Abstract
The steady state of the density matrix is analysed for the two-colour light beam resonantly coupled to the different transitions between the lower
S
1/2 and upper
P
1/2
sodium hyperfine levels,
F=1 and 2. Particularly, the populations, the contributions to the transition dipole moments and the Raman coherences have been calculated for various beam intensities and detunings. The width of the EIT resonance has been found to be entirely due to the collisional destruction of the ground-state Raman coherence and the power broadening. Therefore, the corresponding profile is very narrow for the weak field, as compared with the spectral line shape. In the case of sufficiently strong coupling beam, the dynamical Stark effect is dominant. We have found that the position of the EIT resonance does not depend on the field intensities of the coupling and probe beams.