Threshold nonlinear absorption in Zeeman transitions

Abstract

We experimentally study the absorption spectroscopy from a collection of gaseous 87Rb atoms at room temperature irradiated with three fields. Two of these fields are in a pump–probe saturation absorption configuration. The third field co-propagates with the pump field. The three fields address Zeeman degenerate transitions between hyperfine levels 5S1/2, F = 1 and 5P3/2, F = 0, F = 1 around the D2 line. We find a sub-natural absorption resonance in the counter-propagating probe field for equal detunings of all three fields. This absorption arises in conjunction with the appearance of increased transmission due to electro-magnetically induced transparency in the co-propagating fields. The novel feature of this absorption is its onset only for the blue of 5P3/2, F = 0, as the laser frequency is scanned through the excited states 5P3/2, F = 0, F = 1 and F = 2. The absorption rapidly rises to near maximum values within a narrow band of frequency near 5P3/2, F = 0. Our experimental results are compared with a dressed atom model. We find the threshold absorption to be a result of coherent interaction between the dressed states of our system.