The quantum electrodynamics of synergistic two-photon processes

In this paper it is demonstrated how the phenomena of cooperative Raman scattering and cooperative two-photon absorption may be described in a unified manner, through canonically transforming the system Hamiltonian. The unique features associated with the virtual photon coupling between centres are discussed. In particular it is illustrated how in the near-zone, close proximity leads to two-body two-photon events involving interaction-induced dipoles. In the far-zone, it is shown how the coupling between molecules may be understood in terms of a radiative dipole effect. Attention is focused upon the behaviour of this coupling for distributive and cooperative mechanisms in fluids, and the relative importance of each process discussed, particularly with regard to their very different range dependences. A detailed analysis of the coupling reveals that in two-photon absorption the distributive mechanism dominates, whilst in Raman scattering cooperative energy transfer is favoured. Finally recent progress in the field of synergism is reviewed along with the characteristics associated with synergism in Raman scattering and two-photon absorption.