| Research on liquid crystals, which were discovered a century ago, has been very active over the last two decades. One of these properties, which has drawn much attention recently, is their large optical nonlinearity. With this extraordinarily large nonlinearity, several nonlinear optical processes have been demonstrated using low power lasers.; The subject of this research is the detailed and systematic study of optical nonlinearities of nematic liquid crystals under the influence of an external (AC) electrical field. The Z-Scan technique is employed as the experimental method to observe and monitor the nonlinear effect. The split-step method is used to model the beam propagation in the liquid crystal sample, and hence derive the far-field intensity distribution, from which the theoretical Z-Scan curves are derived.; We have performed a series of Z-Scan experiments to monitor and record the nonlinear effect of liquid crystals. From the experimental results we find that the external voltage can vary the optical nonlinearity of the liquid crystal cell as we expected. We have modeled the beam propagation inside the liquid crystal cell by split-step beam propagation method and have calculated the optical field outside the cell by linear diffraction theory. The simulated on-axis far-field intensity is the typical Z-Scan curve and has a similar shape to experimental Z-Scan curves.; By comparing the experimental and simulated results in detail, it is clear that the modified optically induced molecular reorientation theory describes the behavior of the liquid crystal nonlinearity very well. The external electrical field generates a director distribution inside the liquid crystal cell. The optical field further tilts the directors to create a new nonlinearity profile. Therefore, the optical nonlinearity of liquid crystals is a function of the external voltage. Consequently, the voltage can control the magnitude of n2-eff, for future potential applications. |
