  We investigate the interaction of highly focused linearly polarized optical
beams with a metal knife-edge both theoretically and experimentally. A high
numerical aperture objective focusses beams of various wavelengths onto samples
of different sub-wavelength thicknesses made of several opaque and pure
materials. The standard evaluation of the experimental data shows material and
sample dependent spatial shifts of the reconstructed intensity distribution,
where the orientation of the electric field with respect to the edge plays an
important role. A deeper understanding of the interaction between the
knife-edge and the incoming highly focused beam is gained in our theoretical
model by considering eigenmodes of the metal-insulator-metal structure. We
achieve good qualitative agreement of our numerical simulations with the
experimental findings.
