Competition of Concentration Narrowing and Power Broadening of a Dark Resonance in a Ladder System of Rubidium Atoms: Specificities of Its Occurrence in Thin Spectroscopic Cells

The effect of electromagnetically induced transparency (EIT) in a three-level ladder system 5S(1/2)-5P(3/2) -5D(5/2) of Rb atoms is investigated. The effect of spectral narrowing of a dark resonance (DR) as a function of atomic vapor density and thickness L of a spectroscopic cell containing Rb vapor is demonstrated. Thickness L was varied in the range of 390 nm to 4 mm, while atomic density N was increased up to similar to 10(16) cm(-3); in the process, high-intensity coupling radiation and weak probe radiation were used. The strongest effect, namely, a 22-fold spectral narrowing of the DR, was achieved in a cell of thickness L = 4 mm. The effect of spectral narrowing was decreasing with decrease in thickness L: a 2.4-fold spectral narrowing of the DR was observed at L = 2 mu m. Spectral narrowing was nearly absent at L = 0.8 mu m, and spectral broadening of the DR was observed with further decrease in the cell thickness down to L = 0.4 mu m. A close to 100% DR contrast was achieved in nearly all cases at moderate atomic densities and high intensities of the coupling radiation. An interpretation of the effect of spectral narrowing and broadening of the DR is proposed.