Filterkonzepte für den terahertz-Frequenzbereich

This work addresses filters and modulators for the terahertz regime and studies the fundamentals that lead to new characteristics or new devices. The discussion of the results starts with an alternative approach to fabricate two dimensional filter surfaces. The method is, in comparison to photolithographic techniques, very flexible and additionally allows for the fabrication of freestanding filters. It is based on laser ablation and can remove certain materials from a sample substrate or foil. This work demonstrates the proposed technique and characterizes some well-known devices like band pass filters or polarizers that are fabricated with this technique. In the following sections, different cavities are adapted, utilized and characterized as filters and modulators for the terahertz regime. In particular, Fabry Pérot based filters with very sharp transmission bands and Q-Factors in the region of 500 are discussed. Due to their robust and simple to handle design they can be used very easily. They consist of two metal wire gratings on both surfaces of a semiconductor wafer with very parallel edges. In the setup, they can be tuned in a small frequency region by temperature control or by rotating the sample. Furthermore, at the end of this section, there is given a small outlook that discusses the possibility of using such cavities to enhance sensitivity for probing very thin samples. The following section widens the concept from a filter to a modulator. For this purpose, the optical excitation of free carriers inside the semiconductor substrate is used to achieve an efficient damping of the transmission. The fabricated sample reaches a modulation depth of 90% and a transmission maximum of about 80% at a selected frequency of 942 GHz. Additionally, the discussion includes statements about effects that could improve or deteriorate the modulation characteristics. After that, a thin film is used inside the cavity that can change its phase state from an insulating to a conducting phase via different stimuli. This phase transition can be triggered very fast, but is restricted by a hysteresis in reverse. In testing, the device switches sharp transmission bands by a temperature change and reaches a finesse of about 6. The transition happens in a temperature region smaller than 10°C and possibly allows for future applications to reduce the threshold intensity for a second, yet faster stimulus, which could be optical or electrical. Finally, this work concentrates on filter surfaces which consist of periodically arranged metallic resonators on a dielectric substrate. These unit cells exhibit resonances that are mainly given by the geometry of the resonator. The resonance wavelengths are distinctly smaller than the size of the unit cell and in certain circumstances, the shape of the resonance is asymmetric and therefore often referred to as Fano-like. Initially, a concept is proposed and studied that allows for multiple sharp Fano-like resonances at a time. Individual Fano-like resonances can be excited in some asymmetric resonators. Beyond this, this work shows that slightly changed lattices in chess board geometry can lead to similar resonance characteristics. If both concepts are applied at the same time, it is possible to excite several sharp resonances. For demonstration, two different examples are used in which parameter sweeps and their influence on transmission characteristics and current distributions are investigated. In the future, such frequency-selective surfaces could be used to filter different frequency bands or to build the platform for a novel generation of thin film sensors. Moreover, this work addresses the angular dependent characteristics of the previously proposed filters and investigates an effect which leads to an enhanced sensitivity to the incidence angle. This effect, which is connected to the periodicity of the resonators, is used to achieve a pronounced sensitivity within a selected resonance of a multi band filter. By studying the underlying relationships it is found that there is a region of highest sensitivity to the angle of incidence that can be tuned. Furthermore, it is observed that not all resonator types and not each resonance type is similarly sensitive.