Saturated absorption spectroscopy: elimination of crossover resonances by use of a nanocell

It is demonstrated that velocity selective optical pumping/saturation resonances of reduced absorption in a Rb vapor nanocell with thickness $\mathit{\text{L=}}$ $\lambda$, 2$\lambda$, and 3$\lambda$ (resonant wavelength $\lambda$ = 780 nm) allow the complete elimination of crossover (CO) resonances. We observe well pronounced resonances corresponding to the F${}_g=3$ $\to$ F${}_e=2,3,4$ hyperfine transitions of the ${}^{85}$Rb D${}_2$ line with linewidths close to the natural width. A small CO resonance located midway between F${}_g=3$ $\to$ F${}_e=3$ and F${}_g=3$ $\to$ F${}_e=4$ transitions appears only for $\mathit{\text{L}}$ = 4$\lambda$. The D${}_2$ line ($\lambda$ = 852 nm) in a Cs nanocell exhibits a similar behavior. From the amplitude ratio of the CO and VSOP resonances it is possible to determine the thickness of the column of alkali vapor in the range of 1 - 1000 $\mu$m. The absence of CO resonances for nanocells with $L\sim \lambda$ is attractive for frequency reference application and for studying transitions between Zeeman sublevels in external magnetic fields.