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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Light shift of coherent population trapping resonances

R. Wynands, A. Nagel, Dieter Meschede, B. A. Grishanin, Victor N. Zadkov

Published Feb 25, 1999• 0 citations
Publisher

Abstract

Although the frequency position of the non-coupled state in coherent population trapping does not depend on light intensities, this is not true for the experimentally observed dark resonance minimum. For precision applications, this shift constitutes a major systematic effect. We have measured the light shift in an atomic celsium vapor, as a function of buffer gas pressures. The experimental data for the unbuffered vapor can be reproduced quantitatively with a simple model, taking into account the atomic velocity distribution and the excited state hyperfine structure. The buffered vapor requires a more complicated model.

DOI
10.1117/12.340115

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We have measured the spectral position of the absorption minimum in a coherent population trapping resonance in thermal cesium vapor as a function of light intensity. The dependence of position on intensity is found to be almost linear. We have furthermore studied the dependence of this light shift on neon bufier gas pressure and flnd a strong reduction for higher pressures. So the addition of a bufier gas not only reduces the linewidth of the resonance but also a very important systematic efiect for precision measurements. An atomic three-level system with two low-lying and long-lived levels (a so-called ⁄ system) can exhibit a two-photon resonance when the difierence frequency of two driving flelds matches the splitting of the two lower levels (flg. 1), !2 i !1 = !12 (1). The resonance is called \dark since one-photon absorption and resonance ∞uorescence are strongly reduced because of the trapping of the atomic population in a coherent superposition state of the two lower levels that is no longer coupled to the light flelds (coherent population trapping, CPT) (2). The two-photon resonance can be very narrow (3, 4), with linewidths rivaling those of the microwave-driven magnetic dipole transition between the two lower atomic states. The use of dark resonances for the construction of atomic radio frequency standards (5) or applications for precision measurements, for instance in magnetometry (6), has been suggested, and flrst steps in this direction have been taken (7). It is an essential requirement for such precision applications to understand the in∞uence of experimental conditions on the position and shape of the resonance under investigation. Because of the flnite dephasing rate between the two lower states the position of the absorption minimum at the dark resonance can depend on intensity (8,1), and in experiments a shift of the center of the resonance (light shift, AC Stark shift) is readily observed (9), as was discussed, for instance, for an atomic beam (5) or in connection with a CPT maser (10). In this letter we report measurements of the light shift of the two-photon resonance that couples two hyperflne ground-state levels of the cesium atoms in a thermal vapor both with and without an additional bufier gas.

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Although the frequency position of the non-coupled state in coherent population trapping does not depend on light intensities, this is not true for the experimentally observed dark resonance minimum. For precision applications, this shift constitutes a major systematic effect. We have measured the light shift in an atomic celsium vapor, as a function of buffer gas pressures. The experimental data for the unbuffered vapor can be reproduced quantitatively with a simple model, taking into account the atomic velocity distribution and the excited state hyperfine structure. The buffered vapor requires a more complicated model.
We have measured the spectral position of the absorption minimum in a coherent population trapping resonance in thermal cesium vapor as a function of light intensity. The dependence of position on intensity is found to be almost linear. We have furthermore studied the dependence of this light shift on neon bufier gas pressure and flnd a strong reduction for higher pressures. So the addition of a bufier gas not only reduces the linewidth of the resonance but also a very important systematic efiect for precision measurements. An atomic three-level system with two low-lying and long-lived levels (a so-called ⁄ system) can exhibit a two-photon resonance when the difierence frequency of two driving flelds matches the splitting of the two lower levels (flg. 1), !2 i !1 = !12 (1). The resonance is called \dark since one-photon absorption and resonance ∞uorescence are strongly reduced because of the trapping of the atomic population in a coherent superposition state of the two lower levels that is no longer coupled to the light flelds (coherent population trapping, CPT) (2). The two-photon resonance can be very narrow (3, 4), with linewidths rivaling those of the microwave-driven magnetic dipole transition between the two lower atomic states. The use of dark resonances for the construction of atomic radio frequency standards (5) or applications for precision measurements, for instance in magnetometry (6), has been suggested, and flrst steps in this direction have been taken (7). It is an essential requirement for such precision applications to understand the in∞uence of experimental conditions on the position and shape of the resonance under investigation. Because of the flnite dephasing rate between the two lower states the position of the absorption minimum at the dark resonance can depend on intensity (8,1), and in experiments a shift of the center of the resonance (light shift, AC Stark shift) is readily observed (9), as was discussed, for instance, for an atomic beam (5) or in connection with a CPT maser (10). In this letter we report measurements of the light shift of the two-photon resonance that couples two hyperflne ground-state levels of the cesium atoms in a thermal vapor both with and without an additional bufier gas.
We have measured the spectral position of the absorption minimum in a coherent population trapping resonance in thermal cesium vapor as a function of light intensity. The dependence of position on intensity is found to be almost linear. We have furthermore studied the dependence of this light shift on neon buffer gas pressure and find a strong reduction for higher pressures. So the addition of a buffer gas not only reduces the linewidth of the resonance but also a very important systematic effect for precision measurements.
When neon is introduced as a buffer gas the interaction time of cesium atoms in a vapor cell with resonant laser beams is drastically increased. Using a pair of phase-locked lasers we have observed linewidths as narrow as $42 $Hz for coherent dark resonances in a cesium vapor cell. We study the influence of power and pressure broadening and systematic shifts of the resonance frequency. Our experiments demonstrate that coherent dark resonances could rival direct radio-frequency precision measurements, which have a wide range of applications in physics.
Abstract A theory is developed for the effect of coherent population trapping (CPT) in ‘hot’ atoms under pulsed pumping in an optically dense medium with a buffer gas. The case is considered when the excited level is not degenerate. Based on an analysis of the shape of Ramsey resonances, we show that with increasing optical density of the medium, the dependence of the light shifts of the CPT resonance becomes more and more nontrivial. The dependence of the light shifts of the CPT resonance on the magnitude of the hyperfine splitting of the excited level, concentration of active atoms, temperature, and duration of the dark pause is constructed.
The off-resonant ac Stark shift for coherent population trapping (CPT) resonances probed with Ramsey spectroscopy is investigated experimentally and theoretically. Measurements with laser-cooled 87Rb^{87}\mathrm{Rb}87Rb atoms show excellent quantitative agreement with a simple theory. The shift depends on the relative intensity of the two CPT light fields, but depends only weakly on the total intensity. Since the origin of the shift is through couplings of the interrogation light to off-resonant excited-state hyperfine levels, the size and sign of the shift depend on the specific interrogation scheme. The theory also shows that for several commonly used interrogation schemes it is possible to minimize the off-resonant light shift or its dependence on the CPT intensity ratio by properly selecting the system parameters.
This paper presents a theoretical study of the light-induced shift of the coherent population trapping resonance. An analytical model is proposed that describes the interaction of two radiation components with an atomic system using a ΛΛΛ scheme and takes into account an additional level of excited state. Both weak and strong coupling regimes with off-resonant transitions are considered. It is shown that, in addition to the conventional dynamic Stark shift, an extra shift arises due to the distortion of the resonance line shape when bichromatic laser radiation interacts with off-resonant atomic transitions. An analytical expression for this additional shift is derived in the weak-coupling limit, and its significant impact on the resonance shape and sensitivity to the intensities of the laser field components is demonstrated. It is found that under strong coupling conditions, the additional shift can deviate substantially from a linear dependence on light intensity, suggesting new opportunities for controlling light shifts in precision atomic devices such as quantum frequency standards.