Far field diffraction patterns of laser beam through the cholestric liquid crystal

• Main Authors: Cheng-Yu Wu, 吳承祐
• Other Authors: Ja-Hon Lin
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/546qg3

碩士 === 國立臺北科技大學 === 光電工程系研究所 === 100 === In this work, we use Ti:sapphire laser as light source to study the nonlinear optical characteristics of the liquid crystal (LC). First, we measure the nonlinear refractive indices of LC by the Z-scan measurement. During this measurement, we use linear polarization light beam to measure nematic liquid crystal (NLC) and left-handed circular polarization beam to measure cholestric liquid crystal (CLC) , respectively. Owing to the thermal lensing effect, the measured normalized transmittance reveals peak-valley trace means that the nonlinear refractive index change is negative. Then, we move the LC in z direction before and after the focal point of lens to observe the far field diffraction patterns (FFDP) as Gaussian beam passing through the LC cell. The incident Gaussian beam will have negative wave-front curvature when the LC is put before the focal point of the lens. The FFDP displayed a central bright spot surrounded by a ring with lower intensity when the Gaussian beam passed through a self-defocusing medium. After LC cell moved after the focal point of the lens, the curvature of incident Gaussian beam through the LC will become positive. From FFDPs, the intensity of central bright spot will reduce and the outer ring will increase, resemble to a donut pattern. It is needed to consider the variation of the radius of curvature and beam radius due to apparent change for the Gaussian beam around the focal point. The variation of FFDPs as the Gaussian beam through the LC cell can be calculated using the Kirchhoff diffraction integral. After calculation, we can summarize that the central bright spot can be obtained while the self-defocusing medium is before the focal point and the wave-front curvature is negative. If the self-defocusing medium is after the focal point, the far field diffraction patterns will become a central dark pattern as the wavefront curvature is negative. The investigated results will help us to design practical nonlinear optics limiters of protective devices sensor by LC.