AC Stark shift mitigation in two-photon hydrogen spectroscopy

Precision laser spectroscopy of hydrogen can provide a test of fundamental physics and the ability to determine constants such as the Rydberg constant and proton charge radius. However, to rigorously test theory, multiple transitions need to be measured with a low uncertainty. Two-photon transitions are appealing because they can be made first-order Doppler free. Unfortunately, they also require large laser intensities to obtain adequate signal-to-noise, resulting in a significant AC Stark shift. The AC Stark shift broadens, and in some cases distorts, the resonance in a beam style experiment. Here, we will discuss a method to largely mitigate the AC Stark shift in hydrogen 2S-nS transitions by introducing a second laser field which produces an additional AC Stark shift with similar magnitude, but opposite sign. We will show experimental results that demonstrate the recovery of atomic resonances that approach the natural linewidth of the transition.