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

TL;DR

This paper demonstrates that using a nanocell with specific thicknesses eliminates crossover resonances in saturated absorption spectroscopy, enabling precise frequency references and transition studies.

Contribution

The study shows that nanocells with thicknesses around the wavelength can completely eliminate crossover resonances, improving spectral resolution and potential applications in frequency standards.

Findings

Crossover resonances are eliminated in nanocells with thickness L = λ, 2λ, 3λ.

Resonances have linewidths close to natural width.

The amplitude ratio of resonances allows vapor column thickness measurement.

Abstract

It is demonstrated that velocity selective optical pumping/saturation resonances of reduced absorption in a Rb vapor nanocell with thickness \textit{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 \textit{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.