THEORY OF TWO-PHOTON ABSORPTION

Abstract

The two-photon absorption process is discussed using a density matrix formalism. The electromagnetic field is described by means of coherent states, a technique that permits a simple evaluation of the effect of coherence on the transition probability. It is concluded that the latter is the same for a coherent and an incoherent pulse of radiation. In the case of a stationary field containing many modes, the transition probability is nearly the same in the coherent and incoherent cases, whereas for a single mode the transition probability is twice as high for the incoherent field. These conclusions are independent of the multipole expansion of the interaction Hamiltonian and are valid for electric dipole and higher multipole transitions.