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 = 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 where lambda = 770 nm is the resonant wavelength of the laser). It is shown that despite the huge Doppler broadening (> 0.9 GHz at the cell temperature 170 degrees C), at low laser intensities a narrowing of the resonance absorption spectrum is observed for L = lambda/2 (similar to 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 1.4 cm-sized K cell is presented. The developed theoretical model well describes the experiment.