Near-field effects of focused illumination on periodic structures in scanning tunneling optical microscopy

The influence of a focused polarized Gaussian beam on image formation was s tudied. We show that the position of the tip with reference to the center o f the beam involves asymmetry in the intensity map. A comparison between s and p polarization can be made, owing to the definition of both a three-dim ensional polarized Gaussian beam and a three-dimensional object. This resul t implies that the best way to scan a sample consists in moving it and not the tip; moreover, focusing the incident light to get a higher signal-to-no ise ratio must be done carefully with respect to the sample period. (C) 199 9 Optical Society of America.