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Abstract
The presented PhD thesis deals of the interaction of terahertz radiation with patterned
surfaces. Two aspects of this broad field of research are discussed in more detail: plasmonic
structures supporting THz surface waves and THz metamaterials.
From an electromagnetic point of view, solids can be classified into three categories.
Dielectrics, semiconductors, and metals. At first glance, metals are perhaps the least
interesting group of materials. Their characteristic properties are the very good thermal
and electrical conductivtiy. Due to the filled conduction bands of the metallic bond their
is no transmission of electromagnetic waves in a very wide band of frequencies spanning
from 0 Hz up to the optical range and even beyond. Accordingly, the reectivity is very
high. Hence metal surfaces are classically used as mirrors for optical frequencies. This
relatively simple application is in contrast to the extraordinary importance of metals for
electronic applications. In principle, nearly all optical devices, which involve metals are
only exploiting the high reectivity of metals, i.e. metallic waveguides, which are frequently
used waveguides in the GHz frequency range or all metallic diffractive optics. Further, two
prominent important examples being metallic diffraction gratings or pinhole diaphragms.
Plasmonic applications are based on a different concept.
It has been shown, that on metallic surfaces the free moving electrons can form a
collective charge density oscillation. These oscillations in turn generate an electromagnetic
field which is bound to the surface. The fundamental properties of the phenomenon known
as surface plasmon polaritons (SPP) have attracted a great deal of interest in recent years,
not only in the THz range. As an example, surface plasmon resonance spectroscopy is a
spectroscopic method which exhibits this phenomenon and is mostly used in biochemistry.
Surface plasmons are also discussed for the further miniaturization of photonic circuits,
since they have a property known as wavelength shrinking. Near the so-called surface
plasmon resonance (SPR), the wavelength can be reduced to a tiny fraction of the free-space
wavelength. This phenomenon is dicussed among other interesting properties of SPP to
increase the resolution in imaging methods, or spectroscopic examination of small amounts
of specimen.
Challenging for the THz plasmonics is that most metals behave like ideal conductors
for terahertz frequencies (0;3 - 10 12 Hz .. 5 - 10 12 Hz). Hence, due to the high free charge carrier density, the THz-beam will penetrate the metal surface only marginally. As a result,
THz surface plasmons experience a small attenuation in the direction of propagation and
can thus propagate over many wavelengths. This is an advantage, but a closer look at the
properties of surface waves reveals that they are actually not bound to the surface in this
limiting case. The corresponding solution of Maxwell's equations was already formulated
by Sommerfeld 1899. Hence, this type solution is also called Sommerfeld-Zenneck wave.
Sommerfeld waves can be thought of as plane waves in grazing incidence with respect to
the surface. Without binding to the surface, plasmonic structures for the THz range can
not be realized.
This particular problem can be overcome by the realization that a periodic structuring
of the metal surface in the subwavelength range, such as periodic corrugations, enables
strongly bounnd surface waves. The altered metal surface can be considered as an effective
medium with reduced surface charge density. It is therefore possible to tailor to a certain
extent the properties of the THz plasmon polaritons by structuring the metallic surface.
The resulting designer surfaces are also called plasmonic metamaterials, i.e. artificial,
periodically patterned surfaces. This circumstance draws the line to the second topic of
the presented thesis, the aerosol-jet printed metamaterials. Analogously, metamaterials
are dielectrics with artificial electromagnetic properties. The dielectric properties of these
materials are tailored by surface-applied metallic patterning in the sub-wavelength range.
A variety of exotic optical phenomena, such as the negative refractive index, have already
been demonstrated by metamaterials. In this thesis, almost exclusively THz time-domain
spectroscopy (THz TDS) is used for measuring terahertz field propagation. THz TDS
which is a phase and frequency-resolved broadband spectroscopic measurement method
with an extraordinary high SNR, provides an attractive experimental tool. Additional
measurement methods for the direct determination of the near field are also available
within this frequency range. Further the technologically comparatively simple production of
structured metal surfaces and metamaterials make the terahertz frequency range attractive
for the design of patterned surfaces.
The work is divided into four chapters. The first chapter 2 introduces the reader to the
basics of terahertz time-domain spectroscopy. The chapter itself is divided into three parts:
In the �rst part, section 2.1, the generation and detection of terahertz radiation and the
method of terahertz time-domain spectroscopy is explained briey. The next section 2.2
gives an overview of terahertz beam profile measurements. These are performed using
a variety of detectors, such as a microbolometer-based THz camera, a photoconductive
antenna and a Golay cell. The beam profile measurements are important prerequisits
for e.g. the plasmonic beam-forming element and the THz metamaterials measurements.
Section 2.3 deals with the calculation of the dielectric properties from time-domain measurements
and the appropriate error propagation. Further the inuence of the delay unit on
the measurement uncertainty is discussed.
Chapter 3 introduces the plasmonic Bessel beamformer. A first motivation for studying
these new class of beam-forming THz optics are the interesting underlying physical principles
on its own. A secondary motivation is the reduced space requirement of these types
of devices, especially in the transversal direction. Compared to conventional lenses, the
emitter could be integrated directly into the plasmonic structure. Additionaly, the shown
beam profile, a Bessel beam, is interesting on its own. It has been discussed to use bessel
beams for THz imaging, due to its so-called stalk focus. This type of long range focused beams will help to improve the depth resolution in THz tomographic methods. First, the
theoretical and numerical investigations for engineering the design of the beam former are
presented and later the detailed experimental verification of the structure in the far and
near field is presented.
The grating coupler concept is used in the Bessel beamformer to emit the otherwise
bound surface wave into the free space. The very same concept is exploited in chapter 4,
where a 3D-printed grating coupler for 120 GHz is demonstrated. 3D printing of THz devices
is a current area of research. The attractive manufacturing method reduces drastically
the duration from design, through simulation, to the finished device, to its experimental
characterization. Rapid prototyping for THz devices has never been so fast before. The
chapter begins by discussing the basic properties of dielectric waveguide structures. Subsequently,
the geometry parameters are determinded via simulations. Later the grating
coupler is produced and experimentally characterized with a microwave system.
The chapter 5 presents the research results of the very recent aerosol-jet printed metamaterial
structures. This, for the production of THz metamaterials new technology, makes it
possible to produce almost any conductive structures with line widths in the 10 µm range.
Two additional features make this method interesting. First, in contrast to established methods
such as photolithography, the prototypical production of a large number of different
samples in very small quantities and their iterative improvement is cost-efficient and fast.
Second, the conductive ink can be applied to a vast number of exible substrates. In this
work PET foils are used. The obtained results demonstrate that this printing process can
be used for the production of metamaterial structures in the THz range. To this end a
detailed microscopic examination, including even SEM images, of the fabricated structures
is carried out. Later, the angular dependent THz transmission is measured via THz TDS.
The last chapter 6 gives a summary of the results and an outlook for further research
directions.
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Dates
Created: 2018Issued: 2018-03-13Updated: 2018-03-13
Faculty
Fachbereich Physik
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
DFG-subjects
PlasmonikMessgenauigkeitTherahertz
DDC-Numbers
530
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Jahn, David: Lateral strukturierte Oberflächen zur THz-Strahlmanipulation. : Philipps-Universität Marburg 2018-03-13. DOI: https://doi.org/10.17192/z2018.0093.
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This item has been published with the following license: In Copyright