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arXiv:physics/0204021 · Europhysics Letters (EPL)

Observation of sub-natural linewidths for cold Rb atoms in a magneto-optic trap

Umakant D. Rapol, Ajay Wasan, Vasant Natarajan A

Published Jan 1, 2003• 3 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1209/epl/i2003-00244-6

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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 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 report an experimental observation of doubly dressed states in 87Rb{}^{87}\mathrm{Rb}87Rb atoms cooled and confined in a magneto-optical trap. The doubly dressed states are produced by a strong-coupling laser and a moderate pump laser in a three-level atomic configuration. The absorption spectrum of a weak probe laser reveals a three-peaked spectral profile that can be interpreted by a dressed-state picture and agrees well with calculations based on the density-matrix equations.
Absorption spectra of cold Cs atoms in a magneto-optical trap under the influence of a strong external pump laser field with arbitrary detuning and intensity have been systematically investigated. The spectra featured the Rabi sidebands of each transition, and their behavior was nicely fitted by the dressed-atom theory. A pronounced dispersion-type profile with sub-natural linewidth at the pump frequency may be attributable to the stimulated Raman process by sublevels that are lightshifted and optically pumped by the pump laser.
A spectroscopic investigation is reported for a sample of Cesium atoms which are cooled and confined in a Zeeman shift spontaneous force optical trap.(1'2) From measured absorption spectra for a weak probe beam both in steady state and under transient conditions, we infer that the sample consists of approximately 106 atoms trapped in a spherical volume of radius 400µm and cooled to roughly 300µK. To explore the spectral response of this sample under conditions of strong excitation by the trapping laser beams ("pump" waves), we record transmission as a function of frequency for a low intensity "probe" wave focussed through the trap. As might be expected, we observe probe spectra with broad regions of absorption and amplification characteristic of the Autler-Townes splitting for a "dressed" two-state atom. However, additional narrow features around the pump frequency have linewidths of several hundred kilohertz (well-below the natural linewidth of 5MHz) and exhibit appreciable single pass gain (≥30%) . Our preliminary attempts to describe these features theoretically provide limited qualitative agreement with experiment. In an effort to correlate the spectroscopic characteristics of the trap with its mechanical properties, we have observed recoil of the trapped atoms either along or opposite to the direction of propagation of the probe beam depending upon whether the probe experiences absorption or gain. The direction of recoil can be switched with a change in probe detuning of less than 300kHz.
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.
Abstract The influence of the spectral linewidth of single-frequency lasers on the parameters of the electromagnetically induced absorption resonances in rubidium vapour is examined. We use two counter-propagating pump and probe beams to prepare high-contrast magneto-optical resonances in a Hanle-configuration on Rb atoms contained in a paraffin-coated optical cell. Due to the spectral linewidth of the excitation laser, various groups of atoms are involved in creating the coherent resonance, which changes its parameters. The experimental results show the potential of the system to improve the signal-to-noise ratio for more sensitive measurement of weak magnetic fields.