Rohrbach, David (2023). Enabling Technology and Proof-of-Principle Experiments for Strong Field Terahertz Spectroscopy. (Thesis). Universität Bern, Bern
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Abstract
Electromagnetic radiation in the terahertz (THz) frequency range, from 0.1 THz to 10 THz, encompasses elementary excitations such as lattice vibrations in solids, rotational transitions in molecules, and the dynamics of free electrons. Recent breakthroughs in the generation of ultrafast high-field THz transients have not only enabled selective studies of such excitations, but also the creation of new transient states of matter, opening up a wide range of phenomena to be investigated in chemistry, biology, and materials science. Despite all advances, some experiments require electric or magnetic field strengths beyond what current THz sources can provide. Hence, field enhancement structures have attracted quite some attention. Additionally, control over the THz polarization state is often critical, especially when the symmetry of a particular excitation requires a very specific field direction. In this thesis, we introduce novel concepts and structures to locally enhance the THz field beyond the corresponding values in free space, and demonstrate new technologies to manipulate the polarization state of broadband THz pulses. For experimental characterization, we use a THz time-domain spectrometer based on photoconductive antennas, a spatially resolved near-field electro-optical sampling setup with the capability to measure all three electric field components, and intense THz sources based on optical rectification in lithium niobate or OH1. We complement the experimental findings, unravel underlying principles, and optimize structures by means of numerical simulations. This thesis begins with a demonstration of three-dimensional printing technology as a cost-effective and time-saving tool for fabricating THz wave- and phaseplates. We fabricate and demonstrate simple elements such as quarter- or half-waveplates, as well as more complex structures such as q-plates or spiral phaseplates for generating THz pulses carrying angular momentum. Next, we design field-enhancing sub-wavelength structures and use them for various applications, for example, to develop short-period undulators for future compact x-ray sources based on free electrons, or to study ultrafast mode switching in metamaterials based on field-induced carrier generation in semiconductors. We demonstrate THz Stark spectroscopy as a novel tool for time-resolved studies, allowing for the first time inferences about the static and dynamic electrochemical properties of molecular and, in particular, bio-molecular systems in their natural environment. Finally, we present an integrated THz waveguide platform featuring low loss, vacuum-like dispersion, and local field enhancement. Hence, it not only allows for reshaping-free propagation of single-cycle THz pulses, but also improves THz pump visible probe spectroscopy over an extended interaction length. We then use this platform to demonstrate THz-induced alignment of molecules in the gas phase to an extent that could not be achieved with conventional setups.
| Item Type: | Thesis |
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| Dissertation Type: | Cumulative |
| Date of Defense: | 30 March 2023 |
| Subjects: | 500 Science > 530 Physics 600 Technology > 620 Engineering |
| Institute / Center: | 08 Faculty of Science > Institute of Applied Physics |
| Depositing User: | Hammer Igor |
| Date Deposited: | 06 Apr 2023 14:52 |
| Last Modified: | 30 Mar 2024 23:25 |
| URI: | https://boristheses.unibe.ch/id/eprint/4222 |
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