Light Shift of Coherent Population Trapping Resonances

Abstract

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.