Effective polarization method for controlling a sign of the nonlinear subnatural resonances

We propose the effective and easy method for controlling a sign of narrow resonances of electromagnetically induced transparency (EIT) and absorption (EIA). These nonlinear resonances are the bright examples of the nonlinear effects based on the low-frequency atomic coherences. Here we explore the general light polarizations and various types of atomic transitions. For convenience we use the Hanle configuration for observing the EIT and EIA signals. The scheme is composed of two counterpropagating overlapped light waves with the same frequency and the static magnetic field applied along the cell. The sign of the resonance can be governed by varying either the angle between the polarizations or the ellipticity parameters. The calculations have shown that the sign-reversal effect exists for the dark-type atomic transitions as well as for the bright ones (i.e. F→F+1). We also give some analytical results and clear physical interpretation. Some of the theoretical conclusions have been proved by the experiments (D₁-line in ⁸⁷Rb). Also, it should be specially mentioned that the method proposed can provide the high-contrast EIA resonances even in the presence of a buffer gas, when the collisional depolarization of the exited state takes place.