A Fundamental Limit on Propagation Loss of Waveguides with Subwavelength   Mode Size
  A fundamental trade-off relation between the cross sectional confinement and propagation length of an arbitrary mode of a general waveguide is presented. This limit is a generalization of the well-known diffraction limit for guided modes. The results provide a lower bound on propagation loss of plasmonic waveguides which are attractive for their deep subwavelength mode dimensions. We also introduce a material loss merit factor that sets a criterion for comparing different plasmonic materials for achieving the best trade-off between confinement and loss. 
Amir Arbabi, Ehsan Arbabi, Safieddin Safavi-Naeini
physics.optics