Near-field scanning optical microscopy imaging of multimode interference.

By use of a near-field scanning optical microscope (NSOM) in collection mode, the intensity distribution along a 2 x 2 multimode interference coupler was directly imaged as a function of wavelength. Although calculations can predict the general trend of wavelength dependence and the approximate positions of multiple images in the coupler, the accuracy is poor because of uncertainties in the waveguide width. We show that direct imaging using a NSOM bypasses calculational uncertainties and proves to be a powerful technique for studying these waveguide devices.

Direct imaging of multimode interference in a channel waveguide.

By use of a near-field scanning optical microscope in collection mode, multimode interference was directly measured in an annealed proton-exchanged LiNbO3 waveguide. Periodic transitions from a single-peaked Gaussianlike intensity distribution to a double-peaked intensity distribution were observed. The intensity distribution along the waveguide was calculated, and the results agree well with the experimental observation.

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.