Retarded Electromagnetic Interaction and the Origin of Non-linear   Phenomena in Optics
  The non-linear relation between electric polarization and electric field strength is achieved through introducing the retarded electromagnetic interactions between classical charge particles. The result agrees with the phenomenological theory in current non-linear optics, means that the non-linear phenomena in optics come from the retarded electromagnetic interaction between charged particles. The result slao shows that that most of non-linear phenomenon in optics violate symmetry of time reversal. 
Mei Xiaochun