We discuss level mixing induced transparency (LMIT), which is experimentally found in a thick absorber of FeCO3 containing the 57Fe Mössbauer nuclei. LMIT is observed if two hyperfine levels of the excited state nucleus cross. Absorption of gamma radiation drops by about 25% with respect to the sum of two overlapping absorption lines. The absorption deficit is explained by the mixing of the crossing levels with a symmetry breaking interaction. The level mixing induces a polarization change of the scattered radiation. In such a way, the radiation develops in “normal modes,” which are less absorbed in a thick absorber. We show that, in spite of the absorption drop, the area of the absorption line does not change. This confirms the general knowledge that interference phenomena (destructive or constructive) do not change the lifetime of an excited state particle (atom, nucleus, etc).
We present the results of a pump-probe laser spectroscopic investigation of the Doppler-broadened sodium
D1 resonance line. We find 13 resonances in the resulting spectra. These observations are well described by the numerical predictions of a four-level atomic model of the hyperfine structure of the sodium
D1 line. We also find that many, but not all, of these features can be understood in terms of processes originating in a two-level or three-level subset of the full four-level model. The processes we observed include forward near-degenerate four-wave mixing and saturation in a two-level system, difference-frequency crossing and nondegenerate four-wave mixing in a three-level V system, electromagnetically induced transparency and optical pumping in a three-level lambda system, cross-transition resonance in a four-level double-lambda system, and conventional optical pumping. Most of these processes lead to sub-Doppler or even subnatural linewidths. The dependence of these resonances on the pump intensity and pump detuning from atomic resonance are also studied.
The interaction of a two-level system with a strong standing wave is examined by the probe field resonant to the adjacent transition in the $\ensuremath{\Lambda}$ configuration of Ar II. The known peak of electromagnetically induced transparency is observed in the absorption spectrum at the detuning of the probe field proportional to the detuning of the strong field. Along with it, the different structure has been found at the line center, independently of the strong-field frequency. The calculations clarify that the central structure is induced by the higher-order spatial harmonics of the atomic coherence induced at the probe transition. The effect results in steep frequency dependence of the refractive index at the line center.