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Physical Review Letters

Nonlinear spectroscopy of cold atoms in a spontaneous-force optical trap

J. W. R. Tabosa, G. Chen, Zhongliang Hu, R. B. Lee, Harry J. Kimble

Published Jun 24, 1991• 121 citations
Publisher

Abstract

Probe-wave amplification and absorption spectra are reported for cesium atoms cooled and confined in a Zeeman-shift spontaneous-force optical trap. Novel spectral features are observed with narrow frequency widths and with single-pass gain of 20% in some cases. Negative radiation pressure is demonstrated as the mechanical consequence of optical gain and is investigated as a possible means for optical implosion of a trapped sample.

DOI
10.1103/physrevlett.66.3245

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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.

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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.

Laser Physics Letters•Feb 8, 2010•15 citations
Probe-wave amplification and absorption spectra are reported for cesium atoms cooled and confined in a Zeeman-shift spontaneous-force optical trap. Novel spectral features are observed with narrow frequency widths and with single-pass gain of 20% in some cases. Negative radiation pressure is demonstrated as the mechanical consequence of optical gain and is investigated as a possible means for optical implosion of a trapped sample.
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.
Nonlinear spectroscopy of cold atoms has recently been reported for a sample of cesium atoms confined in a magnetooptical trap.1,2 In particular, we report measurements of the spectral response of the cold sample to a weak probe beam for a range of detunings and power for the saturating trapping beams. Transmission spectra for the probe beam reveal the presence of broad absorption and gain features as well as narrow resonances. The power dependence of the broad part of the spectrum can be reasonably well described by earlier work for two-level atoms in a strong field.3 However, narrow dispersive shaped features around the frequency of the trapping beams are also observed and can have appreciable single pass gain (>20%). An exploration of the correlation between the narrow spectroscopic features and the mechanical properties of the atomic sample leads to the observation of both positive and negative radiation pressure associated with stimulated absorption from and emission to the probe beam.
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Investigations of optical processes for laser cooled and trapped atoms are described. Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap. In addition, numerical absorption and emission spectra for three-level [...], [...], and V systems under intense radiations are calculated. Absorption spectra for a [...] system is used to explain the probe-wave amplification and absorption spectra recorded for Cs atoms cooled and confined in a magneto-optical trap, in which novel spectral features of narrow frequency widths with single-pass gain exceeding 20% are observed. The consequence of the optical gain is demonstrated to lead to negative radiation pressure, which is investigated together with other mechanical forces in the trap. Various alternative trapping schemes in three-level and two-level atoms are proposed as possible means to compress an atomic sample and demonstrated for a two-level magneto-optical trap.
Radiation pressure due to the interaction between a probe light and cold atoms is investigated in a standard cesium magneto-optical trap. The radiation pressure alters the absorption spectroscopy of cold atoms, leading to line shapes and linewidths after resonant interaction that are different for positive and negative probe chirps. The difference is attributed to the radiation pressure of the probe laser, due to which atoms become accelerated at the resonance. The effect of the radiation pressure is also seen in electromagnetically induced transparency (EIT) involving an excited Rydberg level. The density matrix equation accounting for the radiation pressure is used to simulate the experiments. The simulations agree well with the measurements both for absorption and EIT spectra. We find that the effect of the radiation pressure is reduced at low probe intensities, and can be neglected when the probe intensity is smaller than Isat/2 .
Summary form only given. Resonant cold atomic samples are known to provide an efficient medium for the study of nonlinear optical processes such as four-wave-mixing (FWM) and phase conjugation (PC). Although this system is frequently modeled as an homogeneous sample of two-level atoms, the actual nonlinear response strongly depends on the internal structure (Zeeman degeneracy) of the atomic energy levels and consequently on the polarizations of the optical fields. We have studied the spectra of quasi-degenerate PC on the 6S/sub 1/2/(F=4)-6P/sub 3/2/(F=5) transition of cold Cs atoms for different optical polarizations. The atomic sample was obtained in a magneto-optical trap. The spectra where recorded within a 1 ms time interval during which the trapping light beams were blocked and the quadrupole magnetic field was turned off. We used a backwards FWM scheme with two counter-propagating pump beams and a probe beam. The PC signal propagates in opposite direction respect to the probe beam. Typical PC spectra as a function of frequency offset are shown for probe beam polarization parallel and perpendicular to pump polarization. Large differences are observed between these two cases demonstrating the crucial role of the Zeeman degeneracy on the atomic response. The experimental spectra are in good agreement with the theoretical predictions for FWM generation in degenerate two-level atoms.
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.