Sub-Wavelength Plasmonic Solitons in 1D and 2D Arrays of Coupled Metallic Nanowires

In this chapter, we describe a very promising approach to achieve deep sub-wavelength confinement of the optical field guided by plasmonic nanostructures. In the plasmonic nanostructures investigated in our review, namely, one-dimensional (1D) and two-dimensional (2D) arrays of closely spaced parallel metallic nanowires embedded in an optical medium with Kerr nonlinearity, the optical nonlinearity induced by the evanescent component of the guided modes of the nanowires exactly balances the discrete diffraction due to the optical coupling among neighboring metallic nanowires. As a result, nonlinear optical modes, called plasmonic lattice solitons (PLSs), are formed in the plasmonic array. Because the radius of the nanowires and their separation distance could be much smaller than the operating wavelength the size of the PLSs can be deep in the subwavelength regime. We present fundamental (vorticityless) PLSs in both 1D and 2D plasmonic arrays, and also vortical PLSs in 2D arrays, in both focusing and defocusing nonlinear media. We demonstrate that the spatial extent of fundamental and vortical PLSs could be in the deep-subwavelength regime under experimental accessible conditions. Moreover, their existence, stability, and spatial confinement are studied in detail. Our analysis employs a model based on the coupled-mode theory as well as the full set of Maxwell equations, and shows that the predictions of the two models are in excellent agreement for relatively large nanowires separations. We expect that these nonlinear plasmonic modes have important applications to subwavelength nanophotonics. In particular, we demonstrate that the subwavelength PLSs can be used to optically manipulate with nanometer accuracy the power flow in ultra-compact photonic devices.