Terahertz Ultrafast Spectroscopy : A Paradigm for Material Characterization and Light Interaction

Lu, Yuezhen and Degl'Innocenti, Riccardo and Lin, Hungyen (2024) Terahertz Ultrafast Spectroscopy : A Paradigm for Material Characterization and Light Interaction. PhD thesis, Lancaster University.

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Abstract

Recent advancements in terahertz (THz) technology have heralded the development of cutting-edge sources, detectors, and modulators, catalyzing progress in fields such as molecular spectroscopy, 6G wireless communication, and biomedical imaging. Central to these innovations is terahertz time-domain spectroscopy (THz-TDS), which has garnered significant attention in both foundational and applied research. THz-TDS's unique capability to concurrently obtain time-domain and frequency-domain spectral data, along with its non-destructive nature, positions it as an indispensable tool for elucidating the physical and chemical properties of various materials without causing damage. This technique provides profound insights into diverse fields, such as molecular dynamics and vibrational modes of crystal lattices. Leveraging the state-of-the-art measurements enabled by THz-TDS, this thesis delves into the characterization of advanced materials and the examination of light interactions within artificial photonic structures. It showcases a variety of innovative THz-TDS applications aimed at propelling research in diverse areas: Initially, the thesis presents material characterization applications utilizing a standard THz-TDS system configured for both transmission and reflection measurements. Through the analysis of different transmission characteristics of samples, it highlights two exemplary methods: evaluating the surface roughness of 3D-printed metals to within micrometer accuracy, and determining the shielding effectiveness of bio-degradable THz absorbers up to 99.99999%. These methodologies underscore the pivotal role of THz-TDS in advancing next-generation manufacturing technologies. Furthermore, the thesis explores the investigation of artificial photonic structures using a general THz-TDS system. It details the initial measurements of two types of metamaterials—each designed for high Q-factor enhancement and increased light absorption—using both standard and cryogenic THz-TDS. The photonic samples, meticulously engineered through finite element simulations and crafted with cutting-edge microfabrication techniques, demonstrate a seamless correlation between THz-TDS results and simulation predictions, establishing a robust framework for future THz photonic development. In the concluding chapters, specialized THz-TDS techniques, such as aperture near-field scanning microscope (a-SNOM) THz-TDS and high-field THz-TDS, are introduced for detailed analysis of localized light interactions in high Q-factor resonators and high harmonic generation in integrated graphene metamaterials, respectively. The a-SNOM THz-TDS, with its sub-micron spatial resolution and comprehensive time-domain investigation capabilities, facilitates the mapping and quantitative analysis of tightly confined modes, including weakly radiative modes through a cross-polarized configuration. This insight is invaluable for advancing metamaterial-based optoelectronic platforms in THz photonics. High-field THz-TDS is employed to generate tunable, high-power pulses, intensifying the nonlinear effects in materials. Graphene, with its massless electron dynamics and linear band characteristics, emerges as the most effective material for generating high-order harmonic generation in the THz range. The coherent measurement capabilities of high-field THz-TDS unveil distinct high-harmonic trends in graphene-based opto-electronics, showcasing the potential for the development of future THz components.