Microwave Characterization of Plasmonic Transmission Line Through Non-Contact Near-field Imaging at THz Frequencies

The characterization of microwave components at terahertz (THz) to optical frequencies presents a significant challenge due to the lack of conventional microwave characterization equipment. In this study, we propose an alternative approach to address this challenge by employing near-field mapping through electron energy loss spectroscopy (EELS). Unlike conventional vector analyzers, which are typically inaccessible at such high frequencies, EELS offers a practical solution for capturing resonance dynamics and concurrently investigating the spatial distribution of plasmonic modes in plasmonic circuits. To demonstrate the effectiveness of our method, we investigate coplanar waveguide (CPW) transmission line resonators terminated with various loads. Through EELS, we effectively track resonance shifts resulting from changes in termination load impedances. Additionally, we analyze the spatial standing wave profiles within these transmission lines, capturing the load effect on the near-field pattern and providing insights into the mixing of odd and even modes reflected in the EELS measured spectra. Furthermore, we introduce a quantitative method for estimation of transmission line parameters, such as phase constant, based on the analysis of standing wave patterns. This approach enhances our understanding of the transmission line's characteristics and its interaction with loads at THz frequencies.