Subwavelength far-field resolution in a square two-dimensional photonic crystal

It has been suggested that a subwavelength image can be realized by a photonic crystal slab with a square lattice in the lowest band. However, due to the anisotropy of dispersion in such a photonic structure, the image only appears in the near-field region. In this paper, we show that subwavelength far-field focusing and imaging, explicitly following the well-known wave-beam negative refraction law with relative refractive index of -1, can also be realized by a square photonic crystal through choosing suitable scatterers for the lowest valence band.