Dissertation Title:
“ Novel Liquid Crystal Photonic Devices Enabled by Liquid Crystal Alignment Engineering ”
Abstract:
Liquid crystals (LCs) are self-assembled soft materials composed of certain anisotropic molecules with orientational orders. Their widespread applications include information displays and photonic devices, such as spatial light modulators for laser beam steering and tunable-focus lens, where achieving desired LC alignment is pivotal. In general, LC alignment is influenced by several factors, including chemical bonding, dipolar interactions, van der Waals interactions, surface topographies, and steric forces. This dissertation focuses on two new approaches: 1) two-photon polymerization direct-laser writing, and 2) Weigert effect-based reversible photoalignment, for aligning rod-like LC molecules, and highlights the photonic devices enabled by these techniques.
With the help of advanced two-photon polymerization systems, nano-grooves with arbitrary orientations can be easily created on a variety of surfaces. The geometric topography helps align the LC molecules parallel to the groove direction. Alignment on a planar surface, on a curvilinear surface, and even in the bulk can be realized. Based on the patterning ability, three photonic devices are highlighted: a switchable geometric phase microlens array, a tunable compound microlens array, and a polarization-independent phase modulator.
For Weigert effect-based photoalignment, how to achieve space-variant linear polarization field is crucial. Here, two approaches are investigated: the direct projection method and the counter-propagating wave interference exposure method. Using the direct projection method, an LC Dammann grating with pixelized binary phase profile was achieved. Such a method relies on a spatial light modulator and is convenient for creating pixelized alignment that has abrupt changes from pixel to pixel. On the other hand, the interference exposure method can generate continuously and smoothly changing LC alignment. Using such a method, two small f-number microlens arrays are fabricated and assembled into a planar telescope. Further characterizations reveal the high optical quality of the fabricated devices, which not only ensures their adoption in practical applications, but proves the powerful planar alignment patterning capability of the photoalignment materials.
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