Near-field vs. far-field polarization properties of hollow pyramid SNOM tips

We report about the polarization properties of light transmitted by hollow pyramidal SNOM tips. The useof a thin polymeric dichroic film allowed us to clearly distinguish between the behavior in the near-fieldand in the far-field regime and to emphasize strong differences between them: while in the far-field regiona pinning of the polarization can take place, in the near-field region the in-plane polarization always followslinearly that of the light incident on the tip, for all the measured tips. These original findings stressthe very peculiar nature of the evanescent field in the near-field regime, and have important consequencesfor all the applications of polarized light in near-field microscopy.