Surface Plasmon Excitation, Mode Splitting in Slab Wave Guides and Dynamic Oscillation Studies of Liquid Crystals

• Main Authors: Tzong-Shyan Lin, 林宗賢
• Other Authors: Juh-Tzeng Lue
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/52149969599929734277

博士 === 國立清華大學 === 物理學系 === 90 === In this work, we are devoted to study the optical properties of nematic liquid crystal (LC) from the anisotropic birefringence behavior view. These include the shift of minimum attenuated total reflection angle, the mode splitting, and the nonlinear dynamic oscillation. The scenario of this work will be portrayed as follows. Attenuated total reflection (ATR) is a prominent effect in examining the excitation of surface plasmons. The Kretcshmann-Raether configuration is implemented to generate the surface plasmon and to examine the change of the refractive index of LC by an applied field. A mixture of nematic and cholesteric liquid crystal is sandwiched between two glass substrates coated with conducting films. The minimum ATR angle changes less than 1o as the applied voltage increases to as large as 30Vpp, which is controvertible to the calculation based on field induced refractive index change. This fact suggests that the orientation of LC directors adjacent to the electrode surface is unaffected in response to the external field attributing to the strong anchoring effect. Extended Jones matrix simulation imposed with multi-layered structure of LC clearly adducing this fact. Mode splitting is found with a He-Ne laser beam reflecting through a prism-coupled liquid-crystal slab waveguide applied with an electric field. Mode splitting yields stronger manifestation as the imposed voltage passes a critical level, yet it becomes diminished above a critical high voltage. If the voltage increases even higher, mode splitting would disappear, attributing to the turning up of almost all the directors of LC to the surface. Our multi-layered matrix simulation can satisfactorily account for this phenomenon by exploiting the property of the anisotropic optical birefringence of LC under applied voltages. Relaxation oscillations of optical transmittance after the turn-off of the applied electric field of LCs are observed when the detected light transmits through a proper adjustment of the alignment of the polarizer and analyzer. Approximated dynamic simulation, which do not concern the back-flow effect and neglect the inertia terms, of the LC molecule at each layer yields relevant polarization interference of the transmitted light. This effect can satisfactorily portray the oscillation phenomenon. Optics with the extended Jones 4x4 matrix formalism, which is relevant in elucidating the optical properties of anisotropic media, is also implemented to solve this problem.