Nonlinear optical parametric processes are studied in a resonantly driven multilevel system displaying quantum interference effects. It is shown that in such systems a new regime of nonlinear amplification is possible, in which a pair of correlated Stokes and anti-Stokes fields can be generated from infinitesimally small initial values. An atomic coherence grating emerges during this process of efficient nonlinear amplification. The present analysis explains the results of recent optical phase conjugation experiments involving atomic phase coherence.

• Received 17 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.2675