Direct Verification of Backward-Wave Propagation in Subwavelength Waveguide Filled with Split-Ring Resonators

Recently, it has been shown that a rectangular waveguide filled with the split-ring resonators supports the backward- wave propagation below the cut-of frequency of a fundamental TE mode. In the previous studies, the backward- wave propagation below the cut-off was observed indirectly; by comparison of the phases of the transmission coefficients associated with two waveguide samples with different lengths. Here, we report a more intuitive direct method based on the measurements of both microscopic (in-cell) and macroscopic distributions of phase of the magnetic field along the waveguide. Experimental results revealed the positive slope of averaged phase along the waveguide. This proves the existence of the backward-wave in the subwavelength waveguides filled with anisotropic single negative mu-negative (MNG) metamaterial.