We have used the Stark-switching technique to observe two-photon optical free-induction decay in the visible using sodium vapor. Transients were observed with use of both a resonant and a nonresonant intermediate state for the two-photon transition.
A new method of observing two-photon optical transients is proposed, namely, by intensity switching of incident cw laser beams. This method, which is an alternative to the standard dc Stark or frequency-switching techniques, employs the second-order optical (ac) Stark effect to throw the two-photon active levels in or out of resonance by appropriately switching the intensities of the two laser beams. Theoretical treatment for the three basic transients optical nutation, free induction decay, and two-pulse echo in thin samples using intensity switching, is given for both the cases: co- and counter-propagating beams. The feasibility of the method for the $5{S}_{\frac{1}{2}}\ensuremath{-}3{S}_{\frac{1}{2}}$ transition in Na vapor is demonstrated. An important feature is the apearance of an intensity-dependent beat frequency in the optical FID signal.
The observation of two-photon optical nutation and free-induction decay is reported. The experimental result, on a two-photon transition in NH3 with all relevant parameters known, is shown to be in quantitative agreement with the prediction of the two-photon vector model.