Microwave Characterization of Plasmonic Antennas and Transmission Lines at Mid-Infrared Frequencies Through Near-Field Imaging

Overview

Plasmonic antennas operating in the mid-infrared spectral range (f ≈ 20 - 150 THz or λ ≈ 2 – 14 μm) combine strong electromagnetic field confinement with low dissipative losses, making them attractive for sensing, spectroscopy, and energy harvesting applications. However, the mature and systematic microwave approach traditionally applied for antenna design and experimental characterization cannot be straightforwardly applied at mid-infrared frequencies, due to the absence of experimental instrumentation capable of generating well-defined excitations beyond $\sim 1-2$ THz. Conversely, existing optical and spectroscopic characterization techniques generally provide limited insight into circuit-level plasmonic antenna parameters such as input impedance or scattering parameters. This work, for the first time, bridges these two domains by developing a microwave-inspired framework for the quantitative characterization of plasmonic antennas using spectroscopic near-field techniques, with a particular focus on monochromated electron energy-loss spectroscopy (EELS).

The first part of this thesis introduces a theoretical interpretation of monochromated near-field EELS imaging of plasmonic antennas from a microwave perspective. The electron is modeled as a localized, propagating electromagnetic mode, and the measured EELS response is interpreted using an equivalent microwave circuit description that links the signal to scattering parameters. This interpretation is validated on experimentally acquired EELS images of multiple antenna geometries and enables quantitative extraction of antenna input impedance and effective $S_{11}$ and $S_{21}$ directly from the experimental EELS images over 30–150 THz.

We then introduce design-based characterization strategies for coplanar-waveguide (CPW) transmission-line resonator circuits integrated with mid-IR antennas. A resonator-based approach enables extraction of the complex propagation constant $\gamma_{\mathrm{cpw}}$ and the complex $S_{11}$ of a monopole antenna resonant near 60 THz over 40–150 THz, although parameter evaluation is limited to discrete resonance frequencies. Finally, an extended microwave-inspired approach based on multiple CPW resonator devices terminated with open, short, and antenna loads enables broadband extraction of monopole $S_{11}$. The experimentally retrieved complex $S_{11}$ closely agrees with standard full-wave simulations and exhibits a resonance minimum of $-15.5$ dB, indicating efficient CPW–antenna matching.