Numerical study of an apertureless near field scanning optical microscope probe under radial polarization illumination.

An axially symmetric three-dimensional finite element method model is applied to investigate the electromagnetic field distribution in the vicinity of a silver coated glass tip. Under radially polarized illumination, a strongly enhanced field located at the apex of the tip is found due to the constructive interference of surface plasmon propagating at the air/silver interface. The enhancement factor and surface plasmon resonance excitation are analyzed systematically. The optimal condition for field enhancement is investigated through the exploration of different taper angles of the tip and the illumination geometry. The numerical studies show that a significantly enhanced localized electromagnetic field with a full-width-half-maximum of 10 nm is obtainable with 632.8 nm optical excitation.