Spektrale Verbreiterung von Terahertz-Pulsen mittels eines Schottkykontakt-Wellenleiters

Spectroscopic techniques have become ever more important in the field of research and development. Especially THz time-domain spectroscopy is an emerging field with versatile applications allowing for the characterization of the optical and dielectric properties of materials. However, the bandwidth of THz emitters are typically in the range of only a few THz and are limited by exterior factors as the used laser system or the techniques available for the emission and detection of THz radiation. The most common emitter and detector schemes use photoconductive antennas to emit and detect THz radiation. Indeed, the bandwidth of a THz antenna is limited by the design and the used material. Yet, to get the most insight into physical properties it is preferable to have a broad THz spectrum. However, the broader the desired spectrum the more complicated the setup gets. It is shown in this thesis that this limitation can be compensated through a new Schottky waveguide approach. As this tool works independently of the used laser system or emitter, it can therefore be implemented in almost every setup to gain broader THz frequencies. The first part of the dissertation discusses the physics behind the Schottky contact waveguide by applying a phenomenological model to explain the functionality of the device. The underlying principle of the broadening effect corresponds to the rectifying properties of the Schottky contact inside the waveguide. Unambiguous attribution of the spectral extension of waveguidepropagating pulsed radiation is given in a study of a GaAs-goldbased Schottky contact with n-doped intersection and a test sample only consisting of intrinsic and highly doped semiconductor material. On the high frequency edge of the spectrum a signal increase of more than two orders of magnitude could only be observed on samples with rectifying junctions. Not all observed effects could be explained in this work e.g. the shift of the zero-crossing of the gold sample at high electrical fields still remains unsolved. The proposed Schottky contact waveguide approach is scalable and we expect, that by tailoring waveguides with specific properties, this technique to become employable in a wide range of applications where a broad spectrum is needed. This work can be considered as the first step towards THz-whitelight generation independently of the used lasersystem. The thesis contains a second part on experiments which I carried out during a research stay in the group of Prof. Tony Heinz at Columbia University in the City of New York from 2013 to 2014. Again the light-matter interaction of thin material layers is studied. Yet, this work is only loosely linked to the main part of the thesis. Since graphene has been established as an excellent basis for fundamental research and applications, researchers all over the world have been intensively exploring other layered materials that form stable and atomically thin two-dimensional (2D) layers. These 2D materials, such as semiconducting transition metal dichalcogenide monolayers (e.g.MoS2,WS2) or insulating hexagonal boron nitride (h-BN) exhibit very different electronic and optical properties from graphene, while sharing its mechanical robustness and integrity. The focus of the second part of the thesis lies on the light matter coupling of nanosheets of organic inorganic perovskite crystals (OIPC). The OIPCs crystals with thickness down to that of a single unit cell (2.4 nm) were prepared by mechanical exfoliation. These materials differ from other known 2D van-der-Waal layers in being hybrid: Organic compound intrinsically bound into a layered inorganic crystal. To study the electronic and excitonic behaviour of OIPCs we have carried out optical spectroscopy measurements. The most remarkable feature is the extremely strong absorption (25 %) at room temperature at the excitonic transition with a spectral width of about 100 meV. Taking into account the reduction of the effective electric field at the surface of the dielectric substrate we estimate the intrinsic absorption, which we would measure for a suspended layer, to be even higher, i.e. about 37 %. The strong decrease of the layer thickness leads to a significant increase of the exciton binding energy up to 490 meV due to the change in the dielectric environment. The influence of the interface between the substrate and the OIPC modifies the structure of the material. For example an always present structural phase transistion in thick OIPCs which normally occurs slightly below room temperature is surpressed in about half of the measured samples. Our suggestion is that the intimate contact between ultrathin layers and substrate inhibits the expected phase transistion. The unique properties of the OIPC-nanosheets make these materials attractive for investigating novel optical and transport physics as well as studying their surface and interface properties.