Wave front evolution of negatively refracted waves in a photonic crystal

Using a heterodyne near-field scanning microscope, the authors analyze the phase and amplitude of the electric field of an optical wave across the boundary of positive to negative refraction media. The photonic crystal acts as an extremely anisotropic material with a negative curvature of its dispersion surface whose shape is resolved experimentally. This extreme anisotropy results in the beam having a peculiar phase evolution through propagation that does not occur in isotropic media. A focusing wave is produced by negative refraction, which has diverging wave fronts before the internal focus and converging wave fronts after the focus.