Planewave diffraction at the periodically corrugated boundary of vacuum   and a negative phase-velocity material
  Considering the diffraction of a plane wave by a periodically corrugated half-space, we show that the transformation of the refracting medium from positive/negative phase-velocity to negative/positive phase-velocity type has an influence on the diffraction efficiencies. This effect increases with increasing corrugation depth, owing to the presence of evanescent waves in the troughs of the corrugated interface. 
R.A. Depine and A. Lakhtakia