Analysis of near field tip patterns including object interaction using finite-difference time-domain calculations

Numerical simulation of the field patterns in the vicinity of a typical probe tip of a near field scanning optical microscope is performed using the finite-difference time-domain technique. Maps of electric field vectors and Poynting vectors are given for a two-dimensional 120 × 160 cell model of the tip for p-polarized incident illumination with initially no object nearby. A metal stripe object is then placed near the tip and perturbations of the field patterns are displayed, in particular those caused by the induction of surface charge density in the metal object. The metal stripe is scanned across the tip plane at three separation distances and the radiated power, determined by a sum of Poynting vectors across a rear plane, is plotted as a function of lateral position of the object. An interesting edge enhancement effect is seen for the close metal object, but not for a dielectric object.