Laser frequency stabilization to 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. Using a room temperature Rb vapor cell as a reference, we stabilize a first diode laser to the D ₂ transition and a second laser to a transition from the intermediate 5P_3/2 state to a highly excited state with principal quantum number n=19-70. A combined laser linewidth of 280±50 kHz over a 100 μs 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 a direct absorption signal.