  A non-perturbative theory of the spontaneous down conversion (SDC) of surface
plasmon polaritons at a metal-dielectric interface is presented. It is shown
that the process is resonantly enhanced for the characteristic power of
excitation, typically of the order of tens of watts. At a stronger excitation
the yield of SDC decreases rapidly. At a stronger excitation the yield of SDC
decreases rapidly. The reason for this decrease is the high rate of the change
of surface plasmon polaritons by the laser field, exceeding the rate of the
zero-point fluctuations responsible for the SDC process. The obtained results
may help one to construct miniature sources of entangled photons for quantum
communication.
