Flat metasurfaces to collimate electromagnetic waves with high efficiency.

Directional beaming of electromagnetic waves passing through a subwavelength aperture has attracted considerable interests in photonics, but the traditional approach of utilizing gratings to directionally couple surface waves (SWs) to a desired far-field direction faces the low-efficiency issue owing to high-order diffractions. Here we experimentally demonstrate that directional beaming of light can be realized with very high efficiencies, in which two specifically designed metasurfaces (MTSs) are placed at two sides of the aperture to serve as SW to propagating-wave meta-couplers. Different from the grating couplers, the well-designed phase-gradient meta-couplers can freely select the desired diffraction orders by suppressing the undesired diffraction orders. We design and fabricate MTSs with different phase gradients, and perform both far-field and near-field measurements to verify the predicted high-efficiency on/off-axis directional beaming effects. Experimental results are in good agreement with full wave simulations and theoretical analyses.