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 factors. Here, we focus on three alignment techniques for aligning rod-like LC molecules and highlights the photonic devices enabled by these techniques: 1) Two-photon polymerization direct-laser writing-induced alignment, 2) Weigert effect-based reversible photoalignment, and 3) electric field-assisted alignment in polymer-dispersed liquid crystal (PDLC) systems. 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 reversible 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 is 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. By such a method, two miniature high-quality microlens arrays are fabricated and further 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. For a traditional PDLC system, the LC alignment is random from droplet to droplet, and the operation voltage of the active PDLC is too high to be employed in practical applications. Here, we establish a method to perfectly align LC droplets in a PDLC system and use it as a passive film. The well-aligned passive PDLCs exhibit polarization- and angle-dependent light scattering that can be engineered through composition tuning. Two kinds of selective scattering films are demonstrated: The first kind scatters obliquely incident light but is highly transparent for normally incident light, and the second kind scatters normally incident light but is more transparent for obliquely incident light.
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Doctor of Philosophy (Ph.D.)
College of Optics and Photonics
Optics and Photonics
Optics and Photonics
Length of Campus-only Access
Doctoral Dissertation (Open Access)
He, Ziqian, "Novel Liquid Crystal Photonic Devices Enabled by Liquid Crystal Alignment Engineering" (2021). Electronic Theses and Dissertations, 2020-. 510.