A review of unidirectional surface plasmon polariton metacouplers

Efficient and unidirectional excitation of surface plasmon polaritons (SPPs) and their low-frequency counterparts (i.e., spoof SPPs) is highly desired in the research area of photonics and plasmonics, while being at the same time rather challenging. Recent progress in gradient metasurfaces, ultrathin, and planar materials composed of sub-wavelength unit cells, has opened up new possibilities for efficiently coupling of free-space propagating waves to surface waves. In this paper, we will discuss the recent achievements in gradient metasurface-based unidirectional SPP couplers (i.e., metacouplers). Starting with explaining the basic concept of generalized Snell's law, we then introduce some typical examples of unidirectional SPP metacouplers, followed by an overview of polarization-controlled tunable SPP metacouplers. We also describe metacouplers that also feature other functionalities, beyond the unidirectional SPP excitation. The review is concluded with a short summary and perspective for future developments.