  In this paper, we show by experiment that by covering a thin flat nonlinear
lens on the sources, the sub-diffraction-limit observation can be achieved by
measuring either the near-field distribution or the far-field radiation of the
sources at the harmonic frequencies and calculating the inverse Fourier
transformation to obtain the sub-wavelength imaging. Especially, the
sub-wavelength image calculated from measured far-field data demonstrates very
clear resolution. Since metamaterials included with active elements can easily
behave strong nonlinearity under very weak incident electromagnetic powers, the
application of the nonlinear lens proposed in this paper would have important
potential in improving the sub-wavelength resolution in the near future.
