Two-photon polymerisation direct laser writing for liquid crystal photonics

This thesis investigates the use of two-photon polymerisation direct laser writing (2PP-DLW) in liquid crystal materials, with an emphasis on the development of applications in photonics. 2PP-DLW is a 3D printing technique employed in the fabrication of multi-dimensional structures with micro- and nano-scale dimensions. As a maskless microfabrication method, it is typically used as a rapid-prototyping tool for solid polymer objects. However, there is increasing interest in the technique as a way to manufacture functional materials and devices that cannot be produced via other means. Crucial to this effort is the development of smart resins, whereby the physical properties of a resin can be varied and controlled in-situ during the fabrication process. In this thesis, liquid crystal mixtures containing reactive mesogens are shown to be a promising smart resin material due to their anisotropic physical properties and ability to respond to external electric fields.
Stabilisation of the voltage-dependent states of the nematic Fréedericksz cell via the fabrication of polymer micropillars with 2PP-DLW is described and analysed. The polymer structures are studied with polarised optical microscopy and an electrically-tunable visibility phenomenon is characterised and explained. By synchronising the fabrication of polymer features with the application of different voltages, reconfigurable images are inscribed into liquid crystal devices. Potential applications are explored with a demonstration of a prototype anti-counterfeiting security marking.
Topological defects in nematic liquid crystal pi-cells are fabricated and electrically-controlled via the fabrication of topologically discontinuous polymer structures using 2PP-DLW. A control system for electrical-tuning of defects is developed and advanced concepts for confinement of disclination lines are demonstrated. The transport of a microparticle by a defect is presented with the potential for applications in defect templates for micro/nano assembly.
Switchable diffractive optical elements (DOEs) are fabricated with 2PP-DLW in liquid crystals and their diffractive behaviour is characterised. A model of a laser-written diffraction grating is developed using a continuum theory approach to director simulations and a wave-optics approach to the diffraction of light. The 3D capabilities of 2PP-DLW are exploited to fabricate switchable bilayer DOEs and computer-generated holograms.
This work in this thesis has important technological implications in both the development of smart resins for 2PP-DLW as well as in the development of advanced alignment and control techniques for liquid crystal devices.

Keywords: laser microfabrication
smart materials
2PP-DLW
3D printing
3D laser printing
polymerizable liquid crystals
liquid crystals
two photon polymerization
liquid crystal devices
TPP-DLW
nanoprinting
direct laser writing
DOEs
smart resins
3D nanoprinting
laser micromachining
laser fabrication
functional materials
reactive mesogens
liquid crystal polymers
topological defects
diffractive optical elements
Advisor: Morris, S
Elston, S
Booth, M