Polarization effects in reflection-mode scanning near-field optical microscopy

It is shown that the polarization state of light emitted from an aperture-type scanning near-field optical microscopy (SNOM) tip may be significantly altered on reflection from a metal surface in close proximity (5 nm) to the tip, while remaining unchanged at distances of several microns. Proximity to dielectric surfaces produces no discernible change. This effect is discussed and explained theoretically. We demonstrate that optical image contrast of metal samples may be enhanced by using this effect. The mechanism of the enhancement is based on selectively detecting the light emitted from the SNOM aperture and filtering out the light emitted through the sidewalls of the probe. It is also shown that images of a metal grating pattern on glass show strong polarization contrast, which may be understood in terms of simple dipole-image dipole schemes and transmission-line concepts. (C) 1998 American Institute of Physics.