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Light Shift Measurements of Cold Rubidium Atoms using Raman Pump-Probe Spectroscopy

Nathan Souther

Published Jan 1, 2009• 1 citation
OpenAlex

Abstract

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.

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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 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 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 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.
We demonstrate that when laser light is circularly polarized and properly detuned, the ac Stark shift of an alkali-metal atom in its ground state takes the form of a pure Zeeman shift. The condition is satisfied when the laser frequency is between the D1D1D1 and D2D2D2 transition frequencies, with the size of the detuning from the D2D2D2 resonance twice that from the D1D1D1 resonance. The direction of the effective magnetic field is along the laser propagation axis, and its magnitude is proportional to the vector polarizability of the atom and to the laser intensity. We use stimulated Raman spectroscopy on an optically pumped slow atomic beam from a magneto-optical trap to measure the vector polarizability and saturated absorption spectroscopy to show that the scalar polarizability vanishes at the particular detuning.
The energy shift of an atom due to its interaction with the oscillating electric field of an electromagnetic wave, or a laser light, is called the ac Stark shift. It has been studied both in the field of precision spectroscopy and atom optics. When the laser intensity is not constant spatially, its gradient gives rise to a dipole force. The force was used either to trap atoms in an optical trap or to reflect or guide atoms. We studied the ac Stark effect taking into account the fine structure of an alkali atom. We found that when the laser light is circularly polarized and properly detuned between the D1 and D2 transitions, the ac Stark shift can take the form of a pure Zeeman shift, hence the name analogous Zeeman shift.