setReferenceWavevectorComponent(sComponent, fReferenceWavevectorComponent)
 
  Use setReferencePropagationDirection
 
 sComponent = 'X' | 'Y'
 fReferenceWavevectorComponent = k_x^0 | k_y^0
  this is a low level function if you want to chang the
  direction of propagation use other stuff.