Two-photon absorption in strong fields

We present a theoretical formalism to describe simultaneous absorption of two single-mode photons by an atom. The appropriate interaction Hamiltonian, derived by a unitary transformation, is used to obtain a very general master equation for the photon density operator that includes the effects of Stark shifts. The special case in which the atomic levels are in resonance with the field, i.e., zero detuning, is considered. The master equation for the photon probability predicts a linear dependence of absorption on intensity, rather than the usual quadratic dependence, and results in a slower rate of absorption as a function of time. Coherence in the presence of two-photon absorption is also demonstrated. (C) 1999 Elsevier Science B.V. All rights reserved.