Local imaging of photonic structures: image contrast from impedance mismatch

Photonic structures made from square arrays of air holes in Si(3)N(x) membranes are locally imaged by near-field optical microscopy in illumination mode. Holes with diameters smaller than and larger than the wavelength of light are investigated. Counterintuitively, the holes appear dark and the film is bright in transmission images for both hole sizes. Modeling shows that the dominant contrast mechanism is enhanced light emission from the tip when the tip is above the film. Tip emission is enhanced because the tip-air impedance mismatch is reduced when the tip is above the high-index film.