The theory of degenerate four-wave mixing using two-photon transitions is formulated. Two kinds of mechanisms compete in generating conjugate waves leading to two phenomena that do not occur in two-level media: (1) a double-peaked reflection spectrum and (2) coupled-mode oscillation in absorbing media. In addition, because of the dynamic Stark-shifted line center, the reflection coefficient approaches a constant value at large intensities, unlike the two-level coefficient, which bleaches to zero.
© 1980 Optical Society of America
Tao-Yi Fu and Murray Sargent III, "Theory of two-photon phase conjugation," Opt. Lett. 5, 433-435 (1980)