Recovery of narrow resonances in hydrogen laser spectroscopy

Precision spectroscopy of atomic hydrogen can be used as a stringent test of fundamental physics and new physics models. 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. 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. An update of current progress towards an absolute frequency measurement of 2S-nS transitions will also be shown.