  In recent years, metasurfaces have shown extremely powerful abilities for
manipulation of electromagnetic waves. However, the local electromagnetic
response of conventional metasurfaces yields to an intrinsic performance
limitation in terms of efficiency, minimizing their implementation in real-life
applications. The efficiency of reconfigurable metasurfaces further decreases
because of the high density of meta-atoms, reaching 74 meta-atoms per
$\lambda^2$ area, incorporating lossy tunable elements. To address these
problems, we implement strong electromagnetic non-local features in a
\textit{sparse} metasurface composed of electronically reconfigurable
meta-atoms. As a proof-of-concept demonstration, a dynamic sparse metasurface
having as few as $8$ meta-atoms per $\lambda^2$ area is experimentally realized
in the microwave domain to control wavefronts in both near-field and far-field
regions for focusing and beam-forming, respectively. The proposed metasurface
with its sparsity not only facilitates design and fabrication, but also opens
the door to high-efficiency real-time reprogrammable functionalities in beam
manipulations, wireless power transfer and imaging holography.
