Matter System Irradiated by Coherent Light

Abstract

A study is made of quasi-stationary states in a matter system which is under internal exchange-interactions among atoms and coupled to an electromagnetic field, by using a variational method and a unitary transformation; therefrom a photon coherent state is derived, The coherent state is characterized by a cooperative dipole orientation excited in the matter by a coherent light. The matter-field interaction energy term depends on the phase difference between the matter system and the field. When the phase difference equals zero, the coupled system is in the ground state. A thermal equilibrium of the matter system in the molecular field approximation is also studied. It is proved that a phase transition of the second kind occurs. This transition may be followed by coherent resonance scattering processes, even when the dipole excited state decays with a nonradiative relaxation time which is much shorter than the radiative life time.