Influence of defects in opal photonic crystals on the optical transmission imaged by near-field scanning optical microscopy

The electric field intensity above the surface of opal photonic crystals (PCs) and its alteration due to 'crystallographic' defects is investigated by using nearfield scanning optical microscopy (NSOM). The photonic crystals are developed by dip coating in a liquid solution with PMMA opals. Highly regular hexagonal planes with lattice constants of about 260 nm grow on the glass substrate. During the drying process several crack lines are formed that correspond to defects in the crystal structure. The transmitted light intensity at wavelengths inside and outside of the stop band of the PC is studied with NSOM using a tapered fiber tip scanning in all three dimensions. By this technique, a 3D image of the electric field intensity can be measured with a resolution better than 100 nm. The results show that the local optical field distribution is strongly dominated by the defect states in all directions in space over a length scale of several mu m. Above the crack lines, the intensity of light is strongly reduced. Beams of light are observed emerging from the edges of the crack lines and propagate in air with heights of more than 3 mu m. In between two different crack lines, periodic repetitions of the beams are observed. These results are interpreted as light diffraction on a microscopic scale.