Measuring three-dimensional polarization with scanning optical probes

In optical fields with sub-wavelength features the longitudinal field components become considerable. Therefore, the paraxial concept of 2D polarization has to be replaced by the description of the local behaviour of the 3D vector of the electric field. The heterodyne scanning probe optical microscope (heterodyne SPOM) allows one to measure the amplitude and phase of the optical field with sub-wavelength resolution. In this paper we will review some results of earlier work showing clearly the 3D aspect of the polarization in the diffracted field from a 1 mum pitch surface relief grating and present a new set-up for a polarization sensitive heterodyne SPOM. For the correct interpretation of SPOM images of optical fields with sub-wavelength features it is important to be aware of the field conversion by the fibre tip probe.