Eigenvectors of a Raman medium

Summary form only given. Two laser beams whose frequency difference is close, but not exactly equal, to that of a Raman resonance in a molecular medium will drive the resonance so as to establish a phased and, in effect, propagating molecular coherence. We discuss the conditions for the existence of Raman eigenvectors. These conditions are: (1) In analogy to electromagnetically induced transparency (EIT) in atoms, one must use the anti-phased molecular state. (2) There is a requirement on the magnitude of the Raman polarizability as compared to the background dispersive polarizability. (3) The pulse train as a whole (for example 20-ns long for typical Q-switched lasers) must have an energy which is sufficiently large that the total number of photons is large as compared to the number of molecules in the laser path. When these conditions are satisfied, also in analogy to EIT, the Raman coherence is self-consistently prepared by pulses in the front of the pulse train.