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

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

Quantitative video-rate hydration imaging of Nafion proton exchange membranes with terahertz radiation

Nafion membranes are considered as the industry standard electrolyte material for proton exchange membrane fuel cells. These membranes require adequate hydration in order to reach a high proton conductivity. The relatively high sensitivity of terahertz radiation to liquid water enables contrast to be observed for inspecting water presence in Nafion electrolyte membranes. Utilising a commercially available terahertz source and camera, this paper investigates the feasibility of a compact terahertz imaging system for visualising and quantifying liquid water during an ambient air desorption process for Nafion membranes of a wide range of thicknesses – NRE-212 (50 μm), N-115 (127 μm), N-117 (180 μm) and N-1110 (254 μm). We demonstrate that the terahertz imaging system is able to quantify liquid water in the 25–500 μm thickness range, estimate membrane weight change related to liquid water desorption, which correlated well against simultaneous gravimetric analysis and visualise the room temperature liquid water desorption process of a partially hydrated Nafion N-117 membrane

Productive performances of Maremmana young bulls reared following organic rules and slaughtered at 18 and 24 months of age

Fourteen Maremmana young bulls fed on pastures supplemented with mixed hay and concentrates, according to an organic method, were slaughtered at 18 and 24 months of age to evaluate in vita and post-mortem performances. At slaughter, carcass yield was determined and carcass evaluation according to EU rules wasmade. Physico-chemical characteristics of Longissimus thoracis (LT) and Caput longum triceps brachii (TB) muscles were determined. No differences for both carcass yield and quality were found between the two age groups. Animals of 24 months of age presented lighter and more yellow meat as well as higher hue angle and drip loss. TB muscle showed lower protein content and higher shear force, moisture, total lipids and ash than LT. The latter presented a higher percentage of C15iso, whereas TB showed higher values of C18:2n6, C20:3n6, C20:4n6, C18:3n3 and C22:5n3. Maremmana animals, irrespective of age, showed a low percentage of saturated fatty acids, good PUFA/SFA ratio and a great amount of unsaturated fatty acids of the n3 and n6 series, as well as a favorable atherogenicity and thrombogenicity indexes

Tunable Polarization-Induced Fano Resonances in Stacked Wire-Grid Metasurfaces

Stacked metasurfaces are being investigated in light of exploring exotic optical effects that cannot be achieved with single-layered metasurfaces. In this article, we theoretically demonstrate that metallic wire-grid metasurfaces with specific polarization properties have the ability to induce tunable Fano resonances when they are stacked. The developed original model - combining a circulating field approach together with an extended Jones formalism - reveals the underlying principle that gives rise to the polarization-induced Fano resonances. The theoretical frame is validated in an experimental proof of concept using commercially available wire-grids and a terahertz time domain spectrometer. This unexplored possibility opens an alternative path to the realization and control of Fano resonances by using stacked metallic metasurfaces. Furthermore, these findings suggest that the polarization can be used as an additional degree of freedom for the design of optical resonators with enhanced and tunable properties

Extracting the dielectric relaxation of water in thin Nafion membranes by terahertz spectroscopy

Terahertz time domain spectroscopy (THz-TDS) has been demonstrated to be capable of quantifying water uptake and retention properties of Nafion proton exchange membranes (PEMs). With a growing interest in thinner membranes, we analysis thin membranes to reveal its water uptake and retention propertie

Contactless graphene conductivity mapping on a wide range of substrates with terahertz time-domain reflection spectroscopy.

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapour deposited graphene films on sapphire, silicon dioxide/silicon and germanium. We validate the technique against measurements performed with previously established conventional transmission based THz-TDS and are able to resolve conductivity changes in response to induced back-gate voltages. Compared to the transmission geometry, measurement in reflection mode requires careful alignment and complex analysis, but circumvents the need of a terahertz transparent substrate, potentially enabling fast, contactless, in-line characterisation of graphene films on non-insulating substrates such as germanium.H.L. and J.A.Z. acknowledge financial support from the EPSRC (Grant No. EP/L019922/1). P.B.W. acknowledges EPSRC Cambridge NanoDTC EP/G037221/1. R.D., H.E.B. and D. R. acknowledge financial support from the EPSRC (Grant No. EP/J017671/1, Coherent Terahertz Systems). S.H. acknowledges funding from the EPSRC (Grant No. EP/K016636/1, GRAPHTED)