Molecular Design Of Fast Response Liquid Crystals And Research On The Optimization Of Corrector Structures

Liquid crystal wavefront corrector(LCWFC) is a critical component of liquid crystal adaptive optical system, which response speed can directly affect the correction quality of liquid crystal adaptive optical system. The fundamental way to improve the response speed of LCWFC is to improve the performance of liquid crystal materials and optimize the structure of the corrector. Based on the elastic theory of liquid crystal, and to fix the phase modulation as the constraint conditions, the response speed of LCWFC is proportional to the square of birefringence ?n of the liquid crystal material, inversely proportional to the liquid crystal material viscosity γ1. Therefore, the improvement of the material properties of LCWFC is to improve the ?n of the liquid crystal material and at the same time reduce the γ1. The device structure optimization of LCWFC involves the following aspects: how pretilt angle can affect the response speed; how to achieve the optimal cell gap accurately and reliably; the mechanism to improve the response speed using the parallel mode and a comparison with the anti- parallel mode.Firstly, based on the classical theory, a harmonic oscillator model is used to analyze the interaction of light with the liquid crystal molecules, which clearly shows that the birefringence ?n of the liquid crystal is closely related to the anisotropy of the electronic polarizability ?α. The physical mechanism of the birefringence ?n is achieved from a quantum mechanics viewpoint, it has been known that the anisotropy of the electronic polarizability ?α is proportional to the square of the radius of liquid crystal molecule, inversely proportional to the energy difference of the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO). Combining the valence bond theory, in order to reduce the energy difference between HOMO and LUMO, increase the molecular polarizability and ultimately improve the liquid crystal birefringence ?n, the length of conjugated system must be extended. The feasibility of calculation of anisotropic mediums including liquid crystal using the Vulks equation has been studied by comparing the calculated value and experimental results. The effect of size and type of the central group on the birefringence has been studied, on the basis, we have designed a central group which consists of the polycyclic aromatic hydrocarbon. The molecular structures of four kinds of high ?n were determined through the analysis of the influence of lateral axial polar groups, the polar group and the flexible tail chain on the ?n of liquid crystal.A method to study the rotational viscosity and phase state of liquid crystal has been proposed based on the dynamic simulation of liquid crystal molecules and we have provided the detailed theoretical basis for the rotational viscosity calculation. According to the simulation, the liquid crystal molecules of high ?n in the vicinity of 313 K will be in the smectic phase, so we can’t compare their rotational viscosity directly through the simulation of monomer. To solve this problem, a method to compare the rotational viscosity of liquid crystals based on the molecular dynamics of mixtures has been established, which feasibility and accuracy have been verified using literature and experimental data. By comparing the birefringence, rotational viscosity and phase state of the liquid crystal molecules of high ?n, we found that the liquid crystal containing the polycyclic aromatic hydrocarbon have a high ?n and a relatively low rotational viscosity, which response performance can be even better than the liquid crystal containing the phenyl-acetylene benzene. It is predicted roughly that the incorporation of two designed molecules into the mixtures used in our laboratory will improve the response speed of 20%. So the liquid crystal with the polycyclic aromatic hydrocarbon has a great potential in improving the response speed of LCWFC.The effect of pretilt angle on the response speed of LCWFC has been discussed at a fixed phase modulation, and then the following conclusions are drawn: the response speed of LCWFC decreases with the increased pretilt angle, so in order to improve the response speed of LCWFC, the pretilt angle must be reduced as much as possible to about 1 degrees. The physical mechanism and theoretical basis for obtaining the optimal cell gap accurately from a numerical calculation and a single photoelectric measurement have been studied thoroughly and systematically, these methods have been analyzed theoretically and confirmed experimentally. Finally, the mechanism to improve the response speed using the parallel mode and a comparison with the anti- parallel mode have been discussed, and it is concluded that the parallel mode can be used to accelerate the response speed of LCWFC under the condition of accelerating molecular rotation obviously.A systematic study on the improvement of response speed of LCWFC has been performed combining theoretical analysis and experimental analysis, the reference data also included. We believe that the new liquid crystal molecules and the new structure of the device designed in this paper will further improve the response speed of LCWFC, which must greatly contribute to improve the imaging effect of the liquid crystal adaptive optical system.