Collapse and revival of a Dicke-type coherent narrowing in potassium vapor confined in a nanometric-thin cell

A nanometer-thin-cell (in the direction of laser beam propagation) has been elaborated with the thickness of the atomic vapor column varying smoothly in the range of $L = \unit[50-1500]{nm}$. The cell allows one to study the behavior of the resonance absorption over the $D_1$ line of potassium atoms by varying the laser intensity and the cell thickness from $L = \lambda / 2$ to $L = 2 \lambda$ with the step $\lambda/2$ ($\lambda =\unit[770]{nm}$ is the resonant wavelength of the laser). It is shown that despite the huge Doppler broadening ($>\unit[0.9]{GHz}$ at the cell temperature $\unit[170]{^{\circ}C}$), at low laser intensities a narrowing of the resonance absorption spectrum is observed for $L = \lambda/2$ ($\sim \unit[120]{ MHz}$ at FWHM) and $L = 3/2 \lambda$, whereas for $L = \lambda$ and $L =2\lambda$ the spectrum broadens. At moderate laser intensities narrowband velocity selective optical pumping (VSOP) resonances appear at $L = \lambda$ and $L=2\lambda $ with the linewidth close to the natural one. A comparison with saturated absorption spectra obtained in a 1-cm-sized K cell is presented. The developed theoretical model well describes the experiment.