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).
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.
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.
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.
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