Optical Pumping of Mixtures of Rb vapor and N2 Buffer Gas

Optical Pumping of Mixtures of Rb vapor and N₂ Buffer Gas

Link to the DAMOP 2010 Poster

Using the experimental setup shown in Fig. 1, we have studied the optical pumping of mixtures of Rb vapor and N₂ buffer gas. As the frequency of the right-hand circularly-polarized (σ⁺) laser is varied across the D1 absorption profile, the electron spin polarization of the Rb, P_e, is found to take on negative values for small negative values of pump detuning from the absorption profile center (see Fig. 2).

Fig. 1 Apparatus schematic: linear polarizer, quarter-wave plate, beam sampler, and photodiode.

Fig. 2: a) Absorption scan of probe in Rb reference cell. b) Positions of hyperfine transitions, from left to right: ⁸⁷Rb F_g = 2 → F_e = 1, ⁸⁷Rb 2 → 2, ⁸⁵Rb 3 → 2, ⁸⁵Rb 3 → 3, ⁸⁵Rb 2 → 2, ⁸⁵Rb 2 → 3, ⁸⁷Rb 1 → 1, ⁸⁷Rb 1 → 2. c) Measured and calculated polarization of Rb vapor as a function of pump laser frequency. Blue data: 10 Torr N₂, 4.3 • 10¹² cm^-3 Rb; solid curve: 10 Torr N₂, 99.5% light polarization; dashed curve: 10 Torr N₂, 99.95% light polarization. Green data: 1.0 Torr N₂ and 8.8 • 10¹² cm^-3 Rb; curve: 1.0 Torr N₂ and 99.5% light polarization. Red data: 0.1 Torr N₂, 8.4 • 10¹² cm^-3 Rb density; curve: 0.1 Torr, 99.5% light polarization.

The spin reversal phenomenon is due to the underlying hyperfine structure of the optically-pumped Rb vapor. This can be understood by considering the Zeeman structure of ⁸⁵Rb (Fig. 3). When F < I+J, the expectation value of P_e for a given eigenstate of the hyperfine Hamiltonian is proportional to -m_F. Thus, for ⁸⁵Rb, if one could selectively populate the ²S_1/2 F = 2 state with m_F > 0, the polarization of the hyperfine-averaged J-states of the atom would exhibit P_e < 0. To achieve this by pumping the F = 3 --> F = (2,3) transition, however, requires a non-σ⁺ component of the pump laser polarization that can drive vertical or left-going absorptions in order to eliminate population of the F = 3, m_F = +3 dark state.

We have modeled these effects with rate equations for the individual F, m_F ground- and excited-state sublevels for ⁸⁵Rb and ⁸⁷Rb. The predictions of the model are depicted as the solid-line curves in Fig. 2c with the fit parameters of laser power 100 mW and the fraction of left-hand circularly-polarized (σ ^-) light is 0.5%. The model gives reasonable agreement with the data. The dashed-line curve shows the results for 10 Torr with the fraction of σ ^- light equal to 0.05%. As the data sets were taken on different days, the waveplate may have been set slightly differently for the 10 Torr data set or experienced a small rotation around the vertical axis, which would yield different polarizations at the respective optimal settings.

Joan Dreiling, Jorgensen Laboratory of Physics