Controlling energy flow in waveguides has attractive potential in integrated devices from radio frequencies to optical bands. Due to the spin-orbit coupling, the mirror symmetry will be broken, and the handedness of the near-field source will determine the direction of energy transport. Compared with well-established theories about spin-momentum locking, experimental visualization of unidirectional coupling is usually challenging due to the lack of generic chiral sources and the strict environmental requirement. In this work, we design a broadband near-field chiral source in the microwave band and discuss experimental details to visualize spin-momentum locking in three different metamaterial waveguides, including spoof surface plasmon polaritons, line waves, and valley topological insulators. The similarity of these edge waves relies on the abrupt sign change of intrinsic characteristics of two media across the interface. In addition to the development of experimental technology, the advantages and research status of interface waveguides are summarized, and perspectives on future research are presented to explore an avenue for designing controllable spin-sorting devices in the microwave band.

In this paper, a pair of resonators to realize multiple electromagnetically induced transparency-like (EIT-like) transmission peaks operating in the microwave band is designed, where the symmetric double-ellipsoids (SDEs) and the spoof localized surface plasmons (spoof LSPs) are used as the bright and dark modes, respectively. In order to reveal the mechanism of multiple EIT-like, a simplified mechanical model consisting of multiple particles is derived to visualize the metamaterial. The simulation and measurement results are identical showing that the EIT-like metamaterial can produce three transparent EIT-like transmission peaks in the frequency range of 7 ~ 10 GHz. We next proposed a plasmonically induced transparency refractive index sensor working in the infrared regions and analyzed it by simulation to reveal the sensing potential of EIT-like metamaterial. In this sensor, the group delay is treated as a high-Q sensing parameter, which has broad applications in various surface sensing applications based on EIT-like metamaterials.