Optical phase conjugation. Derive an expression for the phase-conjugate reflectivity obtained by degenerate four-wave mixing utilizing the nonlinear response of a collection of "two-level" atoms. You may make the rotating wave and slowly varying amplitude approximations and may assume that the amplitudes of the strong pump waves are not modified by the nonlinear interaction. [Hint: This problem can be solved using the formalism developed in Section 6.3. This problem has been solved in the scientific literature, and the solution is given by Abrams and Lind, 1978.]