Lateral strukturierte Oberflächen zur THz-Strahlmanipulation

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.