Hydrodynamics of Nematic Liquid Crystals for Diffractive Optical Elements

Nematic Liquid Crystals (NLCs) are used widely as adaptive optical materials in devices such as lenses, beam steerers and displays. Usually in NLC Diffractive Optical Elements (DOEs), limitations exist in one or more essential parameters, such as diffraction angle, diffraction intensity, aperture size or adaptive behaviour.
This thesis will investigate an unusual method to create NLC DOEs, through inducing hydrodynamic flow within the materials. Here, several techniques are used to induce periodic flow patterns within the materials, which result in periodic changes to the device’s optical properties. These periodic structures are evaluated for potential as adaptive optical components.
Fraunhofer diffraction theory is introduced as a means to evaluate the potential of various DOEs theoretically. Details of a computer programme developed during the project is presented, which allows calculation of Fraunhofer diffraction patterns. This programme is used to provide quantified analysis of losses in diffraction efficiency caused by imperfect or non-optimized DOEs. The application of these results may be used in aiding DOE device design, which will be discussed.
The first method used to create hydrodynamic domains uses a low frequency electric field applied across the NLC. This induces periodic ion flow within the material, leading to the NLCs adopting a state of electrohydrodynamic instability (EHDI). Of several EHDI modes identified, the 1D Normal Roll (NR) mode was most promising as a DOE. The periods of these gratings are strongly dependent upon device spacing (d). In all calamitic materials, the grating period continuously varied from d to as electric field frequency was increased. This lead to a simultaneous decrease in diffraction efficiency. Elastic constant dependency on EHDI is also investigated, where a material of low k33 is created using a bent dimeric mixture. This displays a desirable property of lower grating period by a factor of around 1.5.
The second method of creating hydrodynamic patterns in NLCs uses bulk and surface acoustic waves. Acoustic wave transmission in bulk NLCs is discussed. Techniques of measuring the speed of sound (vs) in fluids are given, which are used to obtain a value for vs in the NLC mixture E7 of 1720±70ms-1 at ambient temperatures. NLC structural changes under acoustic fields are examined. These investigations are used to create a novel device where the surface acoustic wavelength was varied using a chirped electrode structure. This created a hydrodynamic grating of continuously variable pitch from 100 μm to 450 μm using frequency modulation
The findings and performance of the hydrodynamic gratings investigated are evaluated in the context of currently available DOE technologies. Possible further device improvements and theoretical limits using the results from Fraunhofer diffraction modelling are discussed