Cascaded second-order surface plasmon solitons due to intrinsic metal nonlinearity

We theoretically show the existence of cascaded second-order surface plasmon solitons propagating at the interface between a metal and a linear dielectric. Non-local multipole nonlinearities originating from the free conduction electron plasma of the metal lead to strong interaction between co-propagating surface plasmon polariton beams at the fundamental and second-harmonic frequencies. Finite element numerical modelling for an effective two-dimensional medium explicitly demonstrates soliton formation, confirming the theoretical results. The non-diffractive regime of propagation has been demonstrated at a silica/silver interface for 5λ-wide surface plasmon polariton beams with the loss-limited propagation distance of the order of 100 μm for the 750/1550 nm wavelength pair. Plasmon-soliton formation in phase-matched conditions has been shown to be beneficial for non-diffractive surface plasmon polariton propagation.