Guided plasmonic modes of anisotropic slot waveguides.

We study guided optical modes of a planar plasmonic waveguide filled with a generic anisotropic medium. In particular, we show that both surface and oscillatory modes exist for such waveguides and find their numbers for two specific forms of the anisotropy tensor. We also show that lossless waveguides, characterized by a diagonal permittivity tensor, support simultaneously both the forward and backward propagating modes (surface or oscillatory type) with antisymmetric field patterns. Another family of guided oscillatory modes may be supported by the waveguide if its anisotropy is created by an external magnetic field. These new modes are asymmetric in nature, exhibit extremely low effective refractive indices and may propagate over macroscopic distances without significant attenuation, while remaining strongly confined to the waveguide.