Electromagnetically induced transparency and absorption in a standing wave

A thermal vapor of three-level atoms irradiated by two copropagating laser beams of suitable frequencies exhibits the phenomenon of coherent population trapping, leading to an electromagnetically induced transparency (EIT). When a mirror reflects the beams back onto themselves one finds that depending on the position along the resulting standing wave the fluorescence intensity from the sample (cesium in our case) decreases (EIT) or even increases [electromagnetically induced absorption (EIA)] with a period on the centimeter scale. An intuitive picture is based on the interference of coherent dark states but the explanation of the EIA effect requires consideration of Doppler effects in the thermal vapor. This allows for a quantitative comparison between experimental and calculated results.