Unidirectional surface magnetoplasmons with exotic dispersion at terahertz frequencies

Unidirectional electromagnetic modes can provide unique mechanisms for enabling tunable free-space functionalities based on their exotic dispersion, characterized by a monotonically varying dispersion curve that spans the entire light cone in air. However, beyond the microwave regime, unidirectional modes reported at terahertz to optical frequencies have not exhibited such exotic dispersion. Here, we show that unidirectional surface magnetoplasmons (USMP) with the exotic dispersion can be supported at terahertz frequencies in a semiconductor-silicon-metal layered structure under an external magnetic field, where the interface between the semiconductor and silicon is periodically corrugated at subwavelength scale. A three-dimensional USMP waveguide with exotic dispersion can also be realized using high-impedance surfaces. Such USMP waveguides can achieve desired terahertz free-space functions when appropriate radiation mechanisms are incorporated. Our study shows that a leaky USMP waveguide can achieve broadside radiation and continuously scan a directive beam over a wide range of angles by varying either the frequency or external magnetic field. Moreover, a resonator formed by a ring-shaped USMP waveguide can directly emit terahertz orbital angular momentum (OAM) beam, with the topological charge tunable via frequency or external magnetic field.