title: Enabling Technology and Proof-of-Principle Experiments for Strong Field Terahertz Spectroscopy
Creators: name: Rohrbach, David
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 530 Physics
subject: 620 Engineering

description: 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.