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American Journal of Physics

Measurement of sub-natural linewidth AC Stark shifts in cold atoms: An experiment for an advanced undergraduate laboratory

J. Kleykamp, Andrew J. Hachtel, D. G. Kane, M. D. Marshall, Nathan Souther, Peter Harnish, Samir Bali

Published Nov 29, 2011• 4 citations
Publisher

Abstract

We measure sub-MHz AC Stark shifts, also known as light shifts, in an undergraduate laboratory setting using Raman pump-probe spectroscopy to observe sub-natural linewidth spectral features in the transmission spectrum of a weak probe beam passing through a sample of cold 85Rb atoms confined in a magneto-optical trap. To make this observation a pair of inexpensive fast photodiodes and acousto-optic modulators is needed, in addition to equipment commonly found in advanced undergraduate optics labs with laser cooling and atom trapping setups. A theoretical description of light shifts accessible to junior and senior-level physics majors is provided.

DOI
10.1119/1.3633702

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We measure sub-MHz AC Stark shifts, also known as light shifts, in an undergraduate laboratory setting using Raman pump-probe spectroscopy to observe sub-natural linewidth spectral features in the transmission spectrum of a weak probe beam passing through a sample of cold 85Rb atoms confined in a magneto-optical trap. To make this observation a pair of inexpensive fast photodiodes and acousto-optic modulators is needed, in addition to equipment commonly found in advanced undergraduate optics labs with laser cooling and atom trapping setups. A theoretical description of light shifts accessible to junior and senior-level physics majors is provided.
We have measured light shifts, also known as AC Stark shifts, as a function of laser intensity in cold Rubidium atoms by observing sub-natural linewidth gain and loss features in the transmission spectrum of a weak probe beam passing through the atomic sample. The observed energy-level shifts for atoms in a magneto-optical trap (MOT) are found to be consistently higher than that obtained in optical molasses (i.e., when the magnetic field gradient in the MOT is turned off). Using a simple model of a multilevel Rubidium atom interacting with pump and probe beams, we have calculated the theoretical light shift as a function of intensity. A comparison of these calculated values with the light shift data obtained for molasses reveals good agreement between experiment and theory. Further, our model elucidates the role of the Zeeman shifts arising from the magnetic field gradient in the observed probe transmission spectrum for the MOT.
We have measured light shifts, also known as the A.C. Stark effect, in cold Rubidium atoms using pump-probe spectroscopy.The measurement was made both for atoms in a magneto optical trap (MOT) and for atoms that were in an optical molasses.We show that while the measured light shifts agree with theory for optical molasses there are additional Zeeman shifts in the MOT that the theory does not account for.To the best of our knowledge, this is the first time a careful systematic measurement has been performed in cold atoms of light shift as a function of intensity.
We have studied the absorption of a weak probe beam through cold rubidium atoms in a magneto-optic trap. The absorption spectrum shows two peaks, with the smaller peak having linewidth as small as 28% of the natural linewidth. The modification happens because the laser beams used for trapping also drive the atoms coherently between the ground and excited states. This creates "dressed" states whose energies are shifted depending on the strength of the drive. Linewidth narrowing occurs due to quantum coherence between the dressed states. The separation of the states and the linewidth depends on the trapping-laser intensity and detuning as predicted by this model.
We report on cw measurements of probe beam absorption and four-wave-mixing spectra in a 85Rb^{85}\mathrm{Rb}85Rb magneto-optical trap taken while the trap is in operation. The trapping beams are used as pump light. We concentrate on the central feature of the spectra at small pump-probe detuning and attribute its narrow resonant structures to the superposition of Raman transitions between light-shifted sublevels of the ground atomic state and to atomic recoil processes. These two contributions have different dependencies on trap parameters and we show that the former is inhomogeneously broadened. The strong dependence of the spectra on the probe-beam polarization indicates the existence of large optical anisotropy of the cold-atom sample, which is attributed to the recoil effects. We point out that the recoil-induced resonances can be isolated from other contributions, making pump-probe spectroscopy a highly sensitive diagnostic tool for atoms in a working magneto-optical trap.