Obsevation of Coherence in Two-Photon Excited Rb Vapor

A coherent superposition state of 5S and 5D levels of Rb vapor, when it was excited by two-photon resonant transition between these levels, was observed by applying an interrogation pulse which was non-resonant to the system. The emphasis is laid on this coherence in the atomic excitation, instead of just the population inversion, in explaining the origin of various strong collimated emissions observed by many authors. Saturation of the off diagonal density matrix element between 5S and 5D levels was observed at higher excitation level due to the population change. An analysis of the saturation is given using a simple model for the coherence of the dye laser beam.