  Nonlinear metal-dielectric layered structures are shown to be able to
efficiently generate entangled photon pairs using spontaneous parametric
down-conversion. Increase of electric-field amplitudes in these structures
enhanced by the presence of metal layers is sufficient to compensate for losses
inside thin metal layers. As an example, photon pairs emitted from a structure
composed of alternating nonlinear dielectric GaN layers and metal Ag layers are
analyzed in spectral, temporal as well as spatial domains. Also correlations
and entanglement between two photons in a pair are determined. Very narrow
photon-pair spectra together with strong directionality of photon-pair emission
are observed making the photons suitable for photon-atom interactions. Highly
enhanced electric-field amplitudes provide high photon-pair generation
efficiencies.
