LAUSR

In this work, we show that the concept of phase gradient metasurfaces provides a versatile way to control the diffraction of light through small holes or slits. As an example, we consider a single subwavelength metallic slit surrounded by air grooves of gradient depth that induces the expected phase gradient. It is found that for normal incident light, the phase gradient can enable unidirectional excitation of surface plasmons, which flow directionally toward the slit, resulting in extraordinary optical transmission beyond that in conventional ways. Using this scheme, unidirectional radiation of an optical dipole located inside the slit can be obtained when different phase gradients are applied to both sides of the metal plate.