Superradiant Scattering Limit for Arrays of Subwavelength Scatterers

Electromagnetic scattering bounds on subwavelength structures play an important role in estimating performance of antennas, radio frequency identification 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 two-dimensional array of near-field coupled split-ring resonators and optimize its scattering performance with the aid of a genetic algorithm operating in 19-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 performance. A superradiant criterion is suggested to compare maximal scattering cross sections with the single-channel dipolar limit multiplied by the number of elements within the array. This empirical criterion, which aims to address performance of subwavelength arrays formed by near-field coupled elements, is found to be rather accurate in application to the superscatterer, reported here. Furthermore, the superradiant bound is 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 radar visible.