Sub-Doppler and sub-natural narrowing of an absorption line

We experimentally demonstrate the double dark resonance windows characteristic of a four level tripod system such as the D-2 line of Rb-87 where the F-g = 1, m(g) = -1,0,1 Zeeman sublevels form the bottom three levels and the F-e = 0 state is the upper level. Between the double dark resonance windows, a very narrow absorption resonance is observed whose line-width is shown to be sub-Doppler and subnatural. The sharp absorptive resonance is characterized as a function of applied magnetic field and pump laser power. (C) 2007 Elsevier B.V. All rights reserved.