An optical reference based on a two-photon optical transition with ac Stark shift cancellation is proposed. The reference uses two interrogating laser fields at different frequencies. Compared to conventional optical two-photon references, the new approach offers the possibility for improved short-term stability resulting from a higher signal-to-noise and improved long-term stability due to active ac Stark shift cancellation. We demonstrate the ac Stark shift cancellation method on the $5s\,{ ^2S}_{ 1/2} \rightarrow 5d\,{ ^2D}_{ 5/2}$ two-photon transition in $^{ 87}$Rb.
Categories: physics.atom-ph