Pump-Probe Spectroscopy of Rubidium Vapour

David Smith, Ifan Hughes

We study spectroscopy of Rubidium vapour using probe and counter-propagating pump beams. This page demonstrates that the standard designation of “saturation spectroscopy” is a misnomer in multilevel systems where hyperfine pumping can occur. Hyperfine pumping is the transfer of atoms from one hyperfine ground state into another via absorption and then spontaneous emission. In fact, you'll see that experimental observation dictates that hyperfine pumping is the dominant process in the formation of sub-Doppler features in such a system. In contrast to saturated absorption, the details of the transient solution are crucial and hyperfine pumping leads to a modification of the absorption for detunings of many tens of natural linewidths from resonance.

A published version of this work can be found in the American Journal of Physics: The role of hyperfine pumping in multilevel systems exhibiting saturated absorption, DA Smith and IG Hughes, Am J Phys 72, 631 (2004).

Theory
Top Experimentation Conclusions

A Multi-Level Atom

The energy level diagram shows the ground and excited states that we study in 85Rb. The ground state energy splitting, 3GHz, is much larger than the excited state energy splitting, 213MHz, such that transitions from the lower ground state are extremely far from resonance with a laser beam tuned between the upper ground state and the excited state.Ê Therefore any atom in the lower ground state will be dark to the laser light. The Doppler width is ~500MHz FWHM, so for this system we obtain two separate absorption profiles, since the ground state splitting is 3GHz.

The laser beam, ωL, excites three velocity classes in a Maxwellian velocity distribution, dependent on the detuning from resonance, Δi = ωL- ωi. The transition from the ground F=3 level to the excited F′=4 level is a closed transition, due to electric dipole selection rules, and a hole is burned in the F=3 ground state velocity distribution due to saturation. However, in a multilevel system, hyperfine pumping (the removal of atoms from one ground state into another via absorption and then emission) provides an efficient sink from the original ground state and “canyons” can be burned into the velocity distribution.

Theoretical Spectra

A theoretical transmission spectrum is shown for a probe beam inspecting 85Rb vapour which is subject to a counter-propagating pump beam. Six sub-Doppler features are present. Three correspond to the resonances F=3 to F′=4, 3 and 2; and the other three are crossover resonances. Crucially, these spectra can not be specified solely by a saturation intensity since we are dealing with a multilevel system.

The existence of a dark ground state, accessible within an atom’s transit of the laser beam, means that the transient dynamics of the system are critical. The timescale to reach the steady-state is longer than typical transit times. An atom’s transit time is determined by the temperature of the vapour; the path through the beam; the laser beam’s width and intensity profile; and the excited state lifetime (27ns in this case)!

Hyperfine Pumping “turned off”

In the absence of hyperfine pumping, the theoretical spectrum looks very different.Ê The dominant sub-Doppler peak is the feature corresponding to the F=3 to F′=4 transition, (below).

Saturation “turned off”

In the absence of saturation, the theoretical spectrum appears very similar to experimental observation. The main difference is the absence of the only feature arising exclusively from saturation, the closed F=3 to F′=4 transition.

Experimentation
Top Theory Conclusions

Schematic

Laser light is derived from an external cavity diode laser and split using a thick glass slide into a probe beam (reflected) and pump beam (transmitted). The probe beam is allowed to pass through a room temperature rubidium vapour cell onto a photodiode. Mirrors direct the pump beam such that it counter-propagates the probe beam through the cell. Attenuators are used to vary the power of the pump and probe independently. The pump and probe beams are of identical frequency.

Observations

Spectra obtained with the above experimental set-up show transmission through a rubidium vapour cell - in this case transitions from the 5S1/2 F=3 ground state. Each trace shows a Doppler-broadened transmission profile obtained in the absence of a pump beam (red curve). The observed spectra are shown (in blue) for three different pump powers (increasing top to bottom): 0.5mW, 0.9mW, and 4.5mW respectively. Each subsequent trace shows the previous (in grey) for comparison.

Key Points

There is a dramatic evolution of the sub-Doppler features as the pump power is increased - at higher pump powers some features even merge.

There is not only a change in absorption at resonance frequencies, but also across a large proportion of the Doppler background: this is a direct consequence of the canyons burned into the F=3 ground state velocity distribution.

These experimental spectra do not resemble the theoretical spectra with hyperfine pumping “turned off.” Although saturation does play a part, hyperfine pumping is the dominant mechanism.

Conclusions
Top Theory Experimentation

Hyperfine Pumping dominates the formation of sub-Doppler features in pump-probe spectroscopy with room temperature alkali metal vapours.

The usual nomenclature of “saturation spectroscopy” is a misnomer in sight of this.

The steady-state, two-level atom approach isn't applicable: the multi-level system’s steady state is for all the atoms to be in the dark ground state.

The saturation intensity of the medium is not sufficient to describe the spectra - beam width, intensity profile and atomic velocity distribution are crucial.

Content © David A Smith, Durham University 2005