Exciton Coherency, Its Superradiant Decay And Enhanced Optical Nonlinearity

An exciton, a collective excitation in a crystal, is made by superposing coherently atomic or molecular excitation is each unit-cell over whole the crystal. Therefore the exciton in an ideal large crystal has macroscopic transition dipolemoment and the exciton-polariton is formed due to the strong resonant interaction between the exciton and the radiation field (photon) and the translational symmetry of the system in the 3-dimensional crystal.