Nonlinear Optical Processes In Liquid Crystals And Applications In Optical Switching

Open Access
Author:
Zhao, Shuo
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 26, 2015
Committee Members:
  • Iam Choon Khoo, Committee Chair
  • Iam Choon Khoo, Dissertation Advisor
  • Shizhuo Yin, Committee Member
  • Timothy Joseph Kane, Committee Member
  • Jun Huang, Committee Member
Keywords:
  • Nonlinear Optics
  • Liquid Crystals
  • Optical Switching
Abstract:
This dissertation research completes the exploration and development of the theoretical framework for collective liquid crystalline optical nonlinearities capable of response speeds in the microseconds – nanoseconds scale, which is more than 1000 times faster than the conventional liquid crystal (LC) response speed. Also explored in this dissertation are utilizations of these new discoveries to achieve all-optical switching. This work demonstrates all-optical switching using nonlinear orientational and thermal effects, respectively, in pure and dye-doped twisted nematic liquid crystal (TNLC) cells set between crossed polarizers. In the former case, the flow of liquid crystal molecules is generated by Maxwell stress and thereby exerts a torque on the liquid crystal. The resulting reorientation changes the effective birefringence of the liquid crystal, affecting the overall transmission. In dye-doped twisted nematics, the absorption of dye enhances laser heating in the liquid crystal, which leads to reduction of the order parameter and the corresponding macroscopic birefringence, finally making the transmission drop to zero. Following the sequences of these processes, detailed modeling for collective responses of liquid crystal and the time evolution of transmissions under short laser pulses are presented. Besides theoretical description and modeling, we demonstrate the nonlinear optical switching experimentally. The switching threshold and time are consistent with the simulation results. While dye-doped liquid crystals have a low threshold for nonlinear switching, pure twisted nematics possess high transparency in the entire visible and near-infrared spectrum. These findings are believed to advance the current arsenal of high-performance materials for integration/use in advanced optical systems designed for sensor protection; laser hardening; and other beam/image switching, sensing, and processing operations.