Tailoring and enhancing spontaneous two-photon emission processes using   resonant plasmonic nanostructures
  The rate of spontaneous emission is known to depend on the environment of a light source, and the enhancement of one-photon emission in a resonant cavity is known as the Purcell effect. Here we develop a theory of spontaneous two-photon emission for a general electromagnetic environment including inhomogeneous dispersive and absorptive media. This theory is used to evaluate the two-photon Purcell enhancement in the vicinity of metallic nanoparticles and it is demonstrated that the surface plasmon resonances supported by these particles can enhance the emission rate by more than two orders of magnitude. The control over two-photon Purcell enhancement given by tailored nanostructured environments could provide an emitter with any desired spectral response and may serve as an ultimate route for designing light sources with novel properties. 
Alexander N. Poddubny, Pavel Ginzburg, Pavel A. Belov, Anatoly V.   Zayats, and Yuri S. Kivshar
cond-mat.mes-hall