  The recent surface plasmon entanglement experiment [E. Altewischer et al.,
Nature (London) 418, 304 (2002)] is theoretically analyzed. The entanglement
preservation upon transmission in the non-focused case is found to provide
information about the interaction of the biphoton and the metallic film. The
entanglement degradation in the focused case is explained in the framework of a
fully multimode model. This phenomenon is a consequence of the
polarization-selective filtering behavior of the metallic nanostructured film.
