Effect of local-field correction on a strongly pumped resonance

We present a general formalism that incorporates local-field corrections into Bloch's equations and use it to calculate the strong-field susceptibility of a pressure-broadened gas consisting of two-level atoms. The major effect is to introduce a dynamic Lorentz shift that varies throughout the resonance line shape. We also calculate the susceptibility associated with a weak field (probe) in the presence of a strong field (pump) when the pump and probe are at the same frequency. Here the effect is to introduce both a variable shift and a greater distortion of the line shape.