Laser frequency stabilization to highly excited state transitions using   electromagnetically induced transparency in a cascade system
  We demonstrate laser frequency stabilization to excited state transitions using cascade electromagnetically induced transparency (EIT). Using a room temperature Rb vapor cell as a reference, we stabilize a first diode laser to the D2 transition and a second laser to a transition from the intermediate state to a Rydberg state with principal quantum number n=19 - 70. A combined laser linewidth of 280 kHz over a 0.1 ms time period is achieved. This method may be applied generally to any cascade system and allows laser stabilization to an atomic reference in the absence of strong optical transitions. 
R. P. Abel, A. K. Mohapatra, M. G. Bason, J. D. Pritchard, K. J.   Weatherill, U. Raitzsch and C. S. Adams