Linear susceptibility of a three-level system driven by an intense multimode laser field

A theory of ac Stark splitting at double resonance in a three-level system is presented, the upper and lower transitions being coupled to a near-resonant intense multimode field and a weak monochromatic field, respectively. Linear response to a weak field is considered. The analytical treatment calling into action a diagram technique and the quasistationary-states method is carried out beyond the limits of perturbation theory and the nonresonant approximation. The multimode structure of the radiation spectrum results in a quasi-periodicity of the field-correlation function. It leads to a strong modulation of the absorbtion line shape with a period $\frac{1}{2}\Delta {\nu }_0$ where $\Delta {\nu }_0$ is the intermode interval. The role of different weakening effects such as spontaneous decay of the levels, the finite pulse duration, and instrumental and natural fluctuations of a single mode is studied. The last effect is shown to be the most important, and a possible method of direct measurement of the natural-fluctuations' correlation function is proposed.