Local Imaging of Photonic Structures: Image Contrast from Impedance Mismatch
15 March 2002
Photonic structures constructed from square arrays of air holes in Si3Nx 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. The enhanced tip emission occurs because the tip/air impedance mismatch is reduced when the tip is above the high index film.