Saturated absorption spectroscopy: Elimination of crossover resonances with the use of a nanocell

Abstract

It is demonstrated that velocity selective optical pumping/saturation resonances of reduced absorption in a Rb vapor nanocell with thickness L= λ, 2λ, and 3λ (resonant wavelength λ = 780 nm) allow the complete elimination of crossover (CO) resonances. We observe well pronounced resonances corresponding to the Fg = 3 → Fe = 2, 3, 4 hyperfine transitions of the 85 Rb D2 line with linewidths close to the natural width. A small CO resonance located midway between Fg = 3 → Fe = 3 and Fg = 3 → Fe = 4 transitions appears only for L = 4λ. The D2 line (λ = 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 µm. The absence of CO resonances for nanocells with L ∼ λ is attractive for frequency reference application and for studying transitions between Zeeman sublevels in external magnetic fields.

Giant modification of atomic transition probabilities induced by a magnetic field: forbidden transitions become predominant

“…The most simple and straightforward technique to study such modification of atomic transitions (whose frequency distance belongs to optical domain) is laser spectroscopy of atoms contained in an atomic vapor cell. For B up to  1000 G, the split Zeeman transitions remain overlapped because of Doppler broadening, and sub-Doppler techniques have to be implemented in order to spectrally resolve and study transition probability of individual transition components [7].…”

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

“…3), while it is not the case for SA; moreover, as a rule, a strong cycling transition in the SA spectrum has a smaller amplitude in comparison with that of the weaker non-cycling one; (iii) the SA geometry requires counter propagating beams, while a single-beam transmission spectrum is needed for a NTC; and (iv) the laser power needed for spectral reference applications in case of NTC is more than by an order of magnitude less than that needed for SA. The above mentioned advantages demonstrated earlier for the frequency reference based on a Rb NTC [27] are also valid for a K NTC. The experimental absorption spectra for the atomic transitions F = 2, 1 → F ′ = 1, 2 for the low laser power P L = 2 µW are shown in Fig.…”

Efficient technique for measuring laser frequency stability

“…When the nanocell thickness is L ≈ λ, sub-Doppler peaks of reduced absorption appear in its transmission spectrum. These peaks are centered on the individual hyperfine transitions [20] as in saturated absorption. Unlike in saturated absorption spectroscopy, however, the nanocell technique shows no crossover resonances.…”

“…A hole is drilled in the bottom of the YAG windows for attaching the sidearm. Before filling the nanocell with natural rubidium, the cell was carefully outgassed during 8 h at 400 • C. The wedged gap between the windows allows us to vary the thickness of the vapor column traversed by the laser beam by a vertical translation of the cell, and to adjust the thickness to be L = λ/2 or L = λ, which is important for achieving the narrowest sub-Doppler linewidth [15,19], or for the formation of the reference spectrum [20]. An interferometric mapping of the gap thickness was performed using a He-Ne laser, showing that the thickness of the gap varies between 100 nm and 1000 nm.…”

“…In order to increase the absorption in the nanocell, the side-arm temperature was kept at 130 • C (with a 20 degrees higher window temperature). The lower curve in the figure is a frequency reference spectrum formed by the nanocell with L = λ thickness [20]. The feedback is adjusted to lock the signal to zero voltage.…”