Near-field imaging of optical phase and its singularities

It is generally accepted that by incorporating a scanning near-field optical microscope into one arm of an interferometer it is possible to map both the phase and the amplitude of an optical near field. We show that a straightforward image interpretation is possible only for a field having a narrow polarization range and spatial frequency spectrum, a circumstance that makes the images prone to result in wrong interpretations, especially those pertaining to optical phase singularities. Predicaments when treating phase images are discussed and illustrated by considering the interferometric near-field imaging of co-propagating waveguide modes. A simple method is suggested that enable one to control the extent of distortion of phase information in the imaging process. (C) 2002 Elsevier Science B.V. All rights reserved.