Optical modes in two-dimensionally ordered dielectric spheres

We have experimentally observed the optical modes of periodic two-dimension ally ordered polystyrene spheres with a diameter of 5.15 mu m using the att enuated total reflection method. The dispersion relation of such optical mo des clearly shows that there are two types of propagating modes in the orde red spheres. We point out that one of the two optical modes is related to t he guided modes in optical fibers and the periodicity of the spheres plays an important role in its excitation mechanism. The other mode propagates al ong spherical surfaces.