title: Terahertz Ultrafast Spectroscopy
title: A Paradigm for Material Characterization and Light Interaction
titleType: Subtitle
Creators: name: Lu, Yuezhen
Publisher: name: Lancaster University
Year: 2024
Subjects: Terahertz
graphene
Metamaterial
THz-TDS
superconductor

description: 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.