Nonlinear-optical properties of two-level systems with permanent dipole moments

Abstract
We calculate the transition rates and the linear and nonlinear-optical properties of a homogeneously broadened two-level system with permanent dipole moments in the presence of two pump fields. We present absorption and emission spectra for the system in the steady state, without making the rotating-wave approximation. Expressions for the nonlinear susceptibilities for wave mixing are derived.