Generalized perturbation theory of coherent optical emission

A generalized perturbation theory is developed for an inhomogeneously broadened two-level quantum system interacting with an optical field of arbitrary temporal shape. Solutions of the Liouville equation carried out to nth order in the optical field strength exhibit a rephasing behavior and therefore are interpreted as an n-pulse echo where each pulse corresponds to one power of the field strength. We conclude that at low fields any coherence effect can be interpreted as a generalized stimulated echo phenomenon. Several well-understood coherence effects are explained and a few new ones are predicted, including one example where the light source is incoherent. © 1985 The American Physical Society.