Near-field scattering of longitudinal fields

Longitudinal fields created in strongly focused laser beams are investigated by near-field optical microscopy. Sharp metallic and dielectric tips are raster scanned through the focus of these modes. It is found that regardless of the tip material, the signal scattered by the tip is a measure for the strength of the local longitudinal field. A surprising contrast reversal is observed between the images obtained with a metallic tip and the images obtained with a dielectric tip. The contrast reversal originates from a non-negligible tip-sample interaction. (C) 2003 American Institute of Physics.