Liquid crystal technologies for enhanced optical system functionality
Keywords: analog control of phase
tuneable volume scatterers
laser speckle reduction
speckle suppression
laser projectors
LC spatial light modulators (lc-slms)
optical wavefront manipulation
biomedical microscopes
near-eye displays
liquid crystal technologies
augmented reality
LC diffusers

This thesis aims to investigate and develop novel liquid crystal (LC) technologies that can either manipulate the optical wavefront (phase) of light in a controlled way or improve the imaging quality by reducing the appearance of laser speckle by manipulating the optical wavefront in a pseudo-random way. Control of these properties is of great importance for a vast range of photonics and optics applications; for example, in LC spatial light modulators (LC-SLMs), near-eye displays, laser projectors and biomedical microscopes. Among these applications, laser sources have been an indispensable element in modern imaging applications. This is because they exhibit many essential features (e.g., a high intensity, a narrow spectral linewidth, and directionality). However, due to the high coherence of laser light, the issue of speckle arises, which can badly plague the clarity and the appearance of an image. The essential reason for such degradation is because the coherent laser light will scatter when it interacts with an optically rough surface and forms constructive or destructive interference on the imaging plane.
To combat the formation of speckle and improve the performance of a laser- based imaging system, one approach is to generate as many decorrelated speckle patterns as possible within a short period of time. Existing methods for reducing speckle typically suffer from being either bulky, consisting of motors that introduce vibration to the system, or they do not provide an acceptable degree of speckle suppression. A key part of the research presented in this thesis aims to develop LC diffusers that can reduce the appearance of speckle in a range of different display and imaging applications, including laser projector, laser microscopy, head-up display, holography and polarimetry. The technique involves using highly twisted (helicoidal) liquid crystalline mesophases, doped with various dopants in the form of redox and zwitterionic dopants, to act as tuneable volume scatterers. The recipe for the optimum performance is BL006 + 3 wt.% R5011 + 0.5 wt.% Reichardt’s dye (zwitterion dopant) filled into a 40 µm anti-parallel cell. Not only does this approach provide direct control of the speckle contrast (C), allowing it to be reduced to very low levels, C = 0.07 (93% reduction of the speckle noise), but it does so without the need for bulky mechanical parts, moving components or expensive optical elements.
Another important optical element that this thesis investigates are potential configurations for next-generation LC-SLMs. LC-SLMs are a key element in both imaging and optical wireless communication systems. In general, an LC-SLM can be used to control and shape the wavefront of light propagating in an optical system and hence control the structure of optical illumination, by adjusting the voltage applied to the active LC layer. Among all wavefront correction applications, the phase-only mode is found to be the most promising for applications such as augmented reality near-eye displays and free-space optical beam-steering. The research presented in the latter part of the thesis explores new configurations, such as the nematic Pi-cell, nematic Fréedericksz cell, uniform lying helix chiral flexoelectro-optic LC device, and hybrid aligned nematic cell, that improve the switching speed of an SLM, but also maintain analogue control of the phase (from 0 to 2π) at room temperature via controlled modulation of the optical wavefront. Simulations that take into account factors such as the thickness of the boundary layer, the back-flow effect in the context of the LC hydrodynamic behaviour, and dielectric coupling in a flexoelectro-optic material are examined. These simulations play a crucial role in guiding the optimisation of the device configuration. The development of an analogue, fast-switching phase modulator would enable SLM technology to be used in a wider range of applications.