Control of unlocalized optical nonlinear processes via plasmonic path interference.

Fano resonances can be used to control nonlinear optical processes taking place in the vicinity of a plasmonic hotspot. On top of localization, an extra enhancement appears due to path interference. This scheme, widely studied in the literature, however, has a limited use in practical implementations since substantial frequency conversions actually take place in nonlinear crystals. Here, we raise a different question. Can we control "unlocalized" nonlinear processes, taking place along a crystal body, using path interference effects? We consider metal nanoparticle-quantum emitter dimers embedded in a nonlinear crystal. As our analytical model reveals a rich interference scheme, in parallel, FEM simulations yield a 3-order of magnitude nonlinearity enhancement at the crystal "output" without a remarkable linear and nonlinear enhancement in the "crystal body". Analytical model agrees with the FEM simulations very well. The phenomenon can both be used to enhance frequency conversion without heating the crystal and to provide an additional Fano enhancement on top of the well-known enhancement due to localization.