Transient Two-Photon Coherent Effects
Due to the availability of high power, single-mode laser sources, it is now possible to study coherent interactions of light with atoms and molecules via multi-photon processes. Here, we will describe the first experimental observation of transient two-photon coherent precession — one of the many possible two-photon analogs of well known one-photon coherent effects which have been predicted by the two-photon vector model of Grischkowsky, Loy and Liao. In terms of this vector model, the process can be visualized in a way similar to its familiar one-photon analog: the two-photon polarization vector precesses about the effective field and, depending on their relative phases, induces alternating absorption and emission of light. There are, in addition, features unique to the two-photon case, for example, the optical Stark effects and the Doppler width reduction from counter-propagating beams are shown to be of crucial importance in the experiment.