Classical interaction of the electromagnetic radiation with two-level polarizable matter

The interaction of the classical electromagnetic field with an ensemble of polarizable, identical, atomic particles with two energy levels is investigated, and the coupled non-linear equations of motion for the polarization field and the amplitudes of the level occupancies are solved by a perturbation-theoretical method. A small coupling constant is identified, and the solution is represented as a power series in this coupling constant. Explicit results are given for the leading contributions to the solution. In particular, it is shown that an external electromagnetic field may induce a lasing effect in such an ensemble of particles, by populating the (initially empty) upper level.