of the system is permanently changed by an instant irradiation of polarized light, as a result of third-order nonlinear optical response. In the smectic phase, however, all the components of the response function decay quickly. Our results give a theoretical background at molecular level on the interpretation of the reported experimental observations of peculiar dynamics of liquid crystalline systems at irradiation of laser lights.
INTRODUCTION Since the pioneering work by Wong and Sheng [1], third-order nonlinear optical response (or optical Kerr effect) of liquid crystals has been studied in the last few decades using the advanced experimental techniques utilizing the laser technology at the times [2-9]. In the isotropic phases of liquid crystals, the reorientation of the molecules due to the third-order nonlinear optical response has been found to be much slower than that of typical molecular liquids. This is understood as the result of a cooperative response of the liquid crystalline molecules rather than the simple average of responses by individual molecules. The optical responses have also been measured in the nematic and smectic phases [6,7,3]. However, it is difficult to extract purely optical response of liquid crystals from those experiments because the experimentally obtained signal of the optical response is affected by a collection of various factors such as electrostriction, density wave, flow, and their mutual interactions [6,7]. Molecular dynamics simulation plays an important role in such a subject. In this technique, the optical response of liquid crystalline systems can be analyzed at the condition that there is no external effect such as surface anchoring, elastic force, or thermal effects, which cause density wave and flow. We report in this paper an analysis of the third-order nonlinear optical response of a model liquid crystalline system at its isotropic, nematic, and smectic phases. As the model, we used the Gay-Berne model [10] with parameters ( =3, '=5, =2, and =1). The Gay-Berne model was originally developed as a computationally efficient
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