THE MULTIPHOTON AC STARK-EFFECT

We study the interaction of a two-level atom with two intense lasers: a strong laser of Rabi frequency 2 Ohm on resonance with the atomic tr ansition, and a weaker laser detuned by 7 Ohm/n. i.e. by a subharmonic of the Rabi frequency of the first. The second laser ''dresses'' the dressed states created by the first in an n-photon process. We calcula te the energy levels and eigenstates of this ''doubly-dressed'' atom, and find a new phenomenon: the splitting of the energy levels due to a n n-photon coupling between them, resulting in a multiphoton AC Stark effect. We illustrate this effect in the fluorescence spectrum, and sh ow that the spectrum contains triplets at the subharmonic as well as h armonic resonance frequencies with a clear dependence on the order n o f the resonance and the ratio a of the Rabi frequencies of the lasers. (C) 1998 Elsevier Science B.V.