ECAMP 15

Light shift, or ac Stark shift, plays an important role in vapor cell frequency standards, and substantial efforts have focused on minimizing its impact on frequency stability. Its temperature dependence is, on the other hand, generally considered negligible compared to the pressure shift arising from buffer gas collisions. Thus, it is of primary concern to find the so called inversion temperature at which the first-order sensitivity vanishes by proper choice of buffer gas species. However, in laser-pumped alkali vapor cells, spatial inhomogeneity of laser intensity along the propagation axis can introduce a notable temperature dependence, and the combined effect of collisional and light shifts in coherent population trapping resonances has been observed.
In this work, we present a comprehensive study on how the inversion temperature in a double-resonance Rb clock can be tuned by varying the laser intensity. Our findings reveal that even with a commonly used optical path length of 25 mm, laser attenuation within the cell can induce a significant, non-trivial temperature dependence of the light shift. For a set of the pressures of argon and nitrogen, we observed a transition from a collision-shift-dominated regime to a light-shift-dominated regime as the laser intensity increased, clearly demonstrating the characteristic temperature dependence of the light shift. We developed a simple theoretical model incorporating optical density and absorption line shifts, which was validated by independent measurements of the light shift versus laser frequency. These results offer practical insights for improving the robustness of laser-pumped miniature atomic clocks.