Dark resonances with variable Doppler sensitivity

We have studied dark resonances in a thermal gas of rubidium atoms using a variable angle between the two driving optical beams. The inclined angle was tuned between a Doppler-free situation (i.e., collinear beams) and near 10^-30.3em0exrad, yielding an effective Doppler sensitivity of a far-infrared photon. For the case of buffer gas cells, Dicke narrowing occurs. We can carefully control the transition between the Dicke regime, in which the Doppler broadening is suppressed, and the Doppler-broadened regime, in which the thermal atomic motion strongly couples to the three-level atoms.