Coherently induced hole-burnings in a Doppler broadened four-level atomic system

We investigate the absorption spectra of a probe laser in warm rubidium atoms driven into the four-level N configuration. Our numerical calculations show that there are six (two) coherent hole-burnings and one (two) transparency window(s) in the probe absorption spectra, when the perturbing laser co-propagates (counter-propagates) with the probe laser and both coupling and saturating lasers are exactly resonant with their driving transitions. Gradually weakening the perturbing laser, we can dramatically change the probe absorption spectra to observe more coherent hole-burnings and transparency windows as well as the conversion from transparency windows into coherent hole-burnings. Relevant phenomena can be well understood in the dressed state representation.