Abstract

Scalar wave-fields passing through random media generate random speckle patterns. These patterns contain hot spots but also true zeros of intensity which draw lines in three-dimensional space [1]. These zeros of intensities are surrounded by spiral phase patterns and are thus called optical vortices due to the circulation of the optical current around the nodal line. For optical wave-fields of high numerical apertures, the contribution of all three components of the field must be taken into account. In particular, the axial field cannot be neglected. In STimulated Emission Depletion (STED) microscopy for instance [2], resolution is improved by stimulating fluorescence with a donut-shaped beam exhibiting an optical vortex at its dark center. This beam must be circularly polarized in order to cancel the axial component of the field at the center of the donut. Using the opposite “wrong” circular polarization yields a significant axial component resulting in de-exciting fluorophores located at the donut center.

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