Near-field chiral excitation of universal spin-momentum locking transport of edge waves in microwave metamaterials

Zhixia Xu; Jie Chang; Jinye Tong; Daniel F. Sievenpiper; Tie Jun Cui

doi:10.1117/1.AP.4.4.046004

Abstract

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.

The quantum spin-Hall state is an electronic surface state with immunity against defects.1 The momentum is locked to the intrinsic spin of electrons, and backward transports are suppressed unless the spin flips.2^,3 Analogous phenomena have been widely reported in various uniform and metamaterial waveguides, where theories of universal spin-momentum locking4 and spin-orbit interactions5^,6 have been well studied. Uniform waveguides include closed metal waveguides, fibers, and plasmonic metals, where the unidirectional transmission can be realized based on nano-gratings,7 circularly polarized (CP) dipoles or Huygens dipoles8^,9 supported by resonant particles.10^–14 Metamaterial waveguides consist of various artificial unit cells, where chiral sources carrying spin angular momentum (SAM) and orbital angular momentum (OAM) can interact with unit cells in versatile ways to realize unidirectional coupling.15 Recent research interest is mainly in metamaterial interfaces supporting edge waves. As shown in Fig. 1(a), two metamaterials are stitched together to construct an infinitely thin interface to support edge waves, such as spoof surface plasmon polaritons (SPPs), line waves (LWs), and photonic topological insulators (PTIs). All these kinds of edge waves are related to flipping change of intrinsic characteristics of media across the interface, as shown in Figs. 1(b)–1(d).