  Edge modes of a finite-size woodpile can appear within a complete bandgap on
an <010> edge. The mode area is as small as 0.066 squared
half-in-vacuum-wavelengths, and the propagation loss is small. The field maxima
occur at a dielectric-vacuum interface, like at a metal-dielectric interface
for surface plasmon modes. The edge mode is a subwavelength-scale 2D light
localization mode in non-metallic materials. Analysis of two-mode
co-directional coupling between identical surface Bloch modes suggests that a
large photonic crystal or surface designing would be needed for suppressing the
evanescent field coupling in the woodpile.
