Coherent dips revisited

The semiclassical two-level multiwave coefficients are presented in the limit that the population-difference lifetime T1, is much greater than the dipole lifetime T2. These coefficients are easy to understand analytically and show how pump-probe detuning shifts the phase and the amplitude of pump scattering off induced population pulsations. In particular, a simple formula is found that shows how energy can be exchanged by two waves in an apparently pure-index regime. An FM wave is shown to propagate unchanged through such a medium, and the beat-note component of an AM wave is shown to be unmodified by a thin medium. Simple formulas for T1 of the order of T2 are given that reveal intense pump-dispersive and Lorentzian features at a glance. The results are of use in saturation spectroscopy, modulation spectroscopy, optical instabilities, and phase conjugation.