The precise determination of materials' optical constants in the THz frequency domain is an important new challenge in basic research and is crucial for novel technological applications. Spectroscopic ellipsometry is known as a vital tool for the determination of the materials' dielectric function including its anisotropy. However, ellipsometric measurements at very long wavelengths are difficult due to the lack of reliable sources of sufficient intensity and brilliance. Here we report on our recent advances to use ellipsometry in combination with different electron beam based sources in order to in investigate condensed matter samples in the frequency range from 0.1 to 8 THz. We successfully employ terahertz radiation emitted from two different tunable desktop sources (Smith-Purcell-effect source and a backward wave oscillator) in a polarizer-sample-analyzer ellipsometer scheme. We discuss and present THz range physical material properties due to bound and unbound charge resonances in semiconducting materials. This research will provide important understanding of optical properties for novel materials, inspire new designs, and accelerate development of optical Terahertz devices.