In Apertureless Scanning Near-Field Optical Microscopy, a sharp tip is used to locally enhance the optical tip-sample interaction, permitting to map the electron and/or phonon properties of the samples on the 10-nm scale. In this paper, we present a quantitative study of the depolarization properties of sharp metallic and dielectric tips. Depolarization effects are particularly important for Tip-Enhanced Raman Spectroscopy (TERS) of solid crystals, in which the Raman emission can be strongly polarized according to symmetry-related selection rules, and therefore dependent on the actual polarization state of the incident light. Our results show that metallic tips can depolarize linearly polarized light from a few % up to 25 – 30 %, depending on whether the incident polarization state is parallel or orthogonal to the tip axis. Effects of tip-induced depolarization are studied by investigating the Raman emission of Si (100) at 520 cm^-1. Because of polarization selection rules, the emission from this band is totally depolarized if the sample is illuminated from the far-field with light polarized along the [100] direction. The proximity of the tip, instead, induces a strong component of Raman signal polarized along the incident polarization, which is interpreted as an effect of the tip-induced near-field depolarization. Comparisons between the 1-phonon band at 520 cm^-1 and the 2-phonons bands at 300 and 980 cm^-1, characterized by different polarization selection rules, allow drawing conclusions on the local tip-sample optical interaction, and on the actual field enhancement provided by the tip.

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