Sub-Doppler features due to double dark resonances

A sharp intense sub-Doppler feature is predicted in the probe absorption spectrum of an atomic four-level system when a hyperfine-split state interacts with a single pump. The sub-Doppler feature, which is flanked by two power-broadened transparency windows, is obtained when the frequency of the pump is greater than that of the probe, and the residual Doppler broadening is significant.