HUSSAIN, FIDA (2025) NONLINEAR OPTICAL AND TERAHERTZ RESPONSES IN NANOMATERIALS. [Tesi di dottorato]

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Abstract

This research explores the nonlinear optical (NLO) and terahertz (THz) responses of advanced nanomaterials, with a particular focus on core-shell magnetic nanomaterials (MNMs), 2D antimonene, and transition metal dichalcogenides (TMDs) such as MoS₂ and WS₂. The study employs a combination of experimental techniques, including Z-scan for NLO characterization, THz spectroscopy (THzS) for terahertz analysis, and high-harmonic generation (HHG) to investigate ultrafast nonlinear processes in nanopatterned 2D materials. The first part of the thesis focuses on core-shell Fe₃O₄-MnO₂ magnetic nanomaterials, synthesized through liquid-phase exfoliation and characterized using techniques such as transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and dynamic light scattering (DLS). Z-scan measurements reveal significant third-order nonlinear optical properties, including positive nonlinear refraction (self-focusing) and nonlinear absorption attributed to two-photon absorption (TPA) and saturable absorption (SA). These findings establish core-shell MNMs as promising candidates for all-optical switching and optical limiting applications. The second part investigates the terahertz response of antimonene nanosheets (AMNSs) and quantum dots (AMQDs) prepared by liquid-phase exfoliation and size selection. THz spectroscopy confirms size-dependent carrier dynamics and strong confinement effects in antimonene quantum dots, highlighting their potential for integration into THz modulators. The third component explores HHG in nanopatterned MoS₂ nanosheets, demonstrating enhanced nonlinear conversion efficiencies due to localized field enhancement at nanoscale features. This work confirms the relevance of nanostructuring for controlling strong-field light-matter interactions in 2D materials, paving the way for compact, nonlinear optical devices. This part investigates cyrene-assisted exfoliation of WS₂ and MoS₂, addressing the challenge of sustainable processing of 2D materials. Cyrene, a green solvent, facilitates high-yield exfoliation while maintaining nanosheet quality and stability. This environmentally friendly approach enhances the scalability and sustainability of TMD processing, supporting future applications in photonics. This dissertation provides a comprehensive investigation into the synthesis, characterization, and optical properties of diverse nanomaterials, bridging nonlinear optics, terahertz science, and sustainable material processing. The findings contribute to the development of next-generation various devices, offering new insights into the design and optimization of functional nanomaterials for photonics and ultrafast technologies.

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