Negative Group Velocity: Is It a Backward wave or Fast Light?
  When a negative slope of the dispersion curve is encountered, the propagating light may be either 'fast light' or 'backward propagating'. We show that causality considerations select only one of which for each scenario, and demonstrate that the same photonic (plasmonic) system can support both type of solutions depending on parameters. 
Eyal Feigenbaum, Noam Kaminski and Meir Orenstein