Studies of liquid crystal response time

In this dissertation, the response time issue of the liquid crystal (LC) devices is investigated in meeting the challenges for display and photonic applications. The correlation between the LC director response time and the optical response time is derived theoretically and confirmed experimentally.; This thesis begins with a description of liquid crystal materials and their physical properties, and then introduces the simulation methodologies. These brief but relevant introductory chapters pave the foundation for fully understanding the dynamic response of LC devices. After that, three chapters pertaining to optical response time are presented.; A major contribution of this thesis is that, based on the small angle approximation, we derive rigorous analytical solutions for correlating the LC director response time to its consequent optical response times (both rise and decay) of a vertical-aligned nematic LC cell. Pretilt angle effect on the LC dynamics is studied, and it is found that a modified rotational viscosity is needed in order to explain the experimental results. Grayscale switching is also analyzed numerically. This work successfully fills the gap in the literature of LCD switching dynamics.; An important effect related to response time, backflow is analyzed using a homogeneous LC cell in an infrared wavelength. Due to the relatively high voltage applied in an optical phased array (OPA), the backflow effect which takes place in the first few milliseconds influences the LC response time dramatically. However, the corresponding Leslie viscosity coefficients, which are crucial in investigating the dynamic response of LC devices with backflow, can hardly be found in the literature. A new effective approach to estimate the Leslie coefficients of LC mixtures based on MBBA data is proposed in this dissertation. Using this method, the Leslie coefficients of the LC material under study can be extracted based on its order parameters. The simulation results agree with the experimental data very well. This method provides a useful tool for analyzing the dynamic response including backflow, in order to obtain accurate optical response time under a high biased voltage.; Cell gap is an important factor affecting the LC response time. Usually a thinner cell gap is chosen to achieve faster response time, since normally both rise and decay times are known to be proportional to d 2. However, they are valid only in the Vth < V < 2 Vth region, where Vth stands for the threshold voltage of an LC cell. In the large voltage region where Vpi < V < Vi, the optical decay time is independent of d. In this thesis, we find that between these two extremes the response time is basically linearly proportional to d. Our analytical derivation is validated by experimental results. Therefore, in the whole voltage region, the physical picture of the optical response time as a function of the cell gap is completed. This analysis is useful for understanding the grayscale switching behaviors of the LC phase modulators. With the help of cell gap effect in phase modulator, we can effectively reduce the response time by using a thick cell or double cells to achieve the intended phase retardation in an anticipated operating wavelength.; In conclusion, this dissertation has solved some important issues related to LC optical response time and supplied valuable tools for scientists and engineers to numerically analyze the LC dynamics.