  Electromagnetic scattering bounds on subwavelength structures play an
important role in estimating performances of antennas, RFID tags, and other
wireless communication devices. An appealing approach to increase a scattering
cross-section is accommodating several spectrally overlapping resonances within
a structure. However, numerous fundamental and practical restrictions have been
found and led to the formulation of Chu-Harrington, Geyi, and other limits,
which provide an upper bound to scattering efficiencies. Here we introduce a 2D
array of near-field coupled split-ring resonators and optimize its scattering
performances with the aid of a genetic algorithm, operating in 19th-dimensional
space. Experimental realization of the device is demonstrated to surpass the
theoretical single-channel limit by a factor of >2, motivating the development
of tighter bounds of scattering performances. A super-radiant criterion is
suggested to compare maximal scattering cross-sections versus the
single-channel dipolar limit multiplied by the number of elements within the
array. This new empirical criterion, which aims on addressing performances of
subwavelength arrays formed by near-field coupled elements, was found to be
rather accurate in application to the superscatterer, reported here.
Furthermore, the super-radiant bound was empirically verified with a
Monte-Carlo simulation, collecting statistics on scattering cross sections of a
large set of randomly distributed dipoles. The demonstrated flat superscatterer
can find use as a passive electromagnetic beacon, making miniature airborne and
terrestrial targets to be radar visible.
