Nonglobal model for a near-field scanning optical microscope using diffraction of the optical near field

Diffraction of the localized optical near field by a subwavelength-sized probe (metallic circular aperture) is studied by a nonglobal approach. The problem is closely relevant to the basic principle of a near-field scanning optical microscope (NSOM). We obtain analytic expressions of the diffracted far field from which the transfer function of the NSOM can be calculated. In particular, we find a resonance enhancement in the detection efficiency when the sample size is similar to the probe size. By lateral scanning of the aperture position at a constant height, we also obtain a NSOM image of a dielectric sphere sample.