Microwave Mach-Zehnder Interferometer Sensor Based on Spoof Surface Plasmon Polaritons for Dielectric Material Characterization

In this paper, we propose a microwave Mach-Zehnder Interferometer (MZI) sensor to characterize the dielectric constants of materials. The device consists of a sensing arm and a reference arm, connected by two back-to-back Y-junction power dividers. Both arms are designed with identical spoof surface plasmon polariton (SSPP) waveguide transmission lines (TLs) comprising compact spiral units. SSPP TL's nonlinear dispersion enables the proposed SSPP-based MZI sensor to achieve higher sensitivity than that based on microstrip technology. A material under test (MUT) loading on the sensing arm induces inter-arm phase differences, generating interference nulls in transmission. The spectral positions of these nulls will shift when different MUT are loaded. The spectral position of the interference null near the cut-off frequency shows the maximum displacement and is used to characterize the dielectric materials.