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 D1 line of potassium atoms by varying the laser intensity and the cell thickness (from L = λ / 2 ?> to L = 2 λ ?> with the step λ / 2 ?> where λ = 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 ◦ C ?> ), at low laser intensities a narrowing of the resonance absorption spectrum is observed for L = λ / 2 ?> ( ∼ 120 MHz ?> at FWHM) and L = 3 / 2 λ ?> , whereas for L = λ ?> and L = 2 λ ?> the spectrum broadens. At moderate laser intensities narrowband velocity selective optical pumping (VSOP) resonances appear at L = λ ?> and L = 2 λ ?> 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.