Triplet absorption spectroscopy and electromagnetically induced   transparency
  Coherence phenomena in four-level atomic system, cyclicly driven by three coherent fields, are investigated thoroughly at zero and weak magnetic fields. Each strongly interacting atomic state is converted to a triplet due to a dynamical Stark effect. Two dark lines with a Fano-like profile are arising in the triplet absorption spectrum with anomalous dispersions. We provide the conditions to control the widths of the transparency windows by means of the relative phase of the driving fields and the intensity of the microwave field, that closes the optical system loop. The effect of the Doppler broadening on results of the triplet absorption spectroscopy is analysed in detail. 
F. Ghafoor and R.G. Nazmitdinov
physics.optics quant-ph