Derivation of the three-layer model for surface second-harmonic   generation
  We develop explicit expressions for the surface second-harmonic radiation yield using the three layer model. We derive expressions that can be applied to systems without and symmetry considerations, and then reduce them for the (111), (110), and (100) surface symmetries. 
Sean M. Anderson, Bernardo S. Mendoza
physics.optics cond-mat.mtrl-sci