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 D2 line of 87Rb where the Fg = 1, mg = −1, 0, 1 Zeeman sublevels form the bottom three levels and the Fe = 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 sub-natural. The sharp absorptive resonance is characterized as a function of applied magnetic field and pump laser power.