Unexpected polarization behavior at the aperture of hollow-pyramid

Polarization in the proximity zone beyond the illuminated aperture of a near-field optical microscope is determined by means of a thin dichroic layer of fluorescent molecules used as a near-field polarization analyzer. Near-field probes of the hollow-pyramid type, with a metal coating and about 100 nm apertures, are used. Surprisingly, it is found that the input polarization is always maintained in the near field, independently of the aperture geometry, in spite of the behavior of the transmitted far field, which may result either isotropic or strongly dichroic depending on the ellipticity of the aperture.