Near-field optical microscopy of thin photonic crystal films

Near-field optical microscopy is used to image the light propagating in a Si3N4-on-silica film with a strip of deeply etched submicron holes in the center. Fringe patterns created by interference between incident and diffracted guided modes are observed in the smooth waveguide surrounding the large air-hole photonic film. Observation of the fields in the photonic crystal regions indicates that out-of-plane scattering at the smooth/periodic transitions and leakage caused by grating coupling play major roles. The effects of different air-hole sizes and lattice pitches are also explored.