Phase-sensitive probing of coherent superpositions of lower states in a double-Λ system

Electromagnetically induced transparency and absorption due to competition between coherent dark states associated with two resonant Λ fields have been observed on the D1 line of 87Rb vapor. The properties of these resonances can be manipulated by varying the laser power ratio of the two copropagating Λ fields and their phase difference. The coherent effects have been examined for a closed transition between two ground-state and two excited-state levels without atomic level degeneracy. The results are in good agreement with theoretical predictions.