Erzeugung und Nachweis von Terahertz-Strahlung unter Verwendung von Multimode-Lasersystemen

The enormous potential of terahertz(THz) waves for a variety of scientific applications and for non-destructive sensing drove research efforts to efficient and compact THz systems in recent years. Especially, THz time-domain spectroscopy, employing ultra short femtosecond lasers, was under thorough investigation. This method enables characterizations of the complex dielectric parameters of the sample under investigation over a broad frequency range with just one single measurement. But one has to bear in mind that for the required analysis of the time domain data numerical signal processing is necessary. In this work, three concepts are proposed to advance the versatility of THz spectrometers. The presented advances lay in the field of THz signal data analysis, THz spectroscopy and THz sources. In the first part, the signal processing and data analysis measured by THz spectroscopy is discussed. It is shown that numerical signal processing allows for an accurate extraction of the dielectric characteristics of a sample. Three different analyzing algorithms based on numerical optimization are introduced that simultaneously extract the dielectric material parameter of the sample under investigation and its thickness. The first method is analyzing the time-domain data and suits fast preliminary evaluations, whereas the two latter methods are contemplating the frequency-domain. While the second method offers the best resolution and highest depth of information, the third one combines the information depth of the second with the computational speed of the first by introducing a three dimensional optimization step. Measurements providing all relevant information can now be achieved in real time. In the following second part multimode laser emission is analyzed regarding its suitability for THz generation with photoconductive THz antennas. The correlation between the number of laser modes and the resulting THz waveform and signal quality is investigated. It was found that an increase in laser modes leads to a better frequency resolution but reducing the spectral intensity for each of the individual frequencies at the same time. However, choosing a laser diode with equidistant mode spacing overcomes these limitations and allows for a spectral bandwidth of approx. 1 THz. Besides the high potential for miniaturization this approach offers a significant costs reduction compared to existing THz laser sources and will speed up the introduction of THz spectrometers for the mass market. The thesis will be concluded by a chapter on a novel design for a highly efficient continuous wave THz source operating at room temperature. High infrared laser intensities of an external disc laser are used for a parametric frequency conversion by a nonlinear crystal embedded within the laser cavity. In contrast to photomixing on a semiconductor antenna, the conversion efficiency of the crystal increases with ascending frequency. Thus, this method allows for generation of milliwatt THz powers in the regime above 1 THz at room temperature and emission lines at 1.0 and 1.9 THz, respectively, were exemplarily realized.