Spatial modulation instability of coupled surface plasmon polaritons in a dielectric-metal-dielectric structure

We study the spatial modulation instability (MI) in a system of coupled surface plasmon polaritons propagating at a dielectric-metal-dielectric interface. One of the dielectric media considered is nonlinear and active, which provides enough gain to compensate for the losses in the metal. It is found that the system is unstable for both focusing or defocusing nonlinearity, with more instability in the focusing case. It is noted that nonlinearity is one of the key parameters for instability, with the real part controlling the maximum MI gain and the imaginary part controlling the MI gain saturation. Gain provided by the active medium is also an important parameter for the development of MI in the system. The generated sidebands are amplified as they propagate along the interfaces. We have noted that the defocusing case requires a large gain threshold to exhibit modulation instability for a symmetric solution. (C) 2016 Optical Society of America