All-plasmonic modulation via stimulated emission of copropagating surface plasmon polaritons on a substrate with gain.

We experimentally demonstrate suppressed absorption and stimulated emission of surface plasmon polaritons (SPP) leading to all-plasmonic modulation of an SPP signal propagating at the interface between a metal and a gain medium; these observations are supported by the developed theory. The use of copropagating signal and control waves can provide more than 10 times more efficient SPP stimulated emission compared to out-of-plane pumping and opens up the possibility to realize integratable plasmonic components for active nanophotonic circuitry.