EFFECTS OF SPONTANEOUSLY GENERATED COHERENCE ON THE PUMP-PROBE RESPONSE OF A LAMBDA-SYSTEM

Near-degenerate lower levels in a A system have an additional coherenc e term due to interaction with the vacuum of the radiation field. We r eport the effects of this spontaneously generated coherence on the for mation of a trapped state in the presence of two coherent fields of ar bitrary intensity. We show that such coherence preserves both electrom agnetically induced transparency and coherent population trapping (CPT ) phenomena. However, it changes the time scales associated with the f ormation of the CPT state, and brings about quantitative changes in th e line profiles. We present a clear analytical explanation for our num erical results. We also report the dependence of line shapes on the re lative phase between the two applied fields.