Collision-enhanced Hanle resonances in different nonlinear optical processes

In November 1983, one of the authors ( Y. H. Zou) arrived in Harvard and joined Bloembergen's group. At that time the "collision-induced coherence" ofBloembergen was no longer a puzzling theoretical prediction [ 1) and, even in Harvard, was verified in a series of experiments in Sodium vapor (2]. These experiments may be classified into three types: collision-induced coherence between "unpopulated" excited states, collision-induced population grating and collision-enhanced coherence between initially equally populated states. The third one includes collision -enhanced Zeeman coherence in the atomic ground state and which can be described by a three level model: an optical excited state I e> and two ground Zeeman substates I g> and I g'>. We assume this atomic system is subjected simultaneously to two incident light beams,