Directional beaming of light from a subwavelength metal slit with phase-gradient metasurfaces

In this article, we demonstrate directional beaming of light from a metal nanoslit surrounded with phase-gradient metasurfaces on both sides. Distinct from the grating-based beaming structures, here the momentum mismatch between the surface wave and radiation wave is overcome by the phase-gradient metasurfaces. The deviation angle of the directional beam can be flexibly adjusted by appropriately arranging the phase-gradient of metasurfaces on each side of the nanoslit. The metasurface-based beaming structures also present the ability to operate with high diffraction efficiency and small divergence angle, implying various potential applications in nanophotonics.