A reliable cw Lyman-$\alpha$ laser source for future cooling of   antihydrogen
  We demonstrate a reliable continuous-wave (cw) laser source at the 1\,$S$--2\,$P$ transition in (anti)hydrogen at 121.56\,nm (Lyman-$\alpha$) based on four-wave sum-frequency mixing in mercury. A two-photon resonance in the four-wave mixing scheme is essential for a powerful cw Lyman-$\alpha$ source and is well investigated. 
D. Kolbe, A. Beczkowiak, T. Diehl, A. Koglbauer, M. Sattler, M.   Stappel, R. Steinborn, and J. Walz
physics.optics physics.atom-ph