  We implemented the inverse design method to build a thin near-field lens that
could produce a desired subwavelength focus by manipulating the near fields of
a magnetic dipole source. The flat near-field lens represented by an artificial
impedance surface was fabricated by lumped reactive elements (capacitor and
inductor) with spatially varying values. In the experiment a desired annular
focusing spot with a characteristic size nearly three times smaller than that
allowed by the diffraction limit was obtained. Besides high-resolution imaging,
the proposed near-field plate could be extended for other interesting
applications, such as wireless power transfer or complex wavefront/beam shaper.
