Date of Award

12-2024

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics, Applied

First Advisor

Chandra Yelleswarapu

Second Advisor

Mohamed Gharbi

Third Advisor

Stephen Arnason, Greg Sun

Abstract

The field of nonlinear optics has been crucial in advancing photonic technologies, as high light intensities induce nonlinear responses, enabling diverse advanced applications. Among the various nonlinear optical processes, third-order nonlinearities stand out due to their role in phenomena such as self-focusing, self-phase modulation, optical power limiting, and third-harmonic generation. However, traditional third-order nonlinear optical materials often face limitations in tunability and possess fixed optical properties, constraining their adaptability in dynamic photonic systems. To address these challenges, this thesis explores the development and implementation of electric field-enabled reconfigurable third-order nonlinear optical materials by harnessing the intrinsic properties of liquid crystals and nanomaterials. Plasmonic nanomaterials are distinguished by their strong light-matter interactions, which can be tuned by adjusting the size, shape, and composition of metal nanoparticles (NPs). Localized surface plasmon resonance (LSPR), the collective oscillation of free electrons on NP surfaces, enhances nanoscale electromagnetic fields. While LSPR has been widely studied, its potential for third-order nonlinear optical absorption remains underutilized. This thesis explores the enhancement of third-order nonlinear absorption by dispersing gold nanoparticles (AuNPs) in nematic liquid crystals (5CB). AuNPs forming dimers or structured assemblies exhibit strong plasmon coupling, amplifying local electromagnetic fields and nonlinear optical responses. The elastic and reconfigurable properties of nematic liquid crystals provide a tunable environment for AuNP alignment under external electric fields. Liquid crystal cells with planar-oriented, planar-twisted, and planar-degenerate configurations were prepared to investigate the effects of AuNP alignment on plasmon coupling and nonlinear absorption. Planar-oriented cells showed significantly higher linear and nonlinear optical absorption compared to degenerate cells. Polarization-dependent Z-scan results revealed that plasmon coupling was the primary driver of enhanced nonlinear absorption, with up to a 2-3-fold increase observed when the incident light was polarized parallel to the LC director axis. Other configurations exhibited angle-independent behavior, confirming the absence of directional alignment. By applying external electric fields, the LC orientation and AuNP plasmonic properties were dynamically controlled, enabling tunable nonlinear optical effects. These findings highlight the potential of AuNPs/5CB composites for adaptive photonic technologies, offering insights into plasmonic-LC interactions for advancing optoelectronics and dynamic optical systems.

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