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Optics Letters

Optical probing of cold trapped atoms

Richard W. Fox, H. G. Robinson, S. L. Gilbert, Leo W. Hollberg, John H. Marquardt

Published Sep 1, 1993• 52 citations
Publisher

Abstract

Transitions between excited states of laser-cooled and laser-trapped rubidium and cesium atoms are probed by use of fiber and diode lasers. High-resolution Doppler-free spectra are detected by observation of the absorption and fluorescence of light from the intermediate level of two-step cascade systems. The optical double-resonance spectra show Autler–Townes splitting in the weak probe limit and more complicated spectra for a strongly coupled three-level system.

DOI
10.1364/ol.18.001456

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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.

•Oct 12, 2007•1 citation
Transitions between excited states of laser-cooled and laser-trapped rubidium and cesium atoms are probed by use of fiber and diode lasers. High-resolution Doppler-free spectra are detected by observation of the absorption and fluorescence of light from the intermediate level of two-step cascade systems. The optical double-resonance spectra show Autler–Townes splitting in the weak probe limit and more complicated spectra for a strongly coupled three-level system.
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.
Autler—Townes splitting in absorption spectra of the excited states 6 2 P 3/2 –8 2 S 1/2 of cold cesium atoms confined in a magneto-optical trap has been observed. Experimental data of the Autler—Townes splitting fit well to the dressed-atom theory, by which the fact of the cold atoms dressed by cooling/trapping laser beams is revealed. The results of the theoretical fitting with experiment not only told us the effective Rabi frequency cold atoms experienced, but also could be used for measuring the probability amplitudes of the dressed states.
We present a study of a two-photon double-resonance transition in cold cesium atoms in a magneto-optic trap. A closed five-level density matrix model of the system is developed to characterize the double-resonance Autler–Townes-split absorption line shape. Comparing this model with experimental spectra, we gain insight into the dominant process involved in trapping atoms, in particular, the importance of the interference of the six trapping laser beams. This system is used to probe transient effects in the trap as well as collective effects of the trapped atom cloud resulting from the intentional misalignment of the trapping beams.
The interaction of light with atoms and molecules is of fundamental interest in many branches of science. In atomic physics, this interaction can be used to cool and spatially confine (trap) atoms. These traps can be used as the starting point for other experiments, but the dynamics of the cooling and trapping processes is itself of interest. In order to better understand the physics of trapping atoms in an optical dipole force trap, we have conducted a series of spectroscopic measurements of ultracold rubidium atoms in such a trap. The trap was created at the focus of a Nd:YAG laser beam with wavelength 1064nm and nearly Gaussian spatial mode. For rubidium, the trap light is red-detuned and the atoms are confined to the highest intensity in the beam. In order to probe the atoms confined in the trap, we have performed spectroscopy on the 5S1/2 → 5P3/2 transition. Because the polarizability of the ground and excited states is not the same at the trap wavelength, the spectra exhibit both a shift and inhomogeneous broadening. We have investigated the spectra for both linear and circular polarized traps. We also studied the application of a second laser to couple the excited state (5P3/2) to another higher excited state (5D5/2) to reduce the inhomogeneous broadening of the 5S1/2 → 5P 3/2 transition. Finally, two-photon spectroscopy was performed on atoms in the dipole force trap and compared to spectra taken in a magneto optical trap (MOT). Autler-Townes splitting was clearly observed in the MOT and appears to have been observed in the dipole force trap also.
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.