Velocity Selective Optical Pumping of Rb Hyperfine Lines Induced by a Train of Femtosecond Pulses

We present direct observation of the velocity-selective optical pumping of the Rb ground state hyperfine levels induced by $5S_{1/2}\to 5P_{1/2}$ femtosecond pulse-train excitation. A modified direct frequency comb spectroscopy based on the fixed frequency comb and a weak cw scanning probe laser was developed. The femtosecond pulse-train excitation of a Doppler-broadened Rb four-level atomic vapor is investigated theoretically in the context of the density matrix formalism and the results are compared with the experiment.

Figure 1

Typical time evolution of the ${}^{87}\mathrm{Rb}$ hyperfine state populations, presented as a time average per laser repetition period. $\tau =1.25\mu \mathrm{s}$ represents the average interaction time. (a) ${}^{87}\mathrm{Rb}(5P_{1/2})$ populations. Inset: The results of the direct numerical integration. (b) ${}^{87}\mathrm{Rb}(5S_{1/2})$ populations.

Figure 2

The variations of the ground state populations ${\rho }_{11}$ (solid line) and ${\rho }_{22}$ (dotted line) for different atomic velocity groups with and without fs laser excitation. (a) ${}^{87}\mathrm{Rb}$. (b) ${}^{85}\mathrm{Rb}$. Insets: Schemes of the $5S_{1/2}\mathrm{\text{−}}5P_{1/2}$ four-level system with the corresponding relative transition probabilities.

Figure 3

The comparison of the measured and calculated ${}^{87}\mathrm{Rb}$ $5S_{1/2}\to 5P_{3/2}$ hyperfine absorption line profiles: (a) $F_g=1\to F_e=0,1,2$ line; (b) $F_g=2\to F_e=1,2,3$ line.

Figure 4

The comparison of the measured and calculated ${}^{85}\mathrm{Rb}$ $5S_{1/2}\to 5P_{3/2}$ hyperfine absorption line profiles: (a) $F_g=2\to F_e=1,2,3$ line; (b) $F_g=3\to F_e=2,3,4$ line.