Rivalry of the dark resonance concentration narrowing and field broadening in the ladder system of rubidium atoms: thin spectroscopic cells peculiarities

The effect of electromagnetically-induced transparency in the ladder system of $5S_{1/2}-5P_{3/2}-5D_{5/2}$ levels of Rb atoms is studied. The effect of spectral narrowing of the dark resonance (DR) is shown depending on the density of atomic vapor and the thickness $L$ of the spectroscopic cell, which contains Rb atomic vapor. The thickness $L$ varies from 390 nm to 4 mm, the atomic density $N$ increased to $\sim$10$^{16}$ cm$^{-3}$, and intense coupling and weak probe radiation were used. The maximum effect, namely the 22-fold spectral narrowing of DR, was achieved in a cell with a thickness of $L$ = 4 mm. With decreasing of the thickness $L$, the effect of spectral narrowing becomes weaker: for example, at $L$ = 2 $\mu$m, a 2.4-fold spectral narrowing of DR occurs. The spectral narrowing of DR was practically absent at $L$ = 0.8 $\mu$m, and with a further decrease to $L$ = 0.4 $\mu$m with increasing of atomic vapor density, spectral broadening of DR began to occur. In almost all cases, at moderate atomic densities and high intensities of the coupling radiation, $\sim$100% DR contrast was achieved. The effect of spectral narrowing and broadening of DR is explained.