Uniform unidirectional waveguide with exotic dispersion and its applications at terahertz frequencies

Unidirectional electromagnetic modes can be immune to backscattering, as no backward-propagating mode exists in the system. For broadband unidirectional modes (typically supported by uniform guiding structures), an additional striking property may emerge: exotic dispersion, in which a monotonically varying dispersion curve crosses the entire air light cone. However, in the terahertz regime, uniform unidirectional waveguides proposed previously do not exhibit such behavior. In this work, we show that a semiconductor-silicon-opaque medium structure can support a terahertz unidirectional surface magnetoplasmon (USMP) mode with exotic dispersion under certain conditions. We also reveal why previous USMP waveguides do not exhibit exotic dispersion. USMPs with exotic dispersion enable a class of metasurfaces whose extracted phases and amplitudes can be set arbitrarily and continuously. As examples, we numerically demonstrate terahertz wave focusing and Airy-beam radiation.